Strange radio problems

[peterh337]

Has anybody seen weird radio issues in today's high pressure conditions?

I have

com1: kx155a
com2: kx165a

Com1 is fine.

Com2 (an 8.33 channel spaced radio) suffers for a lot of interference (crackling) on all frequencies when in flight, but not on the ground, even if listening to distant transmitters (e.g. airliners calling up Gatwick Director). Tower radio checks are fine. The crackling is present only while there is an incoming carrier i.e. between transmissions the noise is not there.

Obviously the next stage is to swap the two radios over and do a flight test and see if the problem moves, then check the coax to the aerial, etc. I just have extremely limited facilities for checking RF cabling and radio performance where I am based.

[david viewing]

Teaching you to suck eggs I know but between transmissions you won't hear the crackling because it's below the squelch level... but you knew that.

OK so you've turned the squelch off and still note the crackling when carrier present? I know a radio that does that. I think it must still have an element of AGC regardless of the squelch, so when it detects carrier it turns the gain up. With no carrier the background is actually noisier, so it turns the gain down.

Is the crackling consistent with 'atmospheric' background noise? Or static? Or distant thunderstorms? Or is it a soldered joint inside the radio? A good whack to the front panel might teach it a lesson.

I wish I knew a radio engineer who could fix this stuff. Good luck, David.

[peterh337]

Turning off auto squelch does not change anything. The noise appears the same way.

It could be atmospheric. It is not any kind of "machine" noise.

I was thinking I have a bad RF (antenna) connection but the really weird thing is that listening to distant transmissions when on the ground (with the engine not running - that is something I need to test) works perfectly. And the NAV channels both work equally, with the flag disappearing more or less together, which indicates it is not an RF signal loss.

Nobody I know of in the UK can test a KX165A from first principles. If it is that, I would have to buy an overhauled exchange unit from the USA.

[mad_jock]

Before you do all of that is it foggy and moist where the plane is sitting?

Is it in a hanger?

[Pilot DAR]

foggy and moist where the plane is sitting?


MJ, it is in the UK!

[mad_jock]

I know but there a degrees on moisture and types in the UK.

It can be slashing it down with rain and you won't get any problems but 3 days of fog and you will get all manner of issues with avionics and the like.

The fact there is no problem when its on battery says to me that there is a leak from the earthing side allowing noise from the alt to effect the radio.

This can be for numerous reasons, one of which is that all the connectors are soaked and have condersate on them. A waz round with a can of spray and rerack them and everything will be grand.

Or it could be the back emf thing on the radio DC supply has a water bridge and noise is getting in that way.

If the aircraft is sitting dry in the hanger it would lead me to suspect the radio is fecked internally

[goldeneaglepilot]

I was thinking I have a bad RF (antenna) connection

If that was the case you stand a very good chance of damaging the radio when you transmit. Mad Jock is right on his pointers.

Check the obvious first. Pull the radio out of the rack and fit it back. Is the problem any better? If not start to drill down Mad Jocks suggestions until the problem is resolved.

[Genghis the Engineer]

I once got fed up with the oft repeated line about the Eskimos having lots and lots of words for snow. So, I started trying to work out how many words the British have for rain.

I got as far as 27, and didn't try all that hard.

G

[Jan Olieslagers]

Even if some of the Caledonian vocabulary is very exotic to this poor stupid foreigner, I share MJ's suspicions on the earthing side.

[Pilot.Lyons]

Oh dear....

[wigglyamp]

If it's a ground fault then it's more likely to exhibit problems on Tx when the current draw is higher. Same with a bad antenna - high VSWR will always make the transmitter worse before you notice much effect on rx. Typical VSWR on a general aviation VHF antenna should be better than 2.5:1 at end of the band and ideally less than 1.5:1 at around 127Mhz. The fault could be a duff screen or filter on a magneto P lead. In any case, if the fault can be reproduced on the ground with engine running, it should be isolatable by turning each item of electrical equipment off during a ground run whilst having the squelch open and listening to the noise on a weak distant station ( a distant volmet transmitter is good for this). If the fault is only apparent in flight, look for poor bonding of a flying control or undercarriage door or poor static dischargers.

[silverknapper]

I know it's obvious but are all the static wicks in good shape?

[peterh337]

Many thanks, wigglyamp :ok: and all.

I will do more tests.

Presumably the SWR can be tested by pulling the radio out and sticking a tester into the now exposed RF connector in the back of the tray? Otherwise, the cable runs from the back of the tray to a bulkhead BNC on the centre stack module (that part of the cable, about 3ft long, is inaccessible without pulling the whole centre stack out ;) ) and from the bulkhead it runs to the antenna directly. I know it is not RG400 and should be, but I have not had this problem before.

There seems no evidence that TX is affected.

Also I wonder why a bad static wick or bonding or a bad ignition lead would have no effect on the #1 radio.

I will swap over the two radios, but also do a ground test with distant stations with the engine running (should have thought of that).

The #2 radio is only ~ 2 years old and was bought brand new. Normally the KX1x5A are very reliable radios.

[wigglyamp]

The ideal way to do the VSWR check is to pull the box and use a tester with an integral signal generator that can be plugged intomthe backplate co-ax socket - HR Smith 12-602 or similar. If this isn't available, then a Thruline watt meter can be installed into the cable behind the rack and you use the radio to generate the test signals. On the TB, Ithink there are disconnect plugs that allow the radio stack to be removed so this is where the in-line wattmeter wil go.

Defective bonding of flying controls or poor static wicks can lead to a build up of static on parts of the airframe when airflow is present, and the static build-up can vary with humidity. When it gets really bad, the squelch on VHF can open on it's own just on the electrical noise! it can affect different systems depending on the relative position of the associated VHF antenna to the noise source.

[peterh337]

What puzzles me is that the static wicks are fine, the plane itself is "brand new" (10 years old :) ) and the #1 radio is fine.

I can just about detect a little bit of crackle in the #1 radio, with the auto squelch off.

How can one check bonding, other than by inspection?

Also it has been very humid, which should play against static during flight - or maybe the opposite?

There is 0.5kg of silica gel in the cockpit the whole time, and it is hangared.

The other already mentioned thing is that the noise appears only when somebody is transmitting. There is no trace of it at other times.

[goldeneaglepilot]

You mention that the Coax is the wrong spec, thats a good clue. You also mention that its TX related, checking VSWR is therefore essential before you do damage to the RF front end section of your radio.

[peterh337]

I should have made it clearer that this only just started - 2 days ago in fact.

But I did clearly say it is RX related. TX works fine.

[wigglyamp]

Bonding is checked using a milliOhm meter. The results between each component under test (flying control, gear door, radio unit etc) should be less than 0.05ohm back to main airframe ground (battery earth). When you say the static wicks are good - have you tested them ? Again, the base should be in bond as above. Between the base and the tip should be a resistive value specified in the AMM - it can typically be 2-4Mohm.

[goldeneaglepilot]

It's much harder for you to monitor the quality of the TX signal sitting in the aircraft. If its a coax / connector problem then it will be there during TX, and its a lot easier to check for problems by checking VSWR of the installation. My starting point would be to remove the radios from the rack and plug them back in, you may be surprised, often operating the connectors is enough to "cure" the problem. As is a spray of good quality contact cleaner.

[peterh337]

Between the base and the tip should be a resistive value specified in the AMM - it can typically be 2-4Mohm.

That's interesting... all the wicks I have ever seen were just stranded wire, with no resistor inside.

[goldeneaglepilot]

Why would it need a resistor? The poster said from tip to fixing, just the material itself could give that resistance. Try putting your meter on 12" of Nichrome wire

[peterh337]

Try putting your meter on 12" of Nichrome wire We are digressing a bit but I've been in electronics for over 40 years. Nevertheless I looked up nichrome wire, and the resistance table here (http://www.wiretron.com/nicrdat.html) suggests that you would need an awfully thin nichrome wire to give you the 2-4 Mohm mentioned by Wigglyamp. In fact it would be microscopic.

Anyway, a google on static wicks finds e.g. this (http://en.wikipedia.org/wiki/Static_discharger) which says

Static dischargers are fabricated with a wick of wire or a conductive element on one end, which provides a continuous low resistance discharge path between the aircraft and the air.

OTOH this (http://www.aerospaceweb.org/question/design/q0234.shtml) says they are made of carbon fibre strands.

This (http://navyaviation.tpub.com/14018/css/14018_102.htm) also does not mention any resistive materials in the wick construction.

[stickandrudderman]

I don't want to hijack the thread but since we're having a discussion on technical problems with aerials it seemed like a good time to ask if anyone's got any idea why my set should howl on Tx. Recipients of the Tx can hear it too and complain about it.

[Victorian]

Very interesting discussion about dischargers but it's hard to see how any value of resistance in a discharger - milliohms or megohms - could have any effect on it's ability to discharge the tiny currents and high voltages involved in static build up.

No-one seems to have picked up on Peter's cardinal point: that the crackling is only obvious when there is a carrier present.

Surely this is counter intuitive - with no carrier, the radio's gain will be at a maximum, and therefore most sensitive to noise. When a carrier is detected the radio's AGC will scale back the gain to keep the signal within the radio's dynamic range, avoiding clipping of the am modulation. This reduces sensitivity to noise and should reduce the volume of the crackling - but the opposite happens.

I'm familiar with this same phenomenon in my own aircraft. Can anyone suggest an explanation for it?

[Aerials]

Victorian, I have just read the thread through and arrived at the same conclusion as you. I suspect the agc and squelch circuit too. My only offering though, is the Avionics bench at the maintainer of your choice Peter. I would expect them to find a capacitor breaking down (tantalum beads go a bit funny), dry joint or if there's amplified AGC, that it has gone into oscillation for some reason. VSWR, bonding and static dischargers are all red herrings this time, I think.

Stick, what's the quality of the reception of your transmission with no microphone connected? Sometimes radio frequency energy gets induced into microphone circuits and causes that kind of effect you describe. Has it always been present? Please let us know.

[vulcanised]

Regarding the 'howling', could it be that the rx section of the radio is still active when transmitting and you are getting a howl round effect?

[joris]

Peter read this
http://box703.bluehost.com/~lancairc/media/builderupdates/IV-IVP/StaticWickInstalation.pdf

The are hi resistance....
maybe you where experiencing St Elmo's fire;)

[mad_jock]

Over 50% of my radio problems have been solved by letting the FO do the next few landings.

Maybe if you lifted the aircraft had give it a 6" drop test that would fix it .

[Whopity]

to ask if anyone's got any idea why my set should howl on Tx.RF Feedback into microphone. Can be caused by a poor ground or a ground loop,(multiple grounds). Try a different headset first, disconnect the mic and see if it still howls.

[wigglyamp]

The article from the link below explains more about P static and the function of the static wicks. It also explains the High Resistance nature of static wicks as previously mentioned.


http://box703.bluehost.com/~lancairc/media/builderupdates/IV-IVP/StaticWickInstalation.pdf

[stickandrudderman]

Thanks for the tips and sorry for the hijack!

[Victorian]

Wigglyamp, thanks for posting that but may I draw you back to what Peter actually said:

"a lot of interference (crackling) on all frequencies when in flight .... The crackling is present only while there is an incoming carrier i.e. between transmissions the noise is not there."

Peter is an engineer himself and is not describing static (white noise), he's describing a crackling that only appears when carrier is present. I'm interested in this because I get a similar behaviour in my own aircraft and my engineers can't solve that, either. And they're about as good at listening to the actual question as some contributors on here.

The Lancair article still doesn't get around to describing why their dischargers are resistive, either. Perhaps it's for supression in the same way as resistive plug leads, but since static is not impulsive it's hard to see what difference it can make. But Static supression has been a snake oil subject since Benjamin Franklin fell out with the King and lost us our American empire (http://www.bbc.co.uk/radio4/features/in-our-time/) over it (seriously!). (Starts around 25 min in).

[wigglyamp]

Trying to describe noise/interference can be very subjective. What one owner descibes as a crackle can be described by another as severe magneto interference! We've seem a similar problem to Peter's on a Piper Dakota where it was noise from the magneto harness and only affected Comm 1 - antenna nearest to the engine, yet the frequency didn't vary with engine rpm so not immediately identifiable at engine-related. The noise wasn't sufficient to oist the squelcj so was only heard on top of an incoming radio signal. Beech had problems for quite a while on Bonanzas and Barons when they moved Comm2 antenna onto the belly and put it near the nose-gear door and then got static interference, but only when the gear was down. You need to takle a logical approach starting with engine-off tests using a distant weak station to try and determine when the noise appears and then eliminate causes. It's not unknown for adjacent boxes to interfere with each other either - Narco units suffered from this (12D and 12E). There are so many causes that until you spend time to listen to it, trying to speculate on just the written description can really make no in-roads into solving the issue.

One poster mentioned howling on transmit - unrelated to Peter's issue. A common cause of this - particularly in home-built aircraft, is where the installer has used a triple-core screened cable for the mic-tel sockets with the third core as the common. The sceen jacket has no effect in segregating mic and tel signals so on tx, the tel sidetone, at a much higher signal level than the mic, is induced into the mic cable and causes positive feedback. We're recently re-wired a factory-new certified aircraft for exactly this issue as well.

[horsebox]

The interference is possibly being generated by the ignition system or alternator on the aircraft but probably on the first or second intermediate frequency of the receiver rather than the frequency that the set is tuned to.

Hence the interference is not able to unmute the receiver, but once the receiver is unmuted by a normal signal, the rogue signal can be heard.

Many years ago people used to check the points/timing on a car using the same principe. You would tune an am radio to a weak station, place it near to the engine, rotate the engine carefully by hand then you could hear a click when the points opened/shut.

Do you have a split alternator/battery switch? In flight run on battery only for a few minutes, see if the symptoms change.

I would also be checking the earth bonds around the aircraft.

[peterh337]

What I will do is go a ground engine-running test, listening to distant stations.

That will be quite revealing, because the only difference between that, and getting the noise in flight, is that one is flying :E

If the noise is present, then it is possibly the ignition, and yes I can test it with the alternator field disconnected. It will be quite a puzzle why com2 is doing it and not com1...

If the noise is not present then I will swap the radios and do a flight. If the problem moves with the radio then it is prob99 a duff radio.

There is a particular grounding strap which grounds the rudder to the hull. If this is broken, and it did break once several years ago, then the rudder could be charging up, and the com2 antenna is a lot closer to the rudder than the com1 antenna. Both are on the roof.

This issue started suddenly and strongly. It is nothing gradual. The static wicks are same as they have always been since the plane was new - solid (stranded) cable with zero resistance. I am inclined to suspect the radio...

[peterh337]

Update:

On the ground, with the engine running or not, it works OK with distant stations (people calling LHR etc).

There is no evidence of ignition interference - well not at sea level air pressure.

The #2 radio is slightly more sensitive than the #1 radio, but that's not an issue and in any case one would expect it to be the other way round if there was a cable/antenna issue.

Swapping the radios doesn't produce any meaningful data because I can't replicate the issue on the ground in the first place. But clearly there is nothing wrong with either radio or either antenna as far as sensitivity (gain) goes.

The interesting bit was the airframe bonding. On the TB20, there isn't any... well there is one strap to the rudder (I know because a couple of years ago it came right off) and that one measures fine (below 1 ohm). The ailerons measure ~ 10-20 ohms which is obviously not bonded (and they have static wicks) so that is just the conductivity of the bearings etc. The flaps are well grounded and have wicks.

The elevator is the best bit - it measures over 10 M ohms to the airframe!! It has two static wicks but clearly they will be useless except for the elevator itself.

So, apart from the flaps and their wicks, the parts of the airframe which have wicks are poorly connected to the airframe.

Yet AFAICT this is correct for the TB20/GT. It never had an elevator strap, from new.

I will get an elevator strap done and see what that does.

[peterh337]

Update:

Today we put two bonding straps between the airframe and the elevator, and a test flight shows the interference has totally vanished.

The interesting thing is why the interference did not occur on the flights done since the Annual in January, and those done more recently when it was noticed.

For example I did the JAA IRT during that time, and definitely used both radios on that. I also did some airway flights, and likewise.

The answer may be that the conductivity of the two elevator bearings was intermittent, but it could also have been in the amount of water droplets shed by the aircraft. On the day of the IRT (http://www.wunderground.com/history/airport/EGHI/2012/01/27/DailyHistory.html?req_city=NA&req_state=NA&req_statename=NA&theprefset=SHOWMETAR&theprefvalue=1) the temp/DP was c. 08/02. On the day (http://www.wunderground.com/history/airport/EGBB/2012/03/01/DailyHistory.html?req_city=NA&req_state=NA&req_statename=NA&theprefset=SHOWMETAR&theprefvalue=1) I really noticed it it was c. 07/07 and thus a lot more water droplets were about.

[jxk]

Peter, thanks for posting your latest update. The bonding straps are not something I would have thought could have made the difference - another thing to keep an eye on!

[peterh337]

For me, what swung the likelihood of this being the problem was the almost total open circuit in the two elevator bearings.

You would not think it would be possible but, hey, they obviously did grease them in the Annual exactly as per my requirements :E

Socata, it turns out, sell a bonding kit. I have never heard of anybody who has this installed, but it even includes little straps to ground little inspection covers to the airframe. You have to put a screw into the middle of each inspection cover...

I still don't have bonding on the ailerons but they are quite small. The flaps are well grounded but I can't say this is via bonding or just luck. The rudder was always bonded and I gather this is the only known bonding strap.

The curious thing is that none of these straps, and apparently this is also true for light jets, will carry a lightning discharge. I am aware of gliders crashing because their control linkages have been melted by lightning, and I am sure the same would happen to most "ultralight"-class aircraft (many of which are built to hang glider standards when it comes to linkages) but I have not heard of metal-hull aircraft crashing with welded-up controls. So maybe it turns out that lightning goes via the skin and the spars etc, and if you get hit into the elevator then it goes adequately via the two bearings without welding them up.

[david viewing]

Completely brilliant, Peter! So the crackling sound is the direct sound of water droplets hitting the elevator? (and coupling charge into/out of it, obviously).

Erm, what about the 'only detectable when there's carrier' part? Any thoughts on that?

Anyway, take a Nobel prize (with Oak Leaves) for that!

[peterh337]

I think the crackling was the sound of static charge, picked up by the aircraft as a whole, arcing across (flashing across) the non-conductive elevator bearings to the elevator, from which it escaped via the static wicks as normal.

That's my guess.

But yes it could have been the other way around i.e. static charge picked up by the elevator arcing across its bearings to the airframe, from which it got dissipated via the static wicks on the flaps, and the (somewhat less effective) static wicks on the ailerons.

The COM2 antenna is a lot closer to the elevator than the COM1 antenna, hence...

what about the 'only detectable when there's carrier' part

No idea. These radios are damn complicated digitally synthesised jobs.

[mad_jock]

I reckon you normally wear nylon womens underware thus creating lots of static build up and for the IRT you were wearing cotton proper mans pants.

Seems logical to me.

:ok:

[jxk]

It's quite noticeable that the later PA28 types (Warriors/Archers) that I've come across recently (and with glass cockpits) seem to have many more bonding and static wicks than previous.

[goldeneaglepilot]

I did see in the late 70's a 172 get hit by a lightening (hit the wing) on on short finals. The ailerons were welded solid on the side which took the strike. The only evidence of the strike (other than the welded aileron) was the small area of discolouration on the wing surface.

I guess no amount of bonding strap would have helped with that, but static discharge is a different mater.

Well done Peter

[NutLoose]

I would make sure the jack plugs in the panel are tight and haven't worked loose, also clean your headset plugs, I use a little bit of fine scotchbright.

[wigglyamp]

Hey Peter - good fix!

It seems like a British avionic engineer can actually diagnose a problem correctly (post 11) - I thought you believed only Americans could do anything right.

You know where we are when you finally want the PRNav upgrade - and not for £40K!

[peterh337]

You were right, wigglymap :ok:

BTW I never suggested UK avionics shops are worse than US ones.

It is instrument overhauls, particularly gyro ones, which I would never do in the UK.

[stickandrudderman]

Update to my howling problem:
Mine is a wooden aircraft. I had re-located the aerial inside the fuselage and this was the cause of the problem. When I re-located back outside the problem went away.

[peterh337]

Interesting... how do conventional VHF aerials work on a wooden hull?

Surely they need a ground plane? At least around the aerial base.

Otherwise what you have is a length of coax whose screen suddenly ends in nothing, and the inner conductor goes to the base of the aerial. RF stuff was never my strong area (I used to build pirate ratio transmitters in the late 1960s, with a pair of 807s; we even considered powering it from the 700V DC train rail :E ) but this doesn't seem right. The cable screen needs to go to that ground plane.

[wigglyamp]

On a wooden or composite airframe, the ground plane foe a uni-pole antenna is normally either metallic foil or mesh inside the fuselage. The ground plane should be at least 1/4 wavelength radius and the antenna should be centred to avoid becoming directional. The core of the co-ax goes to the antenna which is insulated from the ground plane and the screen connects to the ground plane. It's also possible to use a 1/2 wave dipole -typically a NAV antenna, or a Comm antenna inside the fin. In this case, the connection to the co-ax uses a matching balun cable and the co-ax core doesn't actually connect to either leg of the dipole.

[stickandrudderman]

What I know about radios can be written on the back of a fag packet using only two letters, one of them being "F".
For the antenna mounted inside the fuselage I used a self-manufactured aluminium ground plane of around 10"x8". For the externally mounted antenna I used the original aluminium ground plane which is about 1/4 of the size.
I would like to eventually return to having an out-of-sight solution but for the moment I have other priorities and am happy that at least the problem has been resolved, even if without any technical understanding.