Crazy Radio Issue, Help Needed.

[hangflyer]

Ok, I have a radio issue that has been plaguing me for a few weeks, I have taken every step possible to try and resolve it and nothing has worked. At this point I have asked every LAME and Radio/Avionics outfit at Jandakot for advice, or they have had a go at locating or resolving the issue.
I am out of ideas and so I am looking for suggestions.



Firstly here is the issue.

There is an intermittent white noise that only appears on VHF (ADF is clean) that reduces a received radio transmission to 2's or 3's, however it is extremely directional and only happens if the station making the transmission is on the nose of the aircraft, if whilst receiving the poor signal I request a radio check they get me 5's If I swing the nose of the aircraft 20 degrees either side of the station the signal becomes perfect again. This happens whether the transmitting station is 500 meters or 10 miles away. It only happens when the engine RPMs are above 2200. There is no noise at 2150rpm and then at 2200rpm it is there in full intensity, it does not increase in any way (pitch/strength) as RPMs increase.

The aircraft has 2 separate radio's connected to 2 separate antennas, they both behave the same way.

Sometimes the noise is there and then it is not. It can be gone for a whole day and then return for the first flight of the next day. I can not correlate it with temperature or humidity or rain or time of day or anything, I have tried! The aircraft is working every day for multiple short flights and the problem comes and goes with annoying regularity, you just never know when it will re-appear. It started about 1 month ago, prior to that is was not present. It did not coincide with any maintenance or event.

The Aircraft is a Aero Subaru (Fuji FA-200) Lycoming IO-360B1B with a McCauley B2D34C53 constant speed 2 bladed prop, this is very similar to the powerplant found in a Piper Arrow. The Fuji is a 4 Place all Metal (Aluminium) low wing aircraft with fixed undercarriage.


Here is what has been tried so far:

Idea 1, the radio antenna is on the rear fuselage, perhaps the front of the aircraft is blocking the signal. Even though the SWRs checked out as perfect we replaced the antenna with a new higher gain one, it made the problem worse! Com 2 with the old poorly positioned antenna now gets a weaker white noise interference than com 1 meaning it is 3's when comm1 is 2's

Idea 2, It is poor earthing of the engine or mags. All the aircraft earthing points cleaned and shielding repaired everywhere. No Change

Idea 3, it is bad plugs. All plugs replaced with new plugs. No Change

Idea 4, the alternator. Engine run with alternator field disconnected (so not energised) and master bus disconnected. No Change.

Idea 5, Static build up, all static bonding points and wicks cleaned and tested. All OK. No change.

Idea 6, Noise filters on the alternator and master bus. Both filters replaced with new units. No Change.

Idea 7, A Loose connection. Every connection under the dash tested and tightened. No Change.

Idea 8, Capacitor in Mags breaking down. Mags overhauled accumulator/capacitors replaced. No Change.

Idea 9, Turn and Bank Coordinator causing noise. Fuse pulled whilst engine running. No Change.

Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change.

Idea 11, Prop making static charge, prop cleaned with solvent and re-painted. No Change.

Idea 12, Radio faulty, radio swapped for new unit, No Change.

Idea 13, Magnetised Crankshaft, but this could not come and go, so discounted.



I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio. The noise does not change with pitch, it is not a whine, it does not get stronger or change frequency. It is just white noise.


If anyone has any ideas, please let me know. No-one seems to have any idea what could be causing this.

[boguing]

I have no knowledge of aircraft engine ancillaries. Cars, motorcycles and boats however...

One thought. If it has a centrifugal ignition advance type distributor, is there any chance that 2,200rpm is the speed at which this one is activating at? And if so, is there some odd metal to metal contact creating noise? 2,200 is way too fast for a normal one.

[Uncle_Jay]

Take the radio out of the A/C does it still make the noise?

Remove the antenna connection - does it still make the noise ?

Same with headphones ?

Do ground stations hear the noise or just inside the plane ?

[ion_berkley]

I'm sure you have tried a lot of things that you didn't write up in your summary so apologies in advance if I'm suggesting something you have covered here.

It's kind of implied that the radio is still squelching correctly, and that you only hear this noise when a station is actively being received and triggering the squelch? If you disable squelch on the radio is the (offending) noise of a constant amplitude regardless of whether a strong signal is being received? I find the directionality you noted very hard to explain.

The change of antenna to a high gain on COM1 causing the noise to become louder indicates strongly that it's RFI coupled via the antenna rather than via power cabling/chassis etc. Does selction of different radio frequency affect the noise amplitude in any way? Noise generated from an engine source is likely to be relatively low frequency and coupling into the last stages of the radio electronics so it's likely unaffected by tuning but affected by audio controls such as volume. Does no antenna at all cause the noise not to be heard?

The fact that the frequency of the noise is completely unaffected by RPM really makes it not sound like classic alternator/magneto noise sources. I know you described the noise as "white noise", but is it really a pure random sounding hiss of wide audio frequency range or does it have some type of structure? What could possibly form a simple "off/on" switch for RFI based on RPM? Position of the governor?...that's a completely mechanical solution surely. And if the noise is frequency independent of the engine RPM then what is the frequency source for the noise?

Can you hear it on a handheld radio? If so you might have some luck sniffing for a source, but that's logistically formidable beyond sweeping the in cabin avionics.

Sorry, scratching around, just trying to give you new ideas.

[hangflyer]

I'll reply to you all at once to save some time!

The prop is Aluminium a McCauley B2D34C53

The electric fuel pump can be turned on and off with no change, could the engine driven mechanical pump create RF?

The Mags were removed, disassembled and overhauled, they check out fine.

The noise in only on recieve, only VHF and only the aircraft hears it.

Having the squelch off makes no difference. 2 Radio's fitted to the aircraft do the same thing. One radio was totally replaced with a new unit that did exactly the same thing.

I have tried taking a hand held on a flight and that never picks up the noise, so when pointing at a station the built in radio will get 2's and the hand held will get 5's I can have the antenna of the handheld quite close to the aircraft antenna and with the same line of sight. This seems to suggest it is something that is leaking into the aircraft fuselage from somewhere.

I have not tried no antenna yet, I will try today.

Thanks for the suggestions.

[glum]

Do you have access to a TDR? (Time domain reflectometer).

It's used to scope coax cables, and check the continuity of the signal wire, as well as the dielectric. If there's any breakdown it will show up.

Does the radio pick it up if you disconnect the aerial input to the T/R?

How about if you disconnect from the aerial, but leave the T/R connected?

Maybe there's something switching on at 2200 on that engine type - a governer perhaps? What's max RPM on it?

Be interesting to get a spectrum analyser on the output of the T/R whilst it's doing this, and see what the frequency band was...

[ion_berkley]

OK, your mention of the fuel pump got me thinking...low wing right? So there's going to be an auxillary electrical fuel pump also...that's going to always run at a constant speed regardless of RPM...question is when is it on? I'd have thought it would be a manual switch only but I'm at the edge of my knowledge base here.

The fact the handheld doesn't hear the noise eliminates something, the noise is not "inband" in the VHF range. Such a typical noise source would be modern and microelectronic/digital in nature..anyhow that possibility is eliminated in my mind now. The handheld most likely doesn't hear the noise source because the internal design of the radio is completely different and so it's potentially sensitive to different frequencies of noise.
(Typical radio designs transform the signal several times to different
frequency ranges before it becomes an audio signal, and each stage generally has a filter that only passes on a certain frequency range to the next radio stage. These frequencies are typically referred to as IF - intermediate frequencies and are generally much lower than the RF signal you are listening to)
I take it the replacement radio's you tried were the exact same design/model?
I am still bothered by the directionality you report....the direction of the noise source relative to the radio/antenna position should not be changing regardless of the position of the station you are receiving.....so why do you only hear the noise when the station is in a certain orientation?...there is only a single antenna per radio, right?

[ampclamp]

I think you must have a bonding issue combined with some antenna shadowing.

Worse when the engine/prop cowls are between the transmitting station and your antennas.
When at right angles to the other station the antenna is probably not being shadowed as much by the fuselage, gear, prop etc allowing a bit more of the other stations audio through.Remember most bonding problems produce fairly weak signals but create localised havoc.
I have had several very tricky bonding problems particularly on VHF comms systems where HF ADF are unaffected.

If it only happens with engine revs it must have something to do with the engine/prop (or relative airspeed with revs at those levels).I also include what engine vibe may be doing to airframe bonding points.

I note no maint action was done to cause it and it is intermitant.This leads me further to the bonding conclusion.Maybe caused by eng vibe that only occurs above 2200.Just enough to break done a poor bonding wire or point.
Silly question, have you ground run the engine in the dark over 2200 to see if you have any flashover anywhere? Hard I know.
Does it only happen at altitude, reproducable on the ground, ever?

Please keep us informed.

[forget]

Ion Berkley. The handheld most likely doesn't hear the noise source because the internal design of the radio is completely different and so it's potentially sensitive to different frequencies of noise.

Ion is right but I think the only way you’ll pin down the source of the problem is to use a radio as a mobile detector. If you can get hold of a connector to the back of one of your radios then wire up only the power lines, the phone output (terminated in a phone socket) and the antenna with six feet of good quality 50 Ohm coax. Plug in a decent antenna to the coax. Tie the antenna onto a length of broomstick.

Power the radio with a battery and take it aboard. In flight, have a passenger listen in to the headset while moving the antenna around the cabin, windows, and into the foot-wells.

If this doesn’t exactly pin anything down it may give you an area for the source of interference.

[ampclamp]

The flashover I refer to may occur in hi tension ccts and can be visible in very low light. Having looked under a few hoods / bonnets sometimes you can see where the electricity is leaking out.Picked this up from old WWII heavy piston guys years ago.
I dont think this is the problem and this kind of testing is not routine.

But I do think it maybe a combo of bonding and perhaps some shadowing.
I'm not betting my gonads on it but am very interested in the fix to further my knowledge.Could dig me out of the poo sometime!
I am avio and have reasonably wide experience also a ham operator.

[mono]

Steady on guys!!

While the attempts to resolve the problem are admirable you can't just start drilling holes or installing unapproved equipment in an a/c.

In my opinion it's almost certainly the co-ax. VSWR can still show within limits for a degraded co-axial cable. Try slaving in a nice freshly made and checked cable and do the checks again. while you're at it check the bonding of the racks and the antenna too.

[ion_berkley]

Mono, I am all for stripping it back to basics with a frustrating problem like this and it's definately worth conclusively eliminating the coax...but bear in mind that he reported an identical problem for both COM1 and COM2...both coax's bad the same way?

Someone also suggested headset...hmm, it's common audio for both radio's...got to be worth eliminating the possibility that it's a fault related to the audio panel too ...though again I think the effect that changing the COM1 antenna had says that the radiator is getting in via the antenna.

[GarageYears]

Re-reading the original post, the problem occurs when:

- Engine RPM is 2200RPM or greater
- Tuned station is directly off the nose
- Is a receive only effect
- Noise does not change in pitch or volume (i.e. it is on or off)
- Noise sometimes is not present, other times it is
- Only on VHF

So, the headset has no idea about any of these things, so let's eliminate that.

So what's possible?

1) Above 2200RPM the received signal strength is significantly reduced?
2) Above 2200RPM something is generating a wideband VHF whitenoise signal? But the problem with this as a theory is what does that have to do with the direction of the source (wanted station) - nothing, so this does not make sense.

If the coax was bad (for both radios!) then how does the orientation of the aircraft w.r.t. to the source station matter? Nope don't think that is it.

What does the hand-held tell us? The fact the hand-held was OK, makes me think this is not generated RF (per [2] above), but to me reinforces the idea that the problem is reduced signal strength when the station is directly off the nose, and related to the aircraft antenna installation w.r.t, the engine/prop orientation.

Unfortunately I really can't come up with a believable theory for this right now. :hmm:

- GY

[ion_berkley]

GarageYears, I agree with your reasoning. I brought up the headset/intercom because it's not entirely clear to me here if we have a problem with an unusual radiator (RFI source) or a shielding problem (RFI sink). I do like your received signal strength thoughts.

What happens if we rewrite your 2) to be: Above 2200RPM something is generating an RF whitenoise signal? (intermittently)

Then we also introduce your concept of an antenna null towards the front of the aircraft (The OP does refer to the antenna installation location as "poorly positioned")

I wish we knew if its the the noise amplitude or the station amplitude that varies intermittently.

[boguing]

GarageYears.

That's why I proposed the distributor theory. Advance happens at a fixed speed. It's in the nose.

And I was wrong about the speeds. 2,200 is right in the band for centrifugal advance changes.

Currently I only have diesel engined cars here, so I can't take a gander, but my memory tells me that the stops for the weights are generally hammered pimples on the backplate.

[ampclamp]

Not sure if you have read my previous posts but you seem to agree it could be a shadowing of the antennas causing reduced received signal strength.But....
There is the issue that it has not always been a problem and apparently not related to recent maintenance , so in addition, I think there maybe a bonding problem causing the white noise that is only a problem when the received signal is low.
I have had an aircraft within 800 metres of the tower, line of sight and no signal. Rotated the aircraft a few degrees and I'd get 5s. The antenna was shadowed almost perfectly by the fuselage and other antennas.We could just hear the tower but they could not get squeak out of us.The aircraft was pointing directly at the tower antennasTrue story.

But given that this issue just cropped up gives reason to believe we have 2 conditions causing the defect.
If the handheld is not getting anything it maybe because it is not grounded to the fuselage which forms part of the bonding circuit.
If parts of the fuselage/ engine / cowls are not bonded there maybe circulating currents / potential differences causing the white noise.

Boguing if what you say is right the dizzy maybe a problem

[stevef]

Sorry to say that the ignition timing is fixed on almost all piston-engined aircraft. The chain is: two gear-driven magnetos timed with each other (typically 20 or 25 degrees btdc on a Lycoming) with points & condensor in each, a rotor/distributor cap assembly, HT leads, then spark plugs. Some magnetos have retard points for easy starting but there's no facility for automatically advancing the ignition with rpm increase.
Rather Neolithic in comparison to automotive engines but simplicity is the key, even to the slight detriment of efficiency.

[boguing]

Oh booger.

Here was I congratulating myself on being an aviation Holmes.

All I can say is 'how quaint'! Fred drift, I know, but do Fadec types embrace the last century?

[FullOppositeRudder]

I've read this thread several times and the problem is very puzzling. There are no obvious answers - only a large number of improbable single events or combinations which might be the cause. Component substitution - where possible - is one way of isolating the issue, but you've already gone some distance down this track.

On balance it seems highly unlikely that the noise should be coming in through the RF side of the system - i.e. antenna / coax combination. A check on the VSWR for each installation would be worth trying. It's accepted that this checks primarily the transmitting side of any given radio installation, but such a check, if satisfactory, should further eliminate the chance of the problem noise coming in through the antenna. This seems especially unlikely anyway given the expected signal strengths in the regime under test. These should be very strong signals, so much so that their being overloaded by an interference source seems highly improbable. Unless of course it's a large arcing issue somewhere which is more likely to produce a cracking sound rather than a (squelch open) white noise.

It could be useful to connect the hand held antenna input to one of the aircraft antennas if that hasn't been tried (and can be). If the noise is still there and on the HH, then the antenna installation is suspect in some way or the noise is coming in through the antenna system.

If this were to be eliminated, it leaves some other problem common to both radios. Is some sort of critical system voltage level (upper limit) being reached at the 2200 RPM level? Is there any other system indicator which betrays something special about that RPM level.

If it were possible, (and safe naturally) I would like to try the radio/ intercom on a DC supply system independent of the A/C DC rail. Come to think of it, is the main A/C DC system nominal 12 or 24 volt? I'm presuming that the radios and com system are on 12 volt. Can other significant electrical loads like lights etc be switched on (or off) when the noise is present? Does this change things - even in minute levels?

Clues may lie in a critical appraisal of the noise itself. Is it exactly the same intensity / pitch / character as what is heard when the squelch is opened manually?

I'm clutching at straws like everyone else. But these are things which I would follow up if the issue were mine. I hasten to add that I'm not in the aircraft maintenance area. My background in this area comes from being an amateur radio operator with a long experience in mobile as well as other fixed installations. I've occasionally had the need to look into 'simple' A/C radio installations. There are a lot of variables in the way these things are set up, rarely are there any circuit diagrams, or explanations at to why things have been set up as they are, and almost every installation is 'different'.

Congratulations on your perseverence up to this point. There has to be solution somewhere, but for the moment anyway I'm as puzzled as everyone else. I do hope you can get to the bottom of it.

Sorry I can't offer more for the moment. It really is a puzzle.

regards
FOR

[crippen]

No experiense of planes,but had a similar "funny noise" on marine radio.

Turned out to be a cable with the screen 'earthed' at both ends. Any screening should only be grounded at one end,preferably at the 'sending' end.:ouch:

[forget]

Any screening should only be grounded at one end.

All radio coax in aircraft is grounded at both ends.

[dixi188]

RF co-ax should be grounded at both ends.
Screened audio wiring should be grounded at one end only.

I will go for a bonding issue with the radio boxes.
It is probably some sort of prop modulation or screening that is always there but does not show up with a good installation.

What is the range of the radios when airborne? Is it a lot less than it used to be?

Do a bonding check with a bonding tester, (they used to have a double probe and a single probe). I think the readings should be 0.02 ohms or less.
Do a check of all bonding including engine to airframe and all components.

Remove all earth connections, clean the metal and re-make the connections if there's any doubt about the bonding.

I'm a bit rusty on this stuff. It's over 30 years since I did installation and maintenance on these things.

Good luck.

[hangflyer]

I can add some more information and deal with some suggestions.

The interference is highly directional and also linked to the RPM somehow. When it initially started it cut in at 1900RPM , after we changed the plugs for new plugs it started at 2100RPM, since the Mags were rebuilt it moved to 2200RPM. As we have totally eliminated anything in the ignition system as the cause of the noise I think it is reasonable to assume that it is linked to the vibration from the engine, the engine has run smoother as the ignition system was modified and improved.

The prop is well balanced and the idea that the prop is somehow out of balance is unlikely.

Yesterday the aircraft had the problem for most of the day whilst it was working, and then I dropped of the last passengers and headed out to the run-up bay to do some testing and the problem was gone. I made 4 attempts over a couple of hours to get the problem to re-appear so that I could do some tests and it never did.

This morning for the first flight of the day the problem came back immediately and was present for every flight during the day. I finished late and did not have a chance to do any testing, tomorrow morning I am planning to do more tests.

The radio guys tested the VSWR of both radios and they were all fine. The other end always gets the transmissions as 5's which would seem to rule out shadowing as the cause. Also there are 2 antennas in different locations on different radio sets that both behave the same way. The only difference being that since we replaced the antenna on com1 with a better larger and higher gain antenna we pick up the interference more strongly with the result that the transmitting station is much harder to understand.

The noise can only be described as a white noise, no distinct pitch or tone, no relation to engine RPM except that it appears and disappears within 50rpm of 2200rpm every time. There is a chop to it, but it does not change with RPMs.

I have run the aircraft with everything except com1 de-energised, including the alternator, with no change.

I think we have ruled out the ignition system as the cause.

We have ruled out the alternator.

We have ruled out every other electrical device.

I have more time tomorrow between flights and I am hoping the problem is still there and I can try some troubleshooting.

Specifically I want to try the radio with the antenna disconnected, with an ungrounded antenna connected, and the hand-held connected to the fixed antenna to try and determine where it might be coming from.

Thanks for all the suggestions so far, I'll keep you updated on my progress.

[forget]

hangflyer, what's your NAV audio like? Affected at all?

Now this really is a long shot - does changing humidity have any affect?

[stevef]

That's strange, forget - I was going to ask the very same question re humidity! Reason being the HT leads. Have they been checked with a lead tester? Humidity has an effect when they're on the way out.

[ChristiaanJ]

hangflyer, what's your NAV audio like? Affected at all?
forget has a point there.... it's the same frequency band.
CJ

[ampclamp]

forget . stevef re humidity , sorry but he answered that in post one.

[Smudger]

Sorry, off - thread, but why has the word "issue" replaced the perfectly suitable word "problem" ? We don't have "problems" now, we have "issues".... I can't STAND IT !!!!!! The two words mean two completely different things... but the word "issue" is now used to mean "problem" ! WHY ???? Apologies for going off - thread ...... but no apologies for making my point!

[mike-wsm]

The noise can only be described as a white noise, no distinct pitch or tone, no relation to engine RPM except that it appears and disappears within 50rpm of 2200rpm every time. There is a chop to it, but it does not change with RPMs.
Might be a clue - can you describe the chop? Frequency, depth, m/s ratio? Any chance of recording it and doing some lab analysis with a scope or spectrum analyser?

[stevef]

Sometimes us over-fifties have the attention span of a geriatric goldfish. :O

[Paradism]

Hangflyer

If all else fails, there is a possibility that you may have an odd static problem. Static can do odd things so I would check that your aileron, flap, elevator and rudder bonding is good. Also check that any static wicks that you have are present and in good condition.

Just an outside chance but who knows?

[mm43]

Having had a close look at the suggestions previously offered, I suspect that the "white noise" is just that. The problem probably relates to a diode or zener diode being triggered into white noise mode by a particular range of circumstances. Humidity playing a part, could also be indicative of some capacitive coupling that changes marginally and aids to trigger the problem.

Now you'll need a "portable" white noise detector to home in on the probable source.

The fact that reception is poor from on the nose, doesn't mean that the source of the interference is forward, in fact the maximum pickup of the noise will be on the side of the antenna, which gives you a huge scope. So, look closely at the airframe wiring schematics and identify the location of any possible culprits. They don't necessarily need to be diodes, a corroded connection could also exhibit similar symptoms.

Your problem, to reproduce the effect on the ground!

[Dimitris]

@mike-wsm (http://www.pprune.org/members/328420-mike-wsm)

How can he do spectrum analysis on recorded audio of the noise? VHF is in the MHz, audio capture in the kHz. If its RF noise he should have to connect spectrum analyser on antenna coax. After the radio 'box' everything is 'translated' (modulated?) in audio spectrum. Frequency of noise would be lost, or would be missleading after modulation in audio freq.

Maybe I'm wrong or misunderstood (in that case delete the post)...

If I'm not, alternative solution:

Get an osciloscope from someone having acess to a univ./school lab, they have 50ohm coax input, have it do an FFT on the antenna input of the radio. No idea how you power the osciloscope in the plane though.
Spectrum analysers are usually bulkier than osciloscopes. at least the ones I have access to. Some osciloscopes save the data also in USB sticks for later process.

[mm43]

DERG

The basic detector is nothing more than a full wave or half wave vhf/uhf pin diode rectifier with an antenna on one side and high impedance meter to measure the output voltage on the otherside. No rocket science, the detection method is looking for a noise source within a meter or two and not miles away. Commonly known as Field Strength Meter.

Simplicity, is an existing VHF airband portable with some means of measuring the signal strength of the input noise on any vacant frequency. Access to the AGC voltage is a means of doing so.

There will be heaps of details on RF rectifier devices (http://www.qsl.net/n9zia/wireless/pics/fs_meter.png) on the www.

[ion_berkley]

@Dimitris, I think the point of @mike-wsm was that spectrum analysis/scope of the audio might give an important clue to the noise source....you got a piece of rotating machinery with dodgy brushes/a spark etc, you're going to see some big spikes proportional to its RPM. Also remember that RF doesn't start in the MHz...radio 4 is still on LW @ 192kHz! It can go MUCH lower than that...

Sure it would be really nice to sniff the actual RF spectrum near the antenna, or the power leads etc too. I have just the portable 5GHz analyzer for the job..but it's not the kind of gear most people have to hand.

It's also important to realize that a great many noise problems are not "in band"..that's is in the frequency range the radio is tuned to receive signals at. The RFI finds it's way into internal circuits that are tuned to work at other typically much lower frequencies and get's mixed in with the real signal there. This noise MIGHT be VHF wideband (i.e throughout the 118-136MHz bands) but very likely it's not (handheld not hearing it was a big clue), it's probably something much narrower band and of much lower frequency.

Hopefully the next batch of experiments disconnecting the coax etc will tell us what path the noise takes into the radios and well be able to eliminate some of the many good suggestions we have.

[ampclamp]

"Sometimes us over-fifties have the attention span of a geriatric goldfish"

Don't worry mate, wrong side of 50 here too. I had to double check the first post myself.The trouble is as threads get longer less newcomers read all that has gone before.End up going in circles.Very easy to forget with so may inputs.To prove this we have the following...

The fact hangflyer has actually moved the noise from 1900 by changing the plugs then from 2100 to 2200 with a mag rebuild must be telling us something.
In the same post hangflyer says we have ruled out the ignition system but so far that is the only thing that has positively done something tangible to the noise apart from a higher gain antenna which made it worse.So whatever it is, it appears to be coming down the antenna and has some relation to the engine ignition system.Maybe not THE cause but it sure looks to be affecting it.
Occham's razor?

[mnttech]

How about changing out the plug harness? I did not see anything in the posts about that one, and a shifting ground may explain the 1900 to 2200 difference. Or with the repaired/rebuilt mags the energy has changed some how.

Changing the Coax to Triax might help too. We used them on the medical helicopters. Ground the outside cover at the transmitter, and the inside as normal.

[Aerials]

Hi chaps, another radio amateur here to just chip in a bit, for what it's worth. The directionality seems to me to be related to the signal strength of the ground station at that particular heading, compared with the on-board noise generator. Weaker in my opinion. OP, have you listened with the squelch wide open on the frequency(ies?) of interest when the ground station ceases transmitting? Do you hear the white noise now? Do you hear it on channels one up and one down with the squelch open? Trying to ascertain this would give us all a clue and prove my signal-to-noise ratio. If the noise is still present it certainly would appear to be on-board as there is a slight doubt in my mind that it is for sure. If it is, I strongly suspect a voltage regulator somewhere and after doing the usual load shedding try switching off each VHF one at a time in case one is affecting the other. I do hope the above can be of some use and hope you find and publish the cause soon! Good luck!

[GarageYears]

Analyzing the audio could certainly be useful - that is exactly what the problem is! Something nasty ending up in the audio - so looking at it in the frequency domain may provide some very helpful clues. If there is any periodicity to the noise then that information may just lead us to the source.

I have a bunch of very nice analysis tools available (my main work is flight simulator audio) and would be happy to see what I can find.

- GY :ok:

[Big Pistons Forever]

Does the noise disappear if the cabin speaker is selected, rather than listening in the headset ?

[Mike-Bracknell]

If it's a specific rpm-band, and changed when items on the engine were refurbed, could it be emanating from the ECU or something controlled by the ECU that only gets activated at a specific RPM - e.g. manifold bypass or something.

(sorry for the vagueness, I come from the world of cars rather than planes)

[mike-wsm]

I come from the world of cars
Er, Bracknell, cars, sounds rather interesting. Looking forward to 13 March I guess? Or 27th?

I think the point about ignition improvements relates to vibration. By improving the ignition and making the engine run smoother the onset of propellor vibration is at a higher rpm. Prop vibration is torsional in nature and is caused by aero engines having no flywheel so producing torque variation as the cylinders take it in turns to go bang. In this airplane the prop vibration band is 2250rpm to 2550rpm. Vibration is affected by prop diameter (mustn't be reduced below 72.5 inches during repairs) and compression ratio (nominally 8.5:1).
.

[good spark]

had a very similar problem to this once on a seneca 3, turned out to be a radio noise filter that was in the wireharness behind the centre panel, btw it illustrates just how directional the shape of the airframe can be, perhaps heli peoples can add a few points here
going back to the seneca we found it after doing a 360 while listening to the noise and at one position hey presto the sound reappeared but inflight it was there all the time, let us know what you find



gs

[FullOppositeRudder]

I'm not sure if I should be doing this, but is this the one we are working with (http://rolfair.com/fjihire.html)? If so, there is another photo on the WWW which shows the two antennas more clearly. It may help in getting a clearer picture of how the fuselage might actually shield a weaker signal. Two days ago I discounted this possibility. Now I'm not so sure.

Was the aircraft exposed to a nearby lightning event prior to the problem presenting?

Fascinating thread. We need to to solve this one - we really do. Very sweet looking machine BTW:)

FOR

[hangflyer]

I'll add an update and answer some more questions.

Firstly FullOppositeRudder (http://www.pprune.org/members/300106-fulloppositerudder) has correctly identified the aircraft, well done! That photo is a few months old and she is now sporting a new front antenna which is one of the large angled fibreglass units, mounted with 4 bolts, and perfectly bonded.

As far as I know the aircraft was never exposed to any lightening. This problem started for no apparent reason on an ordinary day and due to it's intermittent nature is extremely hard to correlate with anything. We have investigated the possibility of a magnetised crankshaft or something else, but everybody seems to agree that this would always have the same (or similar) effect. It could not start and stop.

During the last few days the problem has been present for about 50% of the flights. On one day it was there from cold first thing in the morning and then mid afternoon it stopped during the peak heat of the day (37'C)
On another day it was not there in the morning and started in the afternoon. The following day it was not there all day. It absolutely does not coincide with variations in temperature or humidity, it seems completely random.

Each time I have had time to troubleshoot the issue it seems to stop and so I have not been able to disconnect the coax to the main antenna to see what happens.

When I listen to the audio on the VOR when I am having the issue it seems to be clean, however the VOR antenna is mounted high on the tail and as I am unsure as to the directionality of the VOR antenna or the source of the transmitting station I cannot be sure it is not affected.

The noise is the same whether listening through any headset point and when bypassing the intercom, there is no speaker fitted.

When the problem is there breaking the squelch makes no difference. The noise is there on all VHF channels when receiving a transmission, if pointed straight at the transmitting station.. I have not tried tuning to an unused frequency and breaking the squelch to see what happens, I will try that next time I fly.

I'm leaning towards the dodgy diode/capacitor/connection somewhere in combination with vibration being the cause, but I am running out of places to look!

[mike-wsm]

Canis Loopus

Aaah, lovely photos, I want the 45 minute Scenic Flight plus aerobatics, 4g loop and Immelman, please.

Aerobatic Flights from Jandakot Airport (http://rolfair.com/aerobatic.html)

:D

PS Here's the pic mentioned by FOR above:
Fuji FA-200 VH-FJI - Recreational Pilots Australian Aviation Image Gallery (http://www.recreationalpilots.com.au/gallery/details.php?image_id=3297&sessionid=a59efcbc07fbf41d1f1812ae6dfd9ec7)
.

[Aerials]

Hangflyer, thank you for the update at post #59. Going back over some of the posts the problem is definitely related to engine revs so I wonder if it is possible to temporarily borrow another alternator? I know you've had the engine run with alternator field disconnected and I don't know enough about alternators to be sure that there isn't some residual magnetism keeping the field alive perhaps. Not enough to be able to draw much current but enough to be rectified in the diodes.

What is immediately 'downstream' of the alternator? Is there a voltage regulator that could be temporarily swapped?

Like many others, I feel that antenna shielding, weak received signal (as a consequence of shielding) and the rpm are major factors in this. I'm sure many of us are pleased that the noise is not apparent on ADF or Nav frequencies, this indicates that the noise is of somewhat narrow bandwidth and this can be a sign of a voltage regulation circuit causing the problem.

Keep at it - we will get to the bottom of it in the end!

[Self Loading Freight]

You say the noise is there when you're receiving a transmission and actually reduces the signal strength - I'm assuming you do mean signal strength, and not just readability, and that the noise isn't there when there's no transmission.

This sounds like modulation noise. That happens when the radio signal is getting into some part of the electronics and being modified before getting into the radio's signal processing chain. In domestic receivers, it can sound like a loud hum that only appears when a station's tuned in -- in such cases, the mains rectifiers are getting the signal from the mains lead, modulating it with the mains frequency, and then propagating the resultant mess back into the radio. You can also get similar effects when a completely independent circuit has a fault that makes it marginally unstable, and the radio signal either tips it over into instability or combines with an oscillation or noise source.

Diagnostic characteristics include the problem getting worse (or only appearing) at strong signal strengths and often going away completely for no apparent reason. It needs quite an obscure set of circumstances to kick off.

I think the chances of it being connected with power regulation are high, as such circuits do have a good ability to become wideband, powerful noise sources combined with lots of non-linear junctions that make good modulators and plenty of long wiring runs that act as antennas. Plus, although it's a rare beast, you do seem to have eliminated everything else!

[forget]

Have we been told what model/type radios they are; and serial numbers?

[Gillespie Field]

Sorry if this has been already suggested (I don't have the time to read all the posts).

When the handheld was used was its own rubber antenna used or was a coax link used to hook up to the aux antenna output?
If the Aux Antenna socket wasn't used then could you either, do this or if you don't have a link cable (coax with BNCs professionally fitted on both ends) remove the BNC connector from the back of the radio, bring it into the cabin and connect directly to the handheld. If the noise is still absent from the handheld at all times then your Antenna installation is not causing the problem. If the noise is present as originally described then I would (initially for testing purposes) run a temporary brand new coax cable from the handheld (again with a professionally fitted BNC plug on the radio end) to the antenna connection (removing the original coax temporarily) making sure that both the centre core and the braid are not bridged by loose braid hairs using a magnifying glass and very bright torch to be 100% certain.
If as a result of this new temporary installation the radio test is clear of all interference then you existing Coax cable is causing the problem. Replace it with a brand new cable and professionally fitting a new BNC plug on the radio end.

[Mr Optimistic]

but is there an electrical generator which gives an output dependent on input rpm such that at a certain rpm some voltage or current switched effect occurs eg current dump or voltage regulation ?

Just an ignorant guess.

[mike-wsm]

Another thing to try with a hand-held is to turn down the squelch until you hear the noise and then move it around looking for the noise source. Best to use a loop aerial for this, just a loop of wire soldered to a bnc connector, it should be directional, max noise when its axis is toward the source and zero when at right-angles.

[cone zone]

I don't believe that it is alternator or ignition noise as almost invariably this would come through in your ADF.


Try a new battery, this is the biggest capacitor in the aircraft and may just arrest that noise.

good luck

[GarageYears]

Any more news - while not quite "keeping me up at night", is certainly a mystery I would like to follow to the bitter-end.

There are some points we should not loose - the noise is not evident on the nav aids (were those ever explicitly identified - ADF, ???) and not on the hand-held, but on both aircraft radios.

So far we have not heard what happens if the hand-held is connected to the aircraft antenna...? I believe the aircraft antenna WAS updated at some recent point?

There was also some discussion regarding selection of squelch on/off - was that ever explored?

- GY

[FullOppositeRudder]

Yes, I've been wrestling with all the possibilities for this one also in my spare moments. It continues to elude a simple 'can't miss' diagnosis, and is made all the more frustrating because of its intermittent nature. The points raised in previous posts are worth checking.

I find it hard to latch onto the fact that it occurs only in a narrow RPM range, and that as 'improvements' were made to the ignition systems, the RPM trigger point came up 300 rpm from the previous one. Can this be linked only to vibration, or is there some other factor which causes the noise to appear at that specific figure? Are there other instruments or monitor systems which come into play at that number?

Do the antennas have some sort of static grounding system at the feed point which has reduced the overall RF sensitivity so seriously as to make the direction of the signal source so very narrow and specific? An SWR check would normally betray this, but lets not forget that a 50 ohm resistor at the other end of the coax will quite probably return an excellent VSWR measurement. Is the noise equivalent to an open squelch level and character or is it 'louder' than that?

There's one other factor which the rest of us need to keep in mind. While it hasn't been spelled as such, it seems likely that HF is working at this by himself as both fix-it man and pilot. If that's the case, it adds additional complications into the whole process because flying the aircraft is always the absolute priority, and trying our suggestions will - of necessity - come much further down the list in any given exercise. We all need to be patient as I'm sure he is, but we all share in the frustration which this problem is causing, There has to be a fix somewhere!

FOR

[mike-wsm]

I find it hard to latch onto the fact that it occurs only in a narrow RPM range, and that as 'improvements' were made to the ignition systems, the RPM trigger point came up 300 rpm from the previous one.
The vibration occurs over a wide rpm range, but only above a certain figure. The onset figure was increased by making the engine run smoother. Most likely this is propeller vibration. The normal prop vibration band for this aircraft/engine/propeller combination is 2250rpm to 2550rpm. With the engine running rough the onset would be at a lower rpm.

[FullOppositeRudder]

Thanks for that Mike, I haven't had much to do with the subtleties of aircraft engines. Then might we suspect something in the airframe resonating with this vibration band which is then causing some sort of mechanical partial 'break' or other disturbance to the 'normal' installation?

What sort of coax cable is normally specified in these installations? Elsewhere it would probably be one of the RG-58 species, but whether that's used in aviation systems or some more exotic teflon covered variety might be expected needs checking. It's not impossible that if the coax were routed through some close fitting areas of the structure that it may in time be subject to abrasion or other factors which could wear through the outer coating and then the shield.

What problems might result from this - assuming that only the outer braid might be touching the airframe both of which can be considered 'grounded' anyway - and why it affects both radios remains something of a mystery.

But a mystery is what we've had all along anyway.

FOR
(thinking aloud before the second cup of coffee kicks in ...)

[Phalconphixer]

FOR...
RG.58 cable was indeed an aviation industry standard for many years and may well be present on this aircraft depending on its age. It's use was phased out about 10 years ago when it was realised that the dielectric (pvc) would give off noxious fumes in the event of a fire. It's also a bitch to work with for soldered connectors.

In my last job we removed all RG58 from our fleet and replaced it with either RG.142 or RG.400 depending on the application. Generally speaking both RG142 and RG400 can use the same type of BNC, TNC, N, and C connectors as the old RG58.

As I recall RG.400 can be used for most applications up 1030/1090 Mhz.

Beyond this RG393 is the cable of choice. But RG.393 is a replacement for RG213 / RG214 which was outlawed at the same time and for the same reasons as RG.58.

RG.400 is double screened and proved to be very good at system noise reduction and inter system interference where RF cables for different systems are run close together. Insertion loss is also substantially less than an equal length of RG.58

pp

[FullOppositeRudder]

Thanks for that information PP. :ok:Helpful indeed - I just don't see these alternative coax types in normal land based equipment I work with..

FOR

[Graybeard]

Has the prop spinner been removed for a test?

[hangflyer]

Ok, another update.

Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.

If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.

If you change to an unused frequency and turn off the squelch there is none of the noise.

The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.

The 2 radios have seperate cables and antennaes and they both exibit the same behaviour, with the newer radio with the new antenna being worse ONLY since the antenna was replaced with a higher gain unit.

NAV1 is the second half of COM2 which is a KX175BTSO

All last week the issue was absent, the comms were perfect. On Saturday half the flights were perfect and the other half had a little noise. On Sunday it got worse and we stopped flying at mid day and cancelled the remaining flights as it was becoming a safety issue with stations in front of the aircraft becoming unreadable at times.

This morning the noise is there, it starts at 2450RPM and stops at 2550RPM on the ground, it is still directional only on recieve on both comms.

It is possible to park the aircraft close enough to the tower that there is clear line of sight between the antennae on the top of the aircraft and the tower aerial on the tower, even in this situation the behaviour is unchanged, clean audio until the nose if pointed at the tower then with the nose within 20 degrees of the tower the signal degrades to 2s and then back to 5s again in every other direction.

I'm still going crazy trying to find a solution! I appreciate the suggestions.

[Mr Optimistic]

so the noise is coming in on a cable, either power or antenna feed ? (apologies if this is already known and I am being dumb). But if this is the case, why is it directional ?

[forget]

Oh my God. He gets worse. :ugh: Mods, can you please do something about this buffoon?

[hangflyer]

I have been unable to correlate the issue with any of the following items:

Temperature
Humidity
Cloud Height
Day of Week
Moisture
Potential Static Build Up
Hard Landings
Time of Day
Aircraft Use

Solar Flares!
SpaceWeather.com -- News and information about meteor showers, solar flares, auroras, and near-Earth asteroids (http://www.spaceweather.com/)
Check for yourself!

[MurphyWasRight]

A: The problem occurs over a wide range of recevied signal strengths. This rules out reduction of signal strength by shadowing.

B: Everything but the battery and one radio have been disabled and noise was still there.

C: No hint of it on the handheld.

D: Most telling: The noise is only there when receiving, not in background with squelch disabled.

My theory:
The prop is modulating the Rx signal. This would account for the extreme directional sensititivity, only happpens whe prop is between antenna and source.

I have no idea of the normal grounding(/bonding) path between prop blade
and airplane but suspect as the prop vibration sets in at ~2100 rpm one or more of the blades are intermitantly disconnected.

Given prop length and radio wavelength this could cause a a reflection/phase shift that would modulate the signal.
The impact would also be critically dependant on distance from prop to antenna, which would explain lack of noise on handheld.

One test, not conclusive, would be to measure resistance from each blade to chassis while applying force in every direction to each blade of prop (engine secured!!).

Few more things to try: Is there any hint of the noise when the base station is directly behind you?

Turn to the "edge" of the noise (~15degrees?) and see if problem goes away with airplane pitch. (Another way of moving prop in/out of line of sight of antenna.).

(Added)
Note the theory behind this is that the rapdidly fluctuating Rx signal strength is causing the receivers AGC (automatic gain control) to wig out resulting in incorrect levels at the detector.

More on why not RFI:

Simple shadowing would slowly change the Rx level, much less than flying from .5 miles to 20 miles.

If the inteference was external generated RFI it would be at a (relatively) constant level so a few Db change in Rx level would only matter at some fairly critical distance from base station, farther away it would be pressent all the time, close in would not be there at all.

[Irish Steve]

This may be completely off beam, and I can see no reason for the directional implications, but here goes.

You mentioned that it was there all day, but then wasn't on a test. What changed, in terms of the radio equipment in use, By that, was it a case of 2 people, headsets etc in flight, and only one when testing? I'm thinking around things like headset connectors in the panel for the second person position, maybe an intercom related issue, are you using active noise cancelling headsets, could a socket connection be vibrating with engine speed, or even (perish the thought) is there an issue with the passenger seat bonding that's not the same empty or loaded.

There is also the implication that it's not just radios, there is also possibly an intercom unit/switching unit somwhere in the loop. Has that been eliminated from the possible suspects. Also the wiring to and from headset sockets to the intercom and on to the radios, any chance of a change here, especially related to ground loops in coax and the like, on microphone leads etc.

PTT wiring, any possibility of a chafe on them that is giving airframe contact at certain harmonic frequencies?

Appreciate that this is very much on the crazy end of speculative, but from what I can see, all the obvious and some of the very much not obvious have already been done to death.

Are there any engine instruments or other instruments which can't be switched off that have a voltage regulator as part of their construction.

Are there any maintenance or other access panels that were removed around the time of the start of this, not necessarily forward of the cabin?

what beacons and or strobes are there on the aircraft, and have they been eliminated from the possibilities?

Is there any possibility of harmonic resonance in something like a control cable run causing contact with an ajacent fuselage member?

I'm about out of ideas at this point, best of luck.

[Bigears]

My vote is with Mig15.
I'd suggest that if you try what DERG suggests, then you'd be able to measure the SWR just before the front end gives up due to reflected power returning.
Just my humble opinion.

[mike-wsm]

I agree with Murphy

The problem occurs when the received signal is coming through the prop and the effect is the same on strong and weak signals. This rules out many possibilities.

What may be happening is that the incoming signal is modulated by the prop, quite literally mechanically chopped. This will put strong amplitude modulation onto the carrier. With a two blade prop running at 2400rpm this is 80Hz am. The modulation will be a constant proportion of carrier and so bear a constant relationship to the received signal (also am). Presumably the receiver agc sees the chop and turns the gain down, reducing the amount of wanted signal.

The effect may be specific to a particular type of receiver or even to the mode of operation set on the receiver or the software version loaded. I don't have a schematic so can't even begin to guess what is going on inside the receiver.

[ion_berkley]

I agree with Murphy

The problem occurs when the received signal is coming through the prop and the effect is the same on strong and weak signals. This rules out many possibilities.

What may be happening is that the incoming signal is modulated by the prop, quite literally mechanically chopped. This will put strong amplitude modulation onto the carrier. With a two blade prop running at 2400rpm this is 80Hz am. The modulation will be a constant proportion of carrier and so bear a constant relationship to the received signal (also am). Presumably the receiver agc sees the chop and turns the gain down, reducing the amount of wanted signal.
Kudos @murphy (pending confirmation!!)

The effect may be specific to a particular type of receiver or even to the mode of operation set on the receiver or the software version loaded. I don't have a schematic so can't even begin to guess what is going on inside the receiver.
http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/statusicon/user_offline.gif http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/buttons/report.gif (http://www.pprune.org/report.php?p=6264288) http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/buttons/reply_small.gif (http://www.pprune.org/newreply.php?do=newreply&p=6264288&noquote=1)I think we likely have a winner. I'm not sure if we ever got a result from @hangflyer for the experiment to connect the handheld to the aircraft antenna, but I'm not sure if that would be conclusive anyhow for exactly the reasons @mike-wsm just spelled out, namely that this kind of AM modulation might play havoc with only some AGC designs. @hangflyers other observations now rule out a whole bunch of other possibilities, we don't have an independent noise source. If it proves very difficult to locate the grounding issue for the prop, I wonder if just relocating the antenna to a position that has an aharmonic distance relationship to the prop will work around it? I probably shouldn't be suggesting such kludges...I'm just curious to see if the modulation effect is working the way I imagine.

[hangflyer]

If I park 200M away from the Tower the problem is unchanged.

This is interesting because the VHF antenna has a clear line of sight to the VHF antenna on top of the tower.

At this range I can taxi in circles with the engine RPMs high enough for the problem to be there and the noise starts at +-20 degrees of the nose pointing towards the tower. The rest of the 320 degree rotation the noise is not there at all. When the noise starts coms go from 5s (everywhere else) to 2s for those 20 degrees.
I initially felt the at the signal was going "getting chopped" as it went through the prop, however I can put the aircraft in positions where there is an absolutely clear LOS between the transmitting and receiving antennae and it still happens if the station is aligned with the nose of the aircraft.

One of the major problems I have is that I am not a LAME, as this is a Charter category aircraft I am not free to take things apart myself, I need to bring ideas to the engineers or avionics guys who then charge me a fortune to see if they can fix the problem.

At the risk of sounding unkind, it does seem to me that in most cases today the standard procedure is to replace boxes until the problem goes away, often the problem is determined to be "the box" because a new one fixed it (sometimes because they broke the old one removing it). I am at the point now where they have replaced pretty much all the replaceable components in the system and I still have the issue and seem to be running out of ideas.

There also seems to be a lot of contrary theory amongst the techs.

There are 2 competing companies that have both had a go.

One says that the problem must be shadowing of the antenna by the fuselage in front which would cause a weaker signal and hence the directional nature of the problem.

The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.

Does anyone actually know which one of them is correct?

[hangflyer]

Also we have 2 different brands of radio on 2 different length coax cables on 2 different types of VHF antenna located on 2 different parts of the fuselage that both have the identical problem! Harmonics?

Both radios have been swapped with other units. Not specific to just one unit.

[mike-wsm]

hangflyer

Could you tell us a little more about the aerial change, please? For example why did you decide to fit a new one and when was it fitted? Thanks.

[FullOppositeRudder]

Thanks HF for the additional information. Some other contributions have been very useful, one or two not quite so IMHO (that's all I want to say on that :suspect: )

One says that the problem must be shadowing of the antenna by the fuselage in front which would cause a weaker signal and hence the directional nature of the problem.

The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.

Does anyone actually know which one of them is correct?There is a small amount of truth in each of them but neither would get a satisfactory pass in the real world with my old radio theory tutor.

The first statement might be a consideration if we were dealing in micro-watts over a considerable distance. Unless we have a seriously desensitised reciever (two on them in fact) I suggest that the shielding effect of the fuselage would be minimal given the transmit power of the radio equipment used in the tower ground installations which I would expect to be 'significant'. This theory is further eroded by the fact (if I understand correctly) that even with the aircraft on the ground in close proximity to the tower such that line of sight conditions between the transmitting and receiving antennas, the problem is still evident.

The second statement has some credibility in that if the shield of coax cable to the antenna is bonded to the skin of the aircraft (metal is assumed here) and thus to the airframe (grounded to use the more common terminology) at the antenna feedpoint the entire aircraft becomes the ground (some would say counterpoise) against which the whip of the antenna either radiates or receives a signal. This is the sort of system I would expect in this aircraft (the turtle deck is metal (aluminium) isn't it ?????) .

There is usually some directivity in that the radiation patten of the antenna would normally be the strongest in the direction of the largest area of ground plane. In this instance I think the directivity effect would probably favour signals in front of the aircraft if at all. Others might argue for signals from the rear of the aircraft being stronger in both transmit and receive. A beer or several are needed here to arrive at the correct answer by concensus. But I would be astonished beyond measure if either possibility were serious enough to produce the effect we are seeing here.

The prop modulation possibility as presented and argued by others earlier today is very interesting and seemingly credible. However I am troubled by the intermittent nature of the problem, and that the onset of the problem has been lifted by quite a few hundred RPM as changes were made to engine equipment which improved the performance. Prop modulation is possible perhaps, but surely there would have to be other contributing factors.

Finally, one especially puzzling aspect is that, as I understand it, the noise is stronger on the radio which has the newer, higher gain antenna. That again really suggests some sort of RF interference which is being received by that antenna at a higher received sig strength than the other one. (No it doesn't help a lot - at least I don't think it does - but it's just another question without logical answer)

What is the answer then? I don't know. I'm baffled like almost everyone else but somehow, someway, the cause will be identified and the solution found.

FOR
(all written in hast - with apologies to grammatical and/or technical purists - it's been a long hard day, this might be better attempted after a good sleep) :zzz:

[forget]

Does anyone here remember Fred the Wheel Tapper from an RAF poster of the ‘60s. Fred’s job was to find cracks in the wheels of railway carriages. He’d whack them with his hammer and if they didn’t sound right he’d have them changed. This cost the railway company a fortune until someone discovered that Fred’s hammer was cracked.

So, of all the radios you’ve tried, have they been through the same shop with the same test equipment and same tech? Just curious. :hmm:

[ampclamp]

Prop mod or wavelength is interesting but does not explain, as you say, why it just appeared out of the blue or intermittant.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.

Lots of clues there.

Need to work with what we know, use good radio theory and experience.
When I have had weird radio problems more often than not it is something earthed that should not be, or something that is poorly grounded / bonded/ shielded that should be.

[stevef]

I'm still suspicious of the ignition lead shielding (which prevents wave energy emissions). It's a simple test procedure for your AMO and an inexpensive fix.

[GarageYears]

ampclamp

Also this:

Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.

If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.

If you change to an unused frequency and turn off the squelch there is none of the noise.

The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.More clues here, which have perhaps been glossed over...

The noise only occurs on an active frequency. Note the effect of squelch off on an unused frequency...

Brain working overtime but not coming up with much :ugh:

- GY

[MurphyWasRight]

Ampcamp said
Prop mod or wavelength is interesting but does not explain, as you say, why it just appeared out of the blue or intermittant.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.

Lots of clues there.


One othe big clue is that it happens at -every- distance to base station so almost has to be some modulation effect as opposed to an independant (fixed level) rfi source.

Both the intemitant and RPM sensitivity can be explained by prop vibration, which if I understood a previouse post correctly, is basically a "in the direction of spin" vibration (torque load/unload) that occurs above a certain speed. This onset speed was raised slightly when the ignition system was improved.

My theory is that this vibration is intermitantly grounding/ungrounding one (or both) of the blades, or less likely some mechanically related structure.

Only when ungrounded does the prop blade create a reflection/multipath/interfernce situation.

The signal does not have to go directly "through" the prop for this to happen, think tall buildings and ghosty (analog) TV reception. It does have to be mostly in line, hence the 20 degree range of the effect.

The handheld not seeing the noise can be explained by the effect being larger at "magic" ( ~ 1/4 wavelength multiples) distances from the prop.

Worse with new antenna could be that a slight difference in (electrical/effective) distance to prop.

One interesting test would be to take the handeld outside and slowly move toward the plane when noise is happening.
Not sure one could (safely!!!) get close enough to get the effect or not though. Results of handheld connected to antenna would also be usefull.

[Mr Optimistic]

Not an a/c man (did you guess ?) but is the prop metal ? Reason I ask is that if the spacing between the blades is right then it might be giving you multipath which would modulate the signal. Far enough from the tower and the fraction of the received signal intercepted by the blades would decrease so the effect would vanish. The rpm dependency would have to be explained by agc timeconstant or something. The directionality of a hand-held (unless you want to risk life and limb would prevent that happening to it.

[GarageYears]

Key clues to my mind:

- Non-airframe grounded receiver does NOT 'hear' noise => Noise is not an RF transmission
- Opening squelch on the a/c radios on an open frequency does not 'hear' noise => Infers something is happening to the transmitted signal BEFORE reception by the a/c radios (and reinforces this is not an additional RF signal)
- Only occurs to stations when within +/-20 degrees of being on-the-nose => Something in the front of the a/c is responsible! (Thinks big whirling thing out front...) What angle does the prop diameter subtend to the tower station at 200m. Does the angle of interference appear to change?
- Noise is not always present => See post above by MurphyWasRight! I'm with you on that idea.

Back to thinking some more :8

- GY

[FullOppositeRudder]

Reverting briefly to post 76 from HF if I may...........

Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.Can I please clarify something which I may have missed here or elsewhere?

If the HH is not picking up the noise in the above situation, it is receiving the signal at a good (readable) level - or is nothing being received on the HH at all? That is to say - is the transmission received and understood on the HH inside the aircraft at the same instant as the same transmission is unreadable on the other com radios because of the noise issue?

Gee, this is really frustrating! :confused:

FOR

[hangflyer]

It just gets better.

out of sheer desperation we mounted a new antenna on the underside of the fuselage directly underneath the present COM1 antenna. We disconnected the cable running to the antenna on top of the fuselage and connected it to the one underneath the fuselage.

After 2 days the results so far are that the new antenna on the underside does not suffer from the problem (I'm not relaxing yet though)

I can tune both comm units into the same station in front of the aircraft and the one with the antenna on top of the aircraft gets the noise and the one with the antenna underneath the aircraft does not.

This just ruled out the radio or the cable or the antenna as the issue yet again.

Just to clarify a handheld in the cabin does not pick up the noise,neither does the VOR or ADF.

I think we have established that it has something to do with the airframe acting as part of the antenna and some characteristic of the signal recieved from an antenna on the top part of the fuselage combined with some RPM or vibration related interference being generated in the nose of the aircraft.

The new antenna is the same as one that exibits the problem and they are all equally well bonded, it is just the physical location that has changed.

[cone zone]

Hangflyer,

sounds like a positive note.

I can't help but wonder if it is something to do with the ADF sense antenna just above the two comm antennas.

cheers cone zone

[ABUKABOY]

So, back to the thread...........? In a previous life, back in the 70's, I was tasked with trying to get DC-3's through UK C of A Airtests, eking ANY climb-rate out of them on one engine on a good day, and helping brainstorm all the vagaries of steam-driven radio-gear to boot. One a/c had severe VHF interference, which cured itself when the cockpit-to-fin ADF aerial set up such a vibration that it tore the front mount clean out of the top of the fuselage. It had been resonating in flight, obviously not repeatable on the ground. (Only mentioned because long-wire ADF aerials have been mentioned here in previous threads).
Also, any chance you can obtain/borrow another propeller for a trial fit? (Desperate measure I know, but it sounds like you're ready to try anything). Don't know about pistons, but the props on a brand-new BAS Twotter were so horrifically out of balance, such that they rocked the airframe slightly as they slowly increased speed on start-up, but ran smooth at flight RPM's. A piston-prop achieves a higher rpm much quicker on start-up, and pistons vibrate more than turbines, so you would not notice any slight out-of-balance vibration. (A set of FULLY-balanced Twotter props was charged extra!). If another prop is not available, try removing and checking the balance/blade rock on the prop you have. Might explain at least the rev-dependent aspect of your problem.
Just my pennyworth, will follow with interest.....................

[ampclamp]

I still have some huge plug in crystals that I think came out of a DC3 VHF comm set.Long time ago so not 100%.

Interesting development hangflyer.I PMed you last night before that piece of recent news.

[cone zone]

Forget

I agree with you.

[MurphyWasRight]

I can tune both comm units into the same station in front of the aircraft and the one with the antenna on top of the aircraft gets the noise and the one with the antenna underneath the aircraft does not.


This fits with the prop (or something else towards front/top of plane) becoming unbonded as rpms increase.

To help visualize the possible prop issue think of one element on a yagii (tv style) antenna being rotated, as the elemnt comes in line with the others the gain will increase, at 90 degrres it will decrease.

Noite that it is possible that an ungrounded prop (or whatever) is actually increasing the antenna gain, any fast cycling of Rx strength could upset the AGC and cause the issue.

On the practical side will it work to simply relocate the other antenna or does the bottom location cause other problems?

[stevef]

I'm not sure I see how the prop can become unbonded... There's excellent metal-to-metal contact through the crankshaft flange attachment bolts at the very least.

[MurphyWasRight]

SteveF
I'm not sure I see how the prop can become unbonded... There's excellent metal-to-metal contact through the crankshaft flange attachment bolts at the very least.

I tend to agrre, but do not have any detailed knowledge of how the spinner/blades/pitch controls etc are configured so was thinking of a mechanical path that might "open" with vibration. Certainly a defect and not a day one issue.
Also any change in the bonding path length could affect things, an intermitant engine ground strap etc, although I believe that was checked early on.

While the prop is an obviouse suspect any ~1/8 wavelength (or multiple) conductive element somewhat in line with the antenna could cause a problem if vibrations at higher RPM caused it to vibrate between bonded/unbonded state.

Cone Zone mentioned
I can't help but wonder if it is something to do with the ADF sense antenna just above the two comm antennas.

That could also do it if some element was not tightly bonded and was vibrating between bonded/unbonded.

Note that in any of these scenarios (other than prop which is rotating) the element would have to be vibrating between bonded/unbonded to produce a modulation/noise effect.

If it was just unbonfed the gain might be changed but that would be a static (unchanging) effect that would not cause noise.

It also need to be more or less in-line to produce the pronounced and symetrical directional effect.

[mike-wsm]

Wow! Problem there with antenna topside and gone when antenna is underside.

Can we have a couple more tests, please?
(a) What happens when you fly inverted and try both antennas?
(b) And do you get the same results when flying in the Northern hemisphere?

(my head hurts and I've caught the derg lurgy)

[hangflyer]

One of our initial tests was to disconnect the coax to the ADF sense antenna, but we never removed the whole antenna, now I am starting to wonder if the idea of that antenna vibrating may have some merit. So far this week the new antenna on the underneath has been perfect, whilst the top antenna still suffers from the same problem.

Next week I will try completely removing the ADF sense antenna to see if that changes anything.

Thanks again for the suggestions guys.

[MurphyWasRight]

MikeW (tounge firmly in cheek) suggests:
Can we have a couple more tests, please?
(a) What happens when you fly inverted and try both antennas?
(b) And do you get the same results when flying in the Northern hemisphere?

Actually test A might show provide a clue if something heavy (engine) is
involved the change in gravity vector might exacerbate/reduce the vibration.
Would actually try 90 degrees (on edge) as well if possible.

I would love to volunteer to be radio man for that test but live in wrong hemisphere so have to aggre that idea B should be tried as soon as possible as well :)

While removing the ADF keep a sharp eye out for any other possible intermitant contact areas, cowlings or whatever. Even a thin slot can act as an element in antennas.

While likely would not show anything you could also try ligthly tapping the entire top of plane with a rubber mallet parked pointing to tower ( engine off!!) while listening to radio in an attempt to locate the problem.

Whatever is causing the issue is almost certainly outside of or part of airplane skin and on top half.

There is a chance that it is behind the antennas since it could be acting as a reflective element. The active element in a yagii usually has one larger "reflector" passive element behind it

That would fit a bit with the problem not being heard at base station.

Adding additional point:
I just looked at the picture of the airplane (checking for ADF antenna) and noticed the rear window geometry is such that it could be that directional effect happnes when the antenna is "looking through" the cockpit. This means that the culprit "could" be in the cockpit.

A quick test would be to cover the read window with aluminum foil when the problem is occuring (rear seat passenger needed - again I volunteer).

That said the position of the ADF antenna (if it is where/what I think it is in picture) fits well with the theories, you might try tying some non-conductive fishing line to it and give it a good shaking while on the ground to see if it has an affect on radios.

BTW If it "twangs" when plucked (like a guitar string) at about 40Hz (or multiple) it could be mechanically resonating with the prop vibration in flight. 2400 rpm == 40Hz


Good luck and keep up the reports.

[GarageYears]

Derg:

Let me try to clear because you obviously are missing some of the key points "we" have discovered to date (aside from your somewhat odd-if-not-nonsensical waffle - and FYI, that in itself does not bother me, you are not on my ignore-list - I think your heart is probably about in the right place, just a few of your brain-cells seem a little deranged...):

- The issue is not additional RF emission from spark-plugs, magnetos, or other, since the radios and antenna are NOT directional - such noise would be present irrespective of the aircraft's orientation with respect to the source station.
- The above statement is further backed up by the fact that a hand-held radio does NOT pick-up the noise, nor do nav receivers.
- The fact that the new 'underside' antenna does not suffer the noise, infers the issue is related to the RF line-of-sight path when a station is on the nose, and the location of the VHF antenna and some other factor (bonding, ADF antenna, etc) in that straight-line path.

I have no idea of your background and really don't care, but methodical and thoughtful fault-finding doesn't seem to part of whatever "you" are.

Never mind - I'm sure you won't care a hoot what I have to say... :ugh:

- GY

[MurphyWasRight]

ASFKAP
Its possible that at a certain power setting one or more of the earths is breaking down due to a combination of vibration on an under torqued connection and or contamination or corrosion of the joint which might not show up under static conditions.


Totally agree, almost by definition any bonding fault (assuming that that is the root cause) will not be found by a simple test on the ground since the problem only shows with engine RPM above a certain level.

Even a sensitive (mv) test may/may not find it although it has a better chance.

[MurphyWasRight]

DERG:
GarageYears sums it up pretty well, in part:

- The issue is not additional RF emission from spark-plugs, magnetos, or other, since the radios and antenna are NOT directional - such noise would be present irrespective of the aircraft's orientation with respect to the source station.



There is one other observation that must be considered

The interference is at the same level of "badness" whether the tower is
200 yards or many miles away resulting in very significant difference in Rx levels.

Please explain how any on board RFI source (which has to be at a relativly fixed level having no knowledge of distance to or direction of tower ) can account for that and all the other features of this problem.

[forget]

The interference is at the same level of "badness" whether the tower is 200 yards or many miles away resulting in very significant difference in Rx levels.

Bear in mind it's likely that the ATC transmitting antennas are well remote from the tower.

[ChristiaanJ]

Ancient aeronautical engineer here, with some radio-amateur and electronic background.
Quoting hangflyer's original post.
There is an intermittent white noise that only appears on VHF (ADF is clean) that reduces a received radio transmission to 2's or 3's, however it is extremely directional and ... only happens if .... only happens when the engine RPMs are above 2200. There is no noise at 2150rpm and then at 2200rpm it is there in full intensity, it does not increase in any way (pitch/strength) as RPMs increase.
....the white noise kicks in on the radio. The noise does not change with pitch, it is not a whine, it does not get stronger or change frequency. It is just white noise.
I'll admit immediately that I'm totally baffled by the description of the noise as "white noise", i.e., a "hiss".
With just about anything 'vibrating' (bonding, etc.), I would expect the interference itself being "modulated" by the vibration source, be it propellor, ignition, airframe vibration, or whatever.
From hangflyer's description, this is not the case.

But I would say MurphyWasRight has a point, it's not RFI "as such".
If so, it would also show up on the NAV radios, which are roughly the same fresuency band.

A recording of the noise might help, as suggested before. Somebody might recognise it as something familiar.

CJ

[mike-wsm]

One possibility is that the 'white noise' is generated within the radio receiver and only becomes apparent when the agc turns the gain up. My guess is that there are two consecutive rf gain blocks each with their own agc loop. The 80Hz modulation of the carrier forces the first agc loop to turn the gain down, then the lack of signal to the second block forces the second agc loop to turn the gain up and amplify white noise. Can't prove this without seeing the schematic for the receivers.

PS - Don't knock DERG too much, it's kinda lonely up there in Durham, especially if he's out on the moors.

[MurphyWasRight]

Mike-wsm
The 80Hz modulation of the carrier forces the first agc loop to turn the gain down, then the lack of signal to the second block forces the second agc loop to turn the gain up and amplify white noise.

Not sure we have shown that 80Hz is actualy pressent, that number came from the rpms and prop blade count.

That said if it is related to vibration/rpm it will likely be to fast for AGC to follow but to low to be audible.

hanflyer did add this in a later post:

The noise can only be described as a white noise, no distinct pitch or tone, no relation to engine RPM except that it appears and disappears within 50rpm of 2200rpm every time. There is a chop to it, but it does not change with RPMs.


The chop could be the AGC circuit(s) chasing their tails, not the actual
modulation frequency which if it is anything under 200Hz or so is likely filtered out. (Does the term "motorboating" ring any bells?)

[mm43]

As the RF and AF gains has been mentioned, it is worthwhile to consider the signal strength of the interfering source is strong enough to overcome the natural tendency of the squelch/mute circuit to kill the audio output. That being the case, the noise being "heard" has apparently passed through the front-end, flattened the squelch detection and has probably caused the AGC to clamp its audio level.

The noise is an efficient "jammer", and there is one common thing about RF jammers, they work best when placed between the receiver antenna and the direction of interest. Hangflyer has clearly demonstrated that the "blocking" is ahead, i.e. forward of the antenna, which leads me to suspect that the "spark" transmitter is somewhere on the engine block. Too simple? Yes, but that explains why the hand-held approach is not revealing anything. The hand-held is working with its own pseudo ground/earth, whereas the comms receivers are using the same ground/earth as the spark generator. The energy in an untuned spark transmitter decreases exponentially with distance, and therefore it is possible to explain away the lack of interference in the ADF.

Marconi would have found the source by now, using just any old crystal and a cat's whisker detector!

So having eliminated the magneto, alternator/regulator, and engine block earth straps, I suspect that if the outer shield of the coax at the base of the VHF whip antenna has even a slightly dodgy ground, the input to the receiver will no longer be a low impedance load (50 ohms). The antenna becomes a high impedance untuned long wire, susceptible to any broadband noise generated locally - which in this case happens to be in front of it. The transmitter power effectively recreates a bond, providing the expected matched load.

So is the problem RPM or IAS related? Could even be both.

All idle speculation.

[mike-wsm]

There are some reports of GPS receivers causing interference with radios, it is all to do with harmonics of the GPS IF getting into VHF radio receivers. This could be either in-band or if strong enough an image frequency. Is there a GPS on this airplane?

[Upper Air]

I`m betting on the two COMM boxes.

Like - put COMM 1 and 2 both on the same Freq. Do you still have the same prob?

Or, Is your COMM one and 2 on the same . . . . box, can you seperate them, what I mean is: is there just one switch for the system - why am I asking. . . ? I am trying to seperate COMM 1 from COMM 2.

So if you can ride with one COMM "OFF" then fine, if not then just put both on the same freq. What am I getting at? I am thinking COMM 1 is interfering with COMM 2 and/or vice versa. Why? Because it happens on other aircraft.

Good Luck.

(By the way: We used to get the rythum of the beacon, come through the headsets - it was quite soothing actually)


The have not wired something to the VOR huh. . ?

Bonding. . . ?

vertical / horizontal field . . .of the antenna(e) ? i.e., sticking up vertical out of the ground must be received by equally vertical aeriel on a/c - same for horizontal - but you know that right. . .?

hard iron / soft iron. . . .?

. . .its a right booger this one innit. . .

A wire, missing from the aft of the aircraft to the . . . . roof/top of the aeroplane. . .strung outside the aircraft like a washing line. . . ?


Quadrantal Error keeps looming up in my memory. . . but that refers to NDB reception on Medium Wave. . . .the signal hitting the aircraft at the port quarter was vectored (from) the 6 o`clock position (the tail) giving the impression that the NDB (on Medium Wave) was behind the aircraft - quadrantal error. Could the opposite be true. . ?


HT lead burnt and . . . HT lead . . . signal to power. . . .Has anything with a charge through it like a lead or a cable crossed or passed over or near a lead or box or entity which contains a signal . . . (a stupid and obvious question. . . but with inherent potential). . . potential. . .

windscreen heating element. . .*air con* . .

. . .if you have an NDB on the a/c have they wired something to the NDB?

that`ll be fifty cents, please.

Also the above is FREE so if you don`t like it - then bin it . . .

(may contain nuts)
(do not try this at home)
(always make sure a parent is with you)
(the management accepts no responsibility)
(I know nothing)
(YOU asked!!)
(Mind that Prop!!!)
(Mind that Prop!!!!)
(don`t sue - am poor)

[MurphyWasRight]

The noise is an efficient "jammer", and there is one common thing about RF jammers, they work best when placed between the receiver antenna and the direction of interest. Hangflyer has clearly demonstrated that the "blocking" is ahead, i.e. forward of the antenna, which leads me to suspect that the "spark" transmitter is somewhere on the engine block.


Again I ask anyone proposing an "external" RFI source (spark plugs or whatever) how that explains these observations:

1: Relative "badness" of interference is the same at 200 meters (hangflywer stated direct line of sight to base station antennna) or at a significant distance, 20km would result in 40db Rx level difference.

Hard to see how a fixed level source would always have same impact.

2: Interference present -only- when tuned to an active transmitter.

3: The extreme dirictional sensitivity, even assuming bad shadowing the RX level would only change 6Db or so as the aircraft turned.

4: The strong correlation with RPM - not there at all then fully on just 100 rpm higher.

Yes DERG feel free to answer a direct question or stop complaining about those who dont.

----

BTW On the natue of the noise: Hangflyer is it all all the same as what a distant "on the edge" of recpetion signal sounds like?
Possible the "white noise" without the chop?
Above with interference not active of course.

[mm43]

Hangflyer

The bandwidth being taken up by this thread, which is also full of "noise", is environmentally unfriendly.

Wonder if you have offset it with carbon credits?:D

[mike-wsm]

Short intermission to watch a vid - this one has views of engine and instruments.

lbnqEcn_lOU

And I simply can't resist adding this one.

SGEwQP0DFd8

mm43
Sorry about the additional bandwidth!

[mm43]

Relative "badness" of interference is the same at 200 meters (hangflywer stated direct line of sight to base station antennna) or at a significant distance, 20km would result in 40db Rx level difference.You are implying, as I read it, that at 20km from the base transmitter, the signal is still readable and the apparent noise has reduced relatively by -40dB?

Are we looking at some form of cross modulation, which includes a noise component?

[MurphyWasRight]

Quote:
Relative "badness" of interference is the same at 200 meters (hangflywer stated direct line of sight to base station antennna) or at a significant distance, 20km would result in 40db Rx level difference.

You are implying, as I read it, that at 20km from the base transmitter, the signal is still readable and the apparent noise has reduced relatively by -40dB?

Are we looking at some form of cross modulation, which includes a noise component?


Basically yes, although the 200m is a fact the 20Km is an estimate I dont think hangflyer gave an exact number for max range.
The point is that the signal to (unwanted) noise ratio is staying the same over an impressive range of Rx signal levels.

As I postulated in prior posts I suspect some sort of signal cancelation (or possibly even boosting) effect, either from the prop (unbonded over a critical speed) or some other metalic element vibrating between bonded/unbonded. The ADF antenna is currently one prime suspect.


With just the right dimensions this would cause a significant and rapid Rx level fluctuation that could cause the AGC circuits to wig out.

Importantly this would also account for the extreme directionality of the "noise" since it would only happen if the element (approximatly) in line between the antenna and base station.

[Machaca]

Antennas when problem first described:

http://i337.photobucket.com/albums/n385/motidog/HF-FA-200_1.jpg


After higher-gain antenna installed in place of original:

http://i337.photobucket.com/albums/n385/motidog/HF-FA-200_2.jpg


No photo yet of new antenna mounted on underside, but it reportedly works a charm. Location? Proper base bonding? New coax?

[mm43]

The point is that the signal to (unwanted) noise ratio is staying the same over an impressive range of Rx signal levels.This then is a common mode problem which keeps the levels relative, e.g. it could well be occurring in a mixer, but at some point the noise component is being injected, and probably at an IF frequency. Could the ADF be radiating a small level of LO signal that is then creating an intermodulation probem? I'm not sure if Hangflyer has mentioned that it occurs on only one, or more than one discrete frequencies. We know that it occurs on his local base frequency.

Need to "chew the bone" over this.

[boguing]

Going back to when I was told that there is no advance/retard mechanism in the magnetos.

Web trawl tells me that Bendix 20/200 series are used on this engine.

They have impulse couplings. Which should drop out at 250rpm.

Two things.

1. Are the coupling pivots fitted with snap rings as they should now be? An AD relates.

2. Any chance that the start switch is malfunctioning and staying live?

Before I get my hat and coat, I can't help but focus on the fact that when the mags were replaced, the rpm at which noise occurs rose by a couple of hundred rpm.

[mm43]

Thanks Machaca for the pics.

Location, location ... and an intermodulation product related to the ADF seems now to "have" been the problem.

boguing;
Any chance that the start switch is malfunctioning and staying live?That could explain when the noise cuts in. The snubbing capacitor becomes ineffective above a certain RPM due to the low impedance of the battery hanging across it. But hang on! If a starter is used, it must be a fairly high impedance "impulse" type, otherwise you could kill the permanent magnet.

[Machaca]

Boguing:
I can't help but focus on the fact that when the mags were replaced, the rpm at which noise occurs rose by a couple of hundred rpm.


Yes, that does stand out. The new ignition bits can optimize engine operation and shift the harmonics to a different range, too.

[mike-wsm]

Machaca

Antenna pics show a wire from rear of cabin to top of fin. How does this interact with coms antennas?

[Machaca]

Antenna pics show a wire from rear of cabin to top of fin.

That's the ADF sense antenna that hangflyer said he would remove as a test.


How does this interact with coms antennas?

Perhaps an intermodulation as mm43 has suggested.

[Machaca]

hangflyer -- a few questions if you don't mind:1. Has the problem been present ever since you took possession of the aircraft?

2. Has previous owner of aircraft confirmed/denied this problem?

3. When was the IC-A210 installed?

4. The SoftComm?

5. Does the noise sound like the white, pink or brown noise at simplynoise.com (http://www.simplynoise.com/) ?

[ampclamp]

Hangflyer I sent you a couple of PMs a few days ago.Nor sure it's of any use.

So do we have a mixture of the noise 'x' with another rf source 'y' that puts the resultant beat freq bang into the vhf comms band x + y , x-y etc Harmonics of resultant ? Mind boggles.

Is 'y' an oscillator in the receiver that only operates with received signal?

A spectrum analyser where we could see on a screen the rf frequencies and relative strengths would be useful as would being able to listen to the noise.
Many many years of experience working on the thread now.Not being able to actually experience it and listen is working blindfolded and is frustrating.

[ampclamp]

Mig I believe they have already disabled just about everything to try isolating the noise to a piece of avio/elec equipment.

[mike-wsm]

But have they tried removing the ADF aerial? Not disabling it, actually physically removing it.

[mike-wsm]

Once upon a time, when Design Engineers had permanent jobs and worked in Research Laboratories, our Research Director used to say we had all our best ideas when shaving. And he was right, it was after all the hard work of the previous day and a good night's sleep that a new idea would surface and we would bound into work shouting 'Eureka' and applying for patents.

Nowadays I take advantage of retirement to let my beard grow, so I don't have that shaving experience, but on Sunday mornings I soak in a piping hot bath and that has about the same effect. Today, for example, I was having that long hot soak and dreaming of exquisitely beautiful and sublimely aerobatic Fuji 200s painting elegant patterns in the sky.

One of the Fujis had an ADF aerial from cabin to vert stab and had its coms antennas fuselage topside beneath it. And I saw the radio waves approach that area and vanish.

At this point I have to explain that electromagnetic radiation consists of two perpendicular components which must travel together. One is electric and in this case is vertical, and the other is magnetic and is horizontal.

When the field approaches the tail of the aircraft, the loop consisting of metal fuselage and metal ADF aerial constitutes a shorted turn and prevents any magnetic field from existing in a direction perpendicular to the loop. This kills the loop as a path for the signal so there is no electric field for the coms aerial to pick up. The effect is only strong enough to kill the signal when it somes from directly ahead or astern.

The shorted turn does not have to be a continuous dc path, it only has to be a path at vhf frequencies, so a very small amount of capacitive coupling at each end will be sufficient to make it a short.

My suggestion is to fly (or taxi) with ADF aerial physically removed and check the coms using the remaining topside coms antenna. It shouldn't cost a lot to try and hf can either mail me a pickled wallaby or else have me consigned to the funny farm for demented Engineers.

When our Director told us his the ideas-when-shaving theory, we asked to be paid a shaving allowance, whereupon he went very quiet and then changed the subject.

[Paddybee]

Have scanned this thread and thought that maybe what you have is a Radio Symptom as opposed to a Radio Problem- ie the symptom manifests as a radio issue but the condition is totally unrelated. The various cases where the aircraft was used all day and issue was present, only to disappear when taken for ground testing makes me think: "What if this is caused by presence or absence of front seat pax, or caused by another factor in the flight profile that is concomittant to the RPM and not actually the RPM at all? AOA or trim at that weight/speed condition?" It appears that all the logical steps have been taken (for a radio problem) already. Good luck if this starts a new chain of thought.

[JPI33600]

I hope I'm not suggesting something silly, but would it be possible to perform a test with engine running at 2200 RPM with propeller removed? Depending on the noise still being present or not, that could provide some useful clues.

On the other hand, I suppose removing a propeller is not a trivial operation, at least regarding security aspects. And running an engine without its propeller affixed may just prove impractical...

Mechanics ? What do you say ?

[MurphyWasRight]

I hope I'm not suggesting something silly, but would it be possible to perform a test with engine running at 2200 RPM with propeller removed? Depending on the noise still being present or not, that could provide some useful clues


A related test has been done, From HF's original post:
I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.


Since removing the prop and running engine is not feasible anothe test would be to borrow a "seriouse sound system" from a car nut, put it in the cabin and sweep 20 to 80 hz at aboout 120Db spl.

BTW: that level that is basically just above idling for the class of sound system I am thinking of.
I have never seen one in person but the competions for loudest system are intense, blowing out a windshield is a classic beginners mistake I heard. Needless to say nobody is actually -in- the car during the tests.

[mike-wsm]

I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.

That blows the prop vibration theories.

[MurphyWasRight]

Quote:

I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.
That blows the prop vibration theories.

Not sure if it does or not, unless HF stepped on the clutch :) the engine would still be rotating , pistons up and down etc.
There could still be a resonating effect that might result in increased vibrations in direction of spin.

This however is where my lack of (any!) practical knowledge of the behavior of prop vibration caused by interaction with engine rotating mass is evident.

[mike-wsm]

Yes, still reciprocating masses, but the engine wouldn't be going bang-bang-bangy-bang so no torque impulses to make the prop vibrate.

To be annoyingly pedantic, Lycoming pistons, like VWs and Porsches, go left and right, not up and down. :rolleyes:

[mm43]

I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.Going back to very early on in the thread, I did mention that a zener diode could, if reverse biased, generate the "white" noise. It seems that condition could exist in the alternator regulator when current to the base of the pass transistor is cut off. A capacitor associated with the zener will need to be open circuit for this to happen, certainly a possibility, and something that a standard regulator test would not identify.

[MurphyWasRight]

MM43 wondrs:
Going back to very early on in the thread, I did mention that a zener diode could, if reverse biased, generate the "white" noise. It seems that condition could exist in the alternator regulator when current to the base of the pass transistor is cut off. A capacitor associated with the zener will need to be open circuit for this to happen, certainly a possibility, and something that a standard regulator test would not identify.

Zener or other electical white noise source fails to explain why the interfenrence when pressent is at the same relative level over a 40Db or more Rx signal level range.
(HF did stat 10+ miles range so my 200M to 20Km range is valid.)

From HF original post, a couple more reasons why it is very unlikely to be an external (electically generated) RFI problem:


Idea 4, the alternator. Engine run with alternator field disconnected (so not energised) and master bus disconnected. No Change.

Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change


When looking for caues of a bizzare issue such as this it is important to keep all the clues in mind, even then it can get pretty confusing.

MikeW: Yes I knew that the pistons go left right just let fingers get ahead of brain.
Even was part owner a VW bus once until one of its pistons ended up goind in/out and then allover the place...

[ampclamp]

G'day Mike no I dont think the antenna was physically removed.

Paddybee , I suspect you are on the money.A defect or condition that is evident on the comms but not actually a radio defect as such.

Someone asked what are they using to do bonding checks.I do hope it is a proper boding meter not just an avo or fluke.

[mm43]

Originally Posted by MurphyWasRight;

Zener or other electrical white noise source fails to explain why the interference when present is at the same relative level over a 40dB or more Rx signal level range.My reasoning is that the noise had become equalized in a mixer, or some other non linear device, but that in itself doesn't explain why Rx signals from forward of the antenna are affected markedly more than from other directions.

It has clearly been demonstrated that the noise cuts in at a particular rev range, but also noted that it lacks variation with the revs. That's why, rightly or wrongly I looked at the alternator regulator. You have killed off that idea, so what's the alternative?

The Faraday shorted turn proposal currently doesn't rest well with me.:hmm:

HF has said that the null effect is only forward, and it would need to be the same when going away from the Rx source.

[FullOppositeRudder]

"What if this is caused by presence or absence of front seat pax, or caused by another factor in the flight profile that is concomittant to the RPM and not actually the RPM at all? AOA or trim at that weight/speed condition?" It appears that all the logical steps have been taken (for a radio problem) already. Good luck if this starts a new chain of thought.Thanks for looking in and thinking outside of the box for a while. We need that, we certainly do. One of the problem here - if I read HF's notes correctly is that the problem - when chooses to be evident - can be reproduced on the ground (post #92 applies).

The positive outcome from relocating one antenna to the bottom of the future is a ray of hope, but the over-riding frustrations remain. :{

So what do we know ?
1) The problem suddenly appeared without any apparent reason.
2) Even so, it is intermittent in nature without any identified reason(s) thus far for that factor.
3) Both radios are affected even though they are (I think)of different types.
4) Substitution of one of the radios with a known "good" one still produced the problem as previously.
5) Changing the loads on the DC circuit /bus seems to have no effect on the behaviour of the problem.
6) Improving the engine's running (sorry - I'm sure there's a better way of saying that) by changing spark plugs etc changed the onset of the problem to a point some 300 erpm higher than previously.
7) A different alternator (and presumably) regulator was tried - no change.
8) The problem is directional and appears within the predicted rev range over a wide range of engine operating parameters - but always at the known RPM.
9) Problem cannot be reproduced on a handheld in the cockpit and independent of the aircraft power or antenna circuits.
10) Problem is evident over a very wide range of distances (and therefore signal strengths) - could hardly be a RF signal then could it ...?

I'm still without explanation or answer. It the defies every understanding of radio behaviour that I have ever read about or encountered. The only consolotion is that I'm not alone.

So is the radio system essentially OK - something else altogether is at fault?

Can we establish that this has been observed with a sample of other transmission sources - ie just not the tower or base - other aircraft?

I know nothing about ADF operation. Is there something which "cuts in" within the ADF system within the observed heading range? (but why only at that RPM range also :uhoh:??)

This one really defies logical analysis. I hope I'm still around when it's finally solved once and for all. It's a real worry.

FOR

[mike-wsm]

FOR

Excellent summary of what we have been told. :ok:

Local information:
Au CASA Visual Pilot Guide for Jandakot: (http://www.casa.gov.au/wcmswr/_assets/main/pilots/download/jandakot.pdf)

[GarageYears]

Thanks to Machaca for posting the pics a couple of pages back, having looked at them for a few moments and understanding the fact that the new antenna attached to the underside of the aircraft works perfectly (still true hangflyer?) really does bring into question the interaction of the ADF antenna and the comm antennas.

So a flight with the ADF antenna removed or even just ground tests would be revealing. Somewhat obviously, the fact the ADF antenna is inline with the orientation of the aircraft, which also corresponds to the directional confluence when the problem is heard would seem compelling.

I'm still not sure I can understand how/what the prop RPM setting can have over the behavior though - if the ADF ant. is causing some kind of RF shadow to fall over the VHF upper body antenna, how is RPM (above/below 2250RPM) factored into the equation? I presume some vibration effect?

A real puzzle indeed!

- GY :hmm:

P.S. DERG - yes, VA is a nice place to live, but I'm not sure I'm totally lined up with the tanker decision - though in the end I suspect the Air Force would have been able to make either option work. Living as close to Wash.D.C as I do, there are far more politics in the decision than fit-for-purpose - my perception is Boeing has far stronger lobbyists than EADS could muster, ergo Boeing wins. In the first go-around Boeing simply didn't think it could lose... until it did, hence the re-compete.

[MurphyWasRight]

GarageYears
I'm still not sure I can understand how/what the prop RPM setting can have over the behavior though - if the ADF ant. is causing some kind of RF shadow to fall over the VHF upper body antenna, how is RPM (above/below 2250RPM) factored into the equation? I presume some vibration effect?


It most likely has to be some vibration that cause whatever bit of metal that is the culprit to intermitantly connect/disconnect above a certain prop speed.

This mostly fits with the observation that the "onset" rpm rose slightly (but repeatabley) when the ignition was optimized which (probably) resulted in a smoother running engine.

Either that or the work somehow affected the culprit, access panel cycling or whatever.

Too bad we don't know if the engine off windmilling "onset" speed changed at the same time.
That would remove a smoother engined (and yet again the spark plugs:rolleyes: ) from the suspect list

[mike-wsm]

Here's a generalised schematic of the electrics of a light aircraft. If anyone has a more appropriate one please send me a pm and we can arrange to post it instead.


http://www.free-online-private-pilot-ground-school.com/images/electrical-system-schematic.gif

from Welcome to the free online private pilot ground school (http://www.free-online-private-pilot-ground-school.com)

[Machaca]

This is the stack in VH-FJI :

http://i337.photobucket.com/albums/n385/motidog/HF-FA-200_5.jpg

The map light is partially obscuring the intercom at top.

[boguing]

Mike-WSM.

That diagram implies that there is an alternator on this engine. I stupidly discounted that, assuming that the mags would also handle DC charging/supply.

So apart from my droning on about the mag AD et al, what rpm does this alternator start charging at? My Series III Land Rover is a bit dodgy and it needs about 4,000rpm to kick in. And because it's not A1 condition, I'd bet that its' output is more than a little noisy.

[Machaca]

Hard to discount the contribution corrosion may have in affecting components, switches, cables, etc.

http://i337.photobucket.com/albums/n385/motidog/HF-FA-200_6.jpg

I imagine hangflyer has cleaned things up substantially since acquiring the aircraft, but there's a lot of electrical componentry that will not be so easy to put eyeballs on.

[MurphyWasRight]

HF's original post contains an impressive list of debugging efforts before he even posed the question here.

As to alternators:

Quote:

Idea 4, the alternator. Engine run with alternator field disconnected (so not energised) and master bus disconnected. No Change.

Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change

[boguing]

MWR.

You clearly have better mid term memory than I. You're quite right.

[forget]

Machaca, I don't understand the photographs you've posted. Is that aircraft actually airworthy? I've dealt with flying wrecks in the past but that is a bloody disgrace. Tell me it's not the one we're discussing here.

[mike-wsm]

Year of manufacture 1972

(regosearch, refreshed from casa 22 feb 2011)

[Loose rivets]

I wish I'd spotted this thread from the get-go. Fascinating.

Is it still an ongoing mystery, or is the fitting of the lower aerial considered a final fix/workaround?

I used to commute from the east coast to Luton, often at night in a 4 seat metal aircraft. It was fairly new, but the radio used to get worse as I passed near to Stansted, then improve with distance. I did exactly what I'd want to do with this problem, and measure the AGC voltages. However, in those days, I was able to open the box without breaking hidden seals.

In this case, after a few pages I have to not spend any more time today, but after so much testing and swapping - I was getting a gut feeling (after a C&G and being a telecom tekki) in my yoof, that something exotic was happening. Thinking laterally, I wondered if the basic design of the comms boxes radiated a spurious signal, which was being reflected by by a prop disc that only qualified as a dish at a certain RPM. I'd want to know what happened to that AGC line as the signal was swamped/diminished.

I know it was easy in my day. AGC feedback was a simple voltage that could be measured with an AVO. Now I suppose it's a digital 'statement.' :hmm:

[FullOppositeRudder]

Machaca, I don't understand the photographs you've posted. Is that aircraft actually airworthy? I've dealt with flying wrecks in the past but that is a bloody disgrace. Tell me it's not the one we're discussing here.Yes - I agree. I'm afraid I see almost infinite potential there for poor electrical connections and intermittent operation of equipment - and noise. Somehow it doesn't quite fit in with the sweet looking machine we've seen elsewhere in this this thread.

Another puzzle (hopefully resolved soon) (sigh......)

FOR
PS Please, no-one start posting photos of the bl**dy spark plugs :(

[Machaca]

Was this aircraft resurrected from a serious accident in 1984 (https://www.atsb.gov.au/publications/investigation_reports/1984/aair/aair198403568.aspx) or was the reg number later assigned to another FA-200-180?

[mike-wsm]

Registration details for hangflyer's aircraft are:
Model FA-200-180
Serial FA-200-203
First Registered Date: 07 November 2006

The accident report says the aircraft involved in the 1984 accident was destroyed.

[Machaca]

FA-200 wiring diagram:

http://i337.photobucket.com/albums/n385/motidog/HF-FA-200_8.jpg

Hi-res version here (http://i337.photobucket.com/albums/n385/motidog/HF-FA-200_7.jpg).

[GarageYears]

Thanks for the continued stupidity DERG, you seem to have adopted Charlie Sheen's approach to life ---> "Nutter". :mad:

I presume you are going to explain that these particular cockroaches have some kind of Adonis DNA and can sense when the radio is in-tune with a station and start generating 'white noise'. :D

Since not much useful has been added in 24 hours ('cept the electrical schematics perhaps) I will continue to remind all the folks that can't be bothered to read the ENTIRE thread, that much fault-finding has already been done and the single MOST IMPORTANT point discovered to date are these:

- No white noise with the engine RPM below about 2350 (was originally 2250, but went up a little following replacement of plugs and 'tune-up' bits in the engine
- ONLY NOISE when the tuned station is within +/-20 degrees of the nose, goes away otherwise
- ONLY NOISE when a station IS TUNED, opening the squelch on an open frequency does NOT produce the problem effect
- Noise is NOT heard on a hand-held in the cockpit, or the Nav receivers
- Noise is the SAME whether the station is the tower 200m away or many km distant, the receive volume of the noise is constant
- Noise is heard on BOTH VHF receivers (2 fitted), originally 2 antennas on topside rear of craft
- New antenna installed on the underside of the aircraft does NOT suffer noise

(There might be a couple of other pointers I'm forgetting but I think these are the key ones)

My dis-assembly of all this is:
a) Noise is not RF signal (otherwise hand-held and nav rx would 'hear' it too)
b) Noise is not RF signal (otherwise an open frequency would carry it too)
c) Directionality - prop is in front and effect is linked to RPM.... vibration effect???
d) ADF antenna is directly above topside location of original antennas... interaction?

What am I missing (DREG - again, if you mention cockroaches I have suggestions where to insert them....)

- GY :hmm:

[mike-wsm]

First registered as VH-FJI in 2006 but manufactured in 1972.

Click on: RegoSearch/CASA (http://www.regosearch.com/aircraft/au/FJI).

[Wodrick]

DREG is so much more appropriate than DERG !

[forget]

From a web-site. VH-FJI is maintained in the Charter Category and is probably the finest example of a Fuji in WA.

I'm still baffled by Machaca's photograhs. They must be from a different aircraft - so why post them? :confused:

As I said earlier - 'I've dealt with flying wrecks in the past but that is a bloody disgrace. Tell me it's not the one we're discussing here'.

[mike-wsm]

hangflyer

I would like to suggest a test, please. One of the explanations suggests that there is no external noise, and that the white noise you hear is just the normal internal radio mixer noise heard when the received signal gets very weak and the agc (automatic gain control) turns up the radio gain.

Would it be possible to check reception on a distant station and see if the noise level comes up and whether it sounds the same? To one side and using each receiver in turn.

Many thanks, Mike.

[Machaca]

I'm still baffled by Machaca's photograhs. They must be from a different aircraft - so why post them?

The images I've posted are indeed from photos of VH-FJI -- taken by hangflyer himself last June.

[GarageYears]

The images I've posted are indeed from photos of VH-FJI -- taken by hangflyer himself last June. Bloody hell! And he's worried about the performance of the radios........?!!!:mad:

I'd be more worried about the control column coming off in my hands. :eek:

I really don't get it - the external condition doesn't reflect any of the mess that appears to be inside. :ooh:

To be honest I'm surprised much of anything works. Ever heard the term ground-up rebuild??

- GY

[forget]

Thanks Machaca. There's something very fishy here. No one in their right mind would charter that mess - and surely the aircraft has an independent annual. Who would sign that off? Very very fishy.

I'm out.

[Machaca]

I'd be more worried about the control column coming off in my hands.I would expect since acquiring the aircraft hangflyer has sorted out most mechanical ills and surely followed the technical bulletin (http://www.fhi.co.jp/english/fa200/tb/pdf/TB_200_020_NC_EG.pdf) and replaced the control column tubes.

forget -- a lot of time passed since hangflyer acquired the aircraft and he subsequently offered aerobatic flights for hire. Much work could have been accomplished since last June.

[mm43]

http://i54.tinypic.com/2z71wzr.jpg

It would appear that the alternator is a belt driven Prestolite brand, and the above schematic represents the associated regulator.

Potential noise culprits are the Zener diode Z1 (with open circuit C1) and L1 - a neon. MWR has dismissed the regulator, based on reports from HF, but as nothing has conclusively been determined as the cause, it is still open to conjecture.

[boguing]

mm43, MurphyWR posted #201 a reminder that Hangflyer had run the engine with the field disconnected, to no avail. pm also sent.

[cwatters]

You mentioned it only occurs with the radio tuned to a station. Could this be some sort of coupling between the two radios? Does it only do it when both radios are on?

[MurphyWasRight]

cwatters wonders


You mentioned it only occurs with the radio tuned to a station. Could this be some sort of coupling between the two radios? Does it only do it when both radios are on?


From HF original post:


Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change.

[john_tullamarine]

We appear to have some remaining tech posts interspersed with a range of silly posts. I have had several complaints that the latter are spoiling the game for those more interested in the tech aspects.

I will monitor the thread more closely and, if the standard of some posts doesn't rapidly rise, either move the thread elsewhere or remove some posters as appropriate.

regards,

[mike-wsm]

Potential noise culprits are the Zener diode . . .
Sorry, incorrect. Zener diodes are used in noise generators because they have a broad spectrum, not because they produce any significant amount of noise. In one circuit for a noise generator the zener is followed by +40dB of broadband gain and the output noise is still very low, about -50dBm. A great many electronic components produce noise, this is either specified in the data sheet or easily calculable from the component value. But none are in a range which would produce radio interference when properly designed and packaged. And, as Murphy has pointed out, a constant noise source would not interfere equally with nearby and distant signals. So please, let's put this one to bed. Thanks.

[mike-wsm]

One possible effect is that the ADF aerial in conjunction with the fuselage makes a shorted turn around the comms antenna(s).

I have just run a very crude simulation. I used a small transistor radio receiver on 107MHz (near to the aircraft band) and held a shorted turn of wire so it hung around the radio.

As I rotated the loop, keeping its axis horizontal, it was possible to see changes in signal strength on the tunng LED. There was attenuation of signal and it did vary as the loop was rotated. It appears that a shorted turn around an antenna does cause attenuation of the signal and that this is directional.

When the shorted turn was in the orientation that gave most attenuation the noise came up, as might be expected. The signal drops, the agc increases the gain, and the noise appears to increase. This noise is simply the receiver internal noise in the early stages and the first mixer.

This is not a full explanation of what is happening but it may be contributory. I hope that HF will listen to my requests and at least do a ground test with and without the ADF aerial fitted, in amongst his busy schedule of flights.

If this gets a result then we may be able to add further aspects to explain the remaining symptoms.

[hangflyer]

Ok, time for another update.

After one week with the antenna on the underside of the fuselage I can say that Com1 is definitely no longer suffering from the same issue.

Com2 on the other hand is unchanged, this proves it is something occurring on the top side of the fuselage.

Thanks for the ADF sense wire suggestions, It has been on the aircraft since new, however the wire was replaced in June last year, the problem started several months later.

The ADF wire will be removed for testing to see whether Com2 also behaves perfectly with the wire removed, another update soon.

Those people who are surprised by the age of the aircraft should have a look at the average age of the GA aircraft fleet and you will see this is nothing out of the ordinary. Charter places very strict requirements on the maintenance of the aircraft and nothing that could affect the safety of the aircraft is overlooked.

I was specifically looking for a 4 place aircraft that could be used for charter and was also fully aerobatic with 2 seats filled, the Fuji was the only one I could find.

[GarageYears]

So far the only theory that accommodates the majority of issues is a "shorted-turn" caused by the ADF antenna - I think the majority of those interested in this thread are on that page currently. This lines up with the directional aspect of the problem. How about noise....?

The AGC circuit is probably the culprit. In most receivers the AGC voltage is taken from the detector. The dc voltage is derived to be proportional to the average level of the carrier and adjusts the gain of the radio-frequency (rf) and intermediate-frequency (i-f) amplifiers. The AGC tends to keep the input signal to the audio frequency amplifier constant despite variations in rf signal strength. Remember that all circuits have an inherent "self-noise", so, given we are losing signal due to the shorted-turn effect, the net result is to run the receiver at very high gain, hence the noise.

This theory fits as far as the new antenna on the aircraft underside NOT being so affected (out of the shorted-turn ADF antenna influence).

So what doesn't fit?

The only part I'm perhaps struggling with is why turning off squelch does not seem to produce the same "noise" on an open channel; i.e. tune off station, defeat the squelch and I would have expected a similar noise experience (since the AGC should ramp the receiver to max gain). However I have not looked at the specs or circuit of the radio in question and there are various tricks of circuit design to prevent ''no-carrier-at-all" situations blasting noise... so that may be it.

The last question - why the RPM relationship? Vibration... above X RPM we have some vibration causing the ADF antenna to behave as the theorized shorted-turn. This would probably explain the intermittent effect also, since the exact vibrational mechanism and cause of the shorted-turn behavior are currently unknown.

Thoughts?

[MurphyWasRight]

Garage Years:

The only part I'm perhaps struggling with is why turning off squelch does not seem to produce the same "noise" on an open channel; i.e. tune off station, defeat the squelch and I would have expected a similar noise experience (since the AGC should ramp the receiver to max gain).

This might be explained by the AGC circuits chasing a constantly changing
signal level causing wild swings in level at detector.

Also a weak signal (caused by agc) may actually produce more noticeable noise than no signal at all due to the pressence of a carrier for the noise to modualte.
BTW That is an attempt to describe some very complicated theoretical stuff that I learned much to long ago to be more precise about.

Also it does not necesarily have to be a fully shorted turn, any flapping or intermitantly connected conductor of "magic" length or distance from the antenna might cause rapid level shifts by reflection/cancelation.
The ADF wire certainly looks to be in the right place to be a suspect.

If that is not the culprit I will go back to thinking about how the prop could be unbonded above a certain speed...

[ion_berkley]

The AGC circuit is probably the culprit. In most receivers the AGC voltage is taken from the detector. The dc voltage is derived to be proportional to the average level of the carrier and adjusts the gain of the radio-frequency (rf) and intermediate-frequency (i-f) amplifiers. The AGC tends to keep the input signal to the audio frequency amplifier constant despite variations in rf signal strength. Remember that all circuits have an inherent "self-noise", so, given we are losing signal due to the shorted-turn effect, the net result is to run the receiver at very high gain, hence the noise.

This theory fits as far as the new antenna on the aircraft underside NOT being so affected (out of the shorted-turn ADF antenna influence).

So what doesn't fit?

The only part I'm perhaps struggling with is why turning off squelch does not seem to produce the same "noise" on an open channel; i.e. tune off station, defeat the squelch and I would have expected a similar noise experience (since the AGC should ramp the receiver to max gain). However I have not looked at the specs or circuit of the radio in question and there are various tricks of circuit design to prevent ''no-carrier-at-all" situations blasting noise... so that may be it.Whilst you are surely on the money saying that there is a good chance that an AGC circuit is actively involved in this issue, I think we have to once and for all get away from the idea that this an adative Signal+Noise on the audio problem. Surely the Squelch experiment clearly demonstrates that there is no constantly present unusual noise source, but instead something more akin to an AGC oscillation/instability and/or unusual modulation phenomena somewhere in the signal chain that is induced as a result of some mechanical phenomena near to the actual antenna. The ADF wire removal is going to provide to some very good data.

p.s. THANK YOU John_T!

[GarageYears]

Surely the Squelch experiment clearly demonstrates that there is no constantly present unusual noise source, but instead something more akin to an AGC oscillation/instability and/or unusual modulation phenomena somewhere in the signal chain that is induced as a result of some mechanical phenomena near to the actual antenna. The ADF wire removal is going to provide to some very good data.

I indeed agree and hope to see an update from hangflyer sooner rather than later, especially since the "noise" in this thread has now diminished greatly... almost peaceful here now isn't it? Second the thanks to John_T :D

- GY

[MurphyWasRight]

Whilst you are surely on the money saying that there is a good chance that an AGC circuit is actively involved in this issue, I think we have to once and for all get away from the idea that this an adative Signal+Noise on the audio problem. Surely the Squelch experiment clearly demonstrates that there is no constantly present unusual noise source, but instead something more akin to an AGC oscillation/instability and/or unusual modulation phenomena somewhere in the signal chain that is induced as a result of some mechanical phenomena near to the actual antenna. The ADF wire removal is going to provide to some very good data.



Not sure that GY was saying that there was an "unusual noise source" involved.

I read it that he was puzling over why the max gain (no signal at all) audio did not ramp up the circuit self noise to a point where it at least partially resembled the problem noise.
HF did mention a "chop" to it which I suspect is related to the AGC misbehaviour.

The reason the AGC does not misbehave with no signal is simply that the interlnal self noise is constant, not modulated by whatever is causing the problem.
Even with almost perfect electronics the external background noise would be non directional.

Misbehaving (especially oscilating) agc can cause all sort of weird effects if it causes levels at the detector way beyond design targets.

Historical note: Bell labs used liquid helium cooled amplifiers (and a host of other tricks, google "big ear") for early satelite groundsations, still had noise and eventually discovered the "big bang" cosmic background radiation.


I saw GY.s response after I posted this, concise wins the race :)

BTW: Also heartily agree on thanks to moderator, I have no problem with ignorance (have plenty of my own) but the **** posts were not that...

[john_tullamarine]

We don't always call the shots right first time around .. but we do try our best ...

[Self Loading Freight]

The rust in the pictures is a bit of an eye-opener. Rusty metal can be a semiconductor and thus can create conditions where signals can modulate each other and be re-radiated at a strong enough level to cause local problems. I've had an interference problem - an HF transmitter messing up a VHF receiver - which proved to be a rusty cattle fence in the field next to the house. That took a lot of finding, as the whole fence was radiating: eventually got there with a hand-held receiver and uncharacteristic patience. A spectrum analyser would have been the weapon of choice. A man can dream.

If the corrosion is on a connection carrying noisy DC, either on a power line or a ground, that noise could be superimposed on an ambient radio signal. It wouldn't have to be very much corrosion either. And the orientation of the aircraft could certainly affect the RF level present at the faulty junction and thus the re-radiated field, either through shielding effects or by wiring acting as an antenna.

[mike-wsm]

Yes, a stray wire will re-radiate radio signals, this is the principle used in underground cable detectors, usually in the very low frequency bands which are suitable for long cables and where there are very high power transmitters in full-time operation. But this is a linear effect, there is unlikely to be any measurable intermodulation between signals.

Any effect of HF on a VHF receiver is much more likely to be due to known deficienceies in design, for example breakthrough of excessivly strong images.

[JuergenP]

"We don't always call the shots right first time around .. but we do try our best ..."

The window was opened and the adverisers were happy...all we need now is a chat room...

[chapter22]

Don't know weather this will help or not, but I had a very similar issue on a B737 which we eventually traced to a N2 tacho connector, when we changed the connector the problem went away! the connector was a cannon metal type and I can only assume that it was generating some RF, It would be worthwhile checking in the engine compartment to see if you have any similar metal connectors and if you are able to disconnect it and check to se if the problem goes.
On the 737 it took us over 6 months to find the defect and many things had been changed including the aerial, feeders, tr/rx intercom.
Hope this helps
Dave

[Wodrick]

I thought about mentioning that (N1/2 tacho) I have had similar on a 727 which only cleared by accident when the Tacho Generator was changed for a different defect. Couple of days later smart pilot announced that the noise had gone !
I decided that it wasn't relevant in this case and didn't know what Lycoming did for RPM indication,could be like a car. I have never been involved with anything with less than 80 seats !

[GarageYears]

Indeed - come on hangflyer - we've banished the "evil one" and now the thread goes cold....:sad:

Please let us know what happened when the ADF antenna was removed!

[mike-wsm]

http://mjr.org.uk/pprune-027.gif

[hangflyer]

Ok, time for another update.

Either using the antenna on the bottom of the fuselage or using either of the top antennas with the ADF sense cable antenna removed does not create the problem.

It has been a week now and I can say that with absolute certainty.

The ADF sense antenna is the culprit!

The antenna that was removed does look suspiciously like welding wire to everybody, but it was installed by an approved outfit and is apparantly a supplied cable that comes as part of a kit.

The antenna has been replaced with a stranded copper wire of the type commonly found on HF and ADF sense antennas, and so far so good. I need another week to be certain that the issue is bannished for ever, but I am hopeful.

With any luck my next post will lay this to rest for good.

Thanks again to everyone for the help and suggestions.

[mike-wsm]

Hurrah! Happy flying!

[Gulfstreamaviator]

Does the team think: well done chaps. any more like this, please.

glf

[FullOppositeRudder]

It's been a fascinating trip - a perplexing set of symptoms but it looks like you are nearly there. Happy flying!

FOR

[hangflyer]

Ok, it seems that I was a little premature in thinking the problem was beaten.

The ADF sense antenna was replaced with copper cable and within a couple of flights the issue re-appeared.

Through a process of elimination I now suspect that it might be the stand on which the lower end of the antenna terminates. I am guessing that this is shorting internally and causing the issue. Given that the existing ADF sense antenna has been there in one form or another for the best part of 40 years without issue, it simply has to be some failing component that is causing the problem.
My LAME wants to re-locate the ADF antenna to the belly of the aircraft, but I think it is worth replacing the post on top first.

I'll let you know how it goes!

[MurphyWasRight]

HF suspects

Through a process of elimination I now suspect that it might be the stand on which the lower end of the antenna terminates. I am guessing that this is shorting internally and causing the issue.


If I read the picture correctly that post could be just about the right
length to cause issues if it is alternalty shorted/open with vibration.

The clean flights can be explained by tensions changing a bit after the antenna was swapped.

When you do replace the post keep a sharp eye out for where something might be making intermitant contact, does not matter if it is supposeded to be grounded and coming loose or meant to be isolated getting grounded, either one could do it.

Thanks for keeping us posted, this is indeed one of the more fascinating issues I have followed in a long time.

BTW: Suspect the issue was just waiting for you to claim victory before re-appearing:)

[GarageYears]

Well, glad we seemed to get it right - ADF antenna subsystem. Obviously the wire itself is not the issue, but the fact the wire is connected, so, yes, the lower termination would seem a likely culprit.

Keep us updated!

[mike-wsm]

Dunno anything about actual installations but in theory at least I would expect to see a vhf choke in series with each end of the adf sense aerial. Does the installation kit come with chokes (coils) and are they shown on the installation schematic?

[hangflyer]

After the new copper ADF sense antenna did the same thing as the old steel wire it rules out the antenna itself.

Given that the problem occurred with the Coax disconnected from the antenna itself it seemed to leave only the coax attachment point as the cause of the issue.

I removed the ceramic post that the antenna terminated onto and found that the coax was supposed to join onto the bottom with a nut and washer, the nut and washer were sitting loose in the bottom of the unit. The connection to the coax was suspended just touching the bottom of the antenna mast, but would have vibrated in time with either engine RPMs or antenna movement to create the issue.

The worst vibration was caused by the engine harmonic and hence the RPM dependant part. The directionalility was indeed due to the directional nature of the sense antenna being aligned with the longitudinal axis of the aircraft.

Interestingly it was just a plain nut and washer, no locking mechanism to prevent it from vibrating loose over time.

[mnttech]

Wow, all that work for a lock washer.

[GarageYears]

Thanks for the final update! This has been a rather wild ride...

Are you sure there's nothing related to spark plugs... shhheeshh!

Good work from a few here - you know who you are. :D:ok:

Certainly one or two lessons learned I think.

Cheers, GY :zzz:

[FullOppositeRudder]

Delighted that it's finally fixed. This has provided the weirdest set of symptoms I've ever read about - much less encountered in real life. I've learned a lot from it all, so we both win :ok:. Thanks for keeping us informed and involved.

Regards
FOR