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

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

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.

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

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.....................

abukaboy
 

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.

Forget

I agree with you.

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?

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.

SteveF 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
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.

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)

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.

MikeW (tounge firmly in cheek) suggests:
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.

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

ASFKAP 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.

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

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

Ancient aeronautical engineer here, with some radio-amateur and electronic background.
Quoting hangflyer's original post.
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

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.

Mike-wsm
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 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?)

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.

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?

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)

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.

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

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?

Quote:
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.

Antennas when problem first described:

http://i337.photobucket.com/albums/n...F-FA-200_1.jpg


After higher-gain antenna installed in place of original:

http://i337.photobucket.com/albums/n...F-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?

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.

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.

Thanks Machaca for the pics.

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

boguing;
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.

Boguing:

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

Machaca

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

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


Perhaps an intermodulation as mm43 has suggested.

hangflyer -- a few questions if you don't mind:

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.

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

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

Eureka!!! (maybe)
 

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.