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.