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.

Shorted turn test
 

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.

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.

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?

Garage Years:
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...

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!

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

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...

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

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.

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.

Stuff the MBAs Miss
 

"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...

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

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 !

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!

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.

Hurrah! Happy flying!

and the next question is:>>>>>
 

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

glf

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

FOR

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!

HF suspects
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:)

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!

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?

Final Post. Problem found!!
 

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.

Wow, all that work for a lock washer.

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:

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