Strange radio problems
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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
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Try putting your meter on 12" of Nichrome wire
Anyway, a google on static wicks finds e.g. this 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 says they are made of carbon fibre strands.
This also does not mention any resistive materials in the wick construction.
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.
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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?
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?
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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.
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.
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Peter read this
http://box703.bluehost.com/~lancairc...nstalation.pdf
The are hi resistance....
maybe you where experiencing St Elmo's fire
http://box703.bluehost.com/~lancairc...nstalation.pdf
The are hi resistance....
maybe you where experiencing St Elmo's fire
to ask if anyone's got any idea why my set should howl on Tx.
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Static Wick resistance
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...nstalation.pdf
http://box703.bluehost.com/~lancairc...nstalation.pdf
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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 over it (seriously!). (Starts around 25 min in).
"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 over it (seriously!). (Starts around 25 min in).
Last edited by Victorian; 4th Mar 2012 at 10:21.
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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.
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.
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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.
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.
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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
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...
That will be quite revealing, because the only difference between that, and getting the noise in flight, is that one is flying
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...
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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.
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.
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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 the temp/DP was c. 08/02. On the day I really noticed it it was c. 07/07 and thus a lot more water droplets were about.
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 the temp/DP was c. 08/02. On the day I really noticed it it was c. 07/07 and thus a lot more water droplets were about.
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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!
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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
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.
You would not think it would be possible but, hey, they obviously did grease them in the Annual exactly as per my requirements
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.
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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!
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!