Crazy Radio Issue, Help Needed.
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Ok, another update.
Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.
If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.
If you change to an unused frequency and turn off the squelch there is none of the noise.
The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.
The 2 radios have seperate cables and antennaes and they both exibit the same behaviour, with the newer radio with the new antenna being worse ONLY since the antenna was replaced with a higher gain unit.
NAV1 is the second half of COM2 which is a KX175BTSO
All last week the issue was absent, the comms were perfect. On Saturday half the flights were perfect and the other half had a little noise. On Sunday it got worse and we stopped flying at mid day and cancelled the remaining flights as it was becoming a safety issue with stations in front of the aircraft becoming unreadable at times.
This morning the noise is there, it starts at 2450RPM and stops at 2550RPM on the ground, it is still directional only on recieve on both comms.
It is possible to park the aircraft close enough to the tower that there is clear line of sight between the antennae on the top of the aircraft and the tower aerial on the tower, even in this situation the behaviour is unchanged, clean audio until the nose if pointed at the tower then with the nose within 20 degrees of the tower the signal degrades to 2s and then back to 5s again in every other direction.
I'm still going crazy trying to find a solution! I appreciate the suggestions.
Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.
If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.
If you change to an unused frequency and turn off the squelch there is none of the noise.
The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.
The 2 radios have seperate cables and antennaes and they both exibit the same behaviour, with the newer radio with the new antenna being worse ONLY since the antenna was replaced with a higher gain unit.
NAV1 is the second half of COM2 which is a KX175BTSO
All last week the issue was absent, the comms were perfect. On Saturday half the flights were perfect and the other half had a little noise. On Sunday it got worse and we stopped flying at mid day and cancelled the remaining flights as it was becoming a safety issue with stations in front of the aircraft becoming unreadable at times.
This morning the noise is there, it starts at 2450RPM and stops at 2550RPM on the ground, it is still directional only on recieve on both comms.
It is possible to park the aircraft close enough to the tower that there is clear line of sight between the antennae on the top of the aircraft and the tower aerial on the tower, even in this situation the behaviour is unchanged, clean audio until the nose if pointed at the tower then with the nose within 20 degrees of the tower the signal degrades to 2s and then back to 5s again in every other direction.
I'm still going crazy trying to find a solution! I appreciate the suggestions.
so the noise is coming in on a cable, either power or antenna feed ? (apologies if this is already known and I am being dumb). But if this is the case, why is it directional ?
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I have been unable to correlate the issue with any of the following items:
Temperature
Humidity
Cloud Height
Day of Week
Moisture
Potential Static Build Up
Hard Landings
Time of Day
Aircraft Use
Solar Flares!
SpaceWeather.com -- News and information about meteor showers, solar flares, auroras, and near-Earth asteroids
Check for yourself!
Temperature
Humidity
Cloud Height
Day of Week
Moisture
Potential Static Build Up
Hard Landings
Time of Day
Aircraft Use
Solar Flares!
SpaceWeather.com -- News and information about meteor showers, solar flares, auroras, and near-Earth asteroids
Check for yourself!
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Not RFI
A: The problem occurs over a wide range of recevied signal strengths. This rules out reduction of signal strength by shadowing.
B: Everything but the battery and one radio have been disabled and noise was still there.
C: No hint of it on the handheld.
D: Most telling: The noise is only there when receiving, not in background with squelch disabled.
My theory:
The prop is modulating the Rx signal. This would account for the extreme directional sensititivity, only happpens whe prop is between antenna and source.
I have no idea of the normal grounding(/bonding) path between prop blade
and airplane but suspect as the prop vibration sets in at ~2100 rpm one or more of the blades are intermitantly disconnected.
Given prop length and radio wavelength this could cause a a reflection/phase shift that would modulate the signal.
The impact would also be critically dependant on distance from prop to antenna, which would explain lack of noise on handheld.
One test, not conclusive, would be to measure resistance from each blade to chassis while applying force in every direction to each blade of prop (engine secured!!).
Few more things to try: Is there any hint of the noise when the base station is directly behind you?
Turn to the "edge" of the noise (~15degrees?) and see if problem goes away with airplane pitch. (Another way of moving prop in/out of line of sight of antenna.).
(Added)
Note the theory behind this is that the rapdidly fluctuating Rx signal strength is causing the receivers AGC (automatic gain control) to wig out resulting in incorrect levels at the detector.
More on why not RFI:
Simple shadowing would slowly change the Rx level, much less than flying from .5 miles to 20 miles.
If the inteference was external generated RFI it would be at a (relatively) constant level so a few Db change in Rx level would only matter at some fairly critical distance from base station, farther away it would be pressent all the time, close in would not be there at all.
B: Everything but the battery and one radio have been disabled and noise was still there.
C: No hint of it on the handheld.
D: Most telling: The noise is only there when receiving, not in background with squelch disabled.
My theory:
The prop is modulating the Rx signal. This would account for the extreme directional sensititivity, only happpens whe prop is between antenna and source.
I have no idea of the normal grounding(/bonding) path between prop blade
and airplane but suspect as the prop vibration sets in at ~2100 rpm one or more of the blades are intermitantly disconnected.
Given prop length and radio wavelength this could cause a a reflection/phase shift that would modulate the signal.
The impact would also be critically dependant on distance from prop to antenna, which would explain lack of noise on handheld.
One test, not conclusive, would be to measure resistance from each blade to chassis while applying force in every direction to each blade of prop (engine secured!!).
Few more things to try: Is there any hint of the noise when the base station is directly behind you?
Turn to the "edge" of the noise (~15degrees?) and see if problem goes away with airplane pitch. (Another way of moving prop in/out of line of sight of antenna.).
(Added)
Note the theory behind this is that the rapdidly fluctuating Rx signal strength is causing the receivers AGC (automatic gain control) to wig out resulting in incorrect levels at the detector.
More on why not RFI:
Simple shadowing would slowly change the Rx level, much less than flying from .5 miles to 20 miles.
If the inteference was external generated RFI it would be at a (relatively) constant level so a few Db change in Rx level would only matter at some fairly critical distance from base station, farther away it would be pressent all the time, close in would not be there at all.
Last edited by MurphyWasRight; 22nd Feb 2011 at 19:22. Reason: Typos, clarity. added bit more explanation
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This may be completely off beam, and I can see no reason for the directional implications, but here goes.
You mentioned that it was there all day, but then wasn't on a test. What changed, in terms of the radio equipment in use, By that, was it a case of 2 people, headsets etc in flight, and only one when testing? I'm thinking around things like headset connectors in the panel for the second person position, maybe an intercom related issue, are you using active noise cancelling headsets, could a socket connection be vibrating with engine speed, or even (perish the thought) is there an issue with the passenger seat bonding that's not the same empty or loaded.
There is also the implication that it's not just radios, there is also possibly an intercom unit/switching unit somwhere in the loop. Has that been eliminated from the possible suspects. Also the wiring to and from headset sockets to the intercom and on to the radios, any chance of a change here, especially related to ground loops in coax and the like, on microphone leads etc.
PTT wiring, any possibility of a chafe on them that is giving airframe contact at certain harmonic frequencies?
Appreciate that this is very much on the crazy end of speculative, but from what I can see, all the obvious and some of the very much not obvious have already been done to death.
Are there any engine instruments or other instruments which can't be switched off that have a voltage regulator as part of their construction.
Are there any maintenance or other access panels that were removed around the time of the start of this, not necessarily forward of the cabin?
what beacons and or strobes are there on the aircraft, and have they been eliminated from the possibilities?
Is there any possibility of harmonic resonance in something like a control cable run causing contact with an ajacent fuselage member?
I'm about out of ideas at this point, best of luck.
You mentioned that it was there all day, but then wasn't on a test. What changed, in terms of the radio equipment in use, By that, was it a case of 2 people, headsets etc in flight, and only one when testing? I'm thinking around things like headset connectors in the panel for the second person position, maybe an intercom related issue, are you using active noise cancelling headsets, could a socket connection be vibrating with engine speed, or even (perish the thought) is there an issue with the passenger seat bonding that's not the same empty or loaded.
There is also the implication that it's not just radios, there is also possibly an intercom unit/switching unit somwhere in the loop. Has that been eliminated from the possible suspects. Also the wiring to and from headset sockets to the intercom and on to the radios, any chance of a change here, especially related to ground loops in coax and the like, on microphone leads etc.
PTT wiring, any possibility of a chafe on them that is giving airframe contact at certain harmonic frequencies?
Appreciate that this is very much on the crazy end of speculative, but from what I can see, all the obvious and some of the very much not obvious have already been done to death.
Are there any engine instruments or other instruments which can't be switched off that have a voltage regulator as part of their construction.
Are there any maintenance or other access panels that were removed around the time of the start of this, not necessarily forward of the cabin?
what beacons and or strobes are there on the aircraft, and have they been eliminated from the possibilities?
Is there any possibility of harmonic resonance in something like a control cable run causing contact with an ajacent fuselage member?
I'm about out of ideas at this point, best of luck.
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My vote is with Mig15.
I'd suggest that if you try what DERG suggests, then you'd be able to measure the SWR just before the front end gives up due to reflected power returning.
Just my humble opinion.
I'd suggest that if you try what DERG suggests, then you'd be able to measure the SWR just before the front end gives up due to reflected power returning.
Just my humble opinion.
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I agree with Murphy
The problem occurs when the received signal is coming through the prop and the effect is the same on strong and weak signals. This rules out many possibilities.
What may be happening is that the incoming signal is modulated by the prop, quite literally mechanically chopped. This will put strong amplitude modulation onto the carrier. With a two blade prop running at 2400rpm this is 80Hz am. The modulation will be a constant proportion of carrier and so bear a constant relationship to the received signal (also am). Presumably the receiver agc sees the chop and turns the gain down, reducing the amount of wanted signal.
The effect may be specific to a particular type of receiver or even to the mode of operation set on the receiver or the software version loaded. I don't have a schematic so can't even begin to guess what is going on inside the receiver.
The problem occurs when the received signal is coming through the prop and the effect is the same on strong and weak signals. This rules out many possibilities.
What may be happening is that the incoming signal is modulated by the prop, quite literally mechanically chopped. This will put strong amplitude modulation onto the carrier. With a two blade prop running at 2400rpm this is 80Hz am. The modulation will be a constant proportion of carrier and so bear a constant relationship to the received signal (also am). Presumably the receiver agc sees the chop and turns the gain down, reducing the amount of wanted signal.
The effect may be specific to a particular type of receiver or even to the mode of operation set on the receiver or the software version loaded. I don't have a schematic so can't even begin to guess what is going on inside the receiver.
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I agree with Murphy
The problem occurs when the received signal is coming through the prop and the effect is the same on strong and weak signals. This rules out many possibilities.
What may be happening is that the incoming signal is modulated by the prop, quite literally mechanically chopped. This will put strong amplitude modulation onto the carrier. With a two blade prop running at 2400rpm this is 80Hz am. The modulation will be a constant proportion of carrier and so bear a constant relationship to the received signal (also am). Presumably the receiver agc sees the chop and turns the gain down, reducing the amount of wanted signal.
Kudos @murphy (pending confirmation!!)
The effect may be specific to a particular type of receiver or even to the mode of operation set on the receiver or the software version loaded. I don't have a schematic so can't even begin to guess what is going on inside the receiver.
The problem occurs when the received signal is coming through the prop and the effect is the same on strong and weak signals. This rules out many possibilities.
What may be happening is that the incoming signal is modulated by the prop, quite literally mechanically chopped. This will put strong amplitude modulation onto the carrier. With a two blade prop running at 2400rpm this is 80Hz am. The modulation will be a constant proportion of carrier and so bear a constant relationship to the received signal (also am). Presumably the receiver agc sees the chop and turns the gain down, reducing the amount of wanted signal.
Kudos @murphy (pending confirmation!!)
The effect may be specific to a particular type of receiver or even to the mode of operation set on the receiver or the software version loaded. I don't have a schematic so can't even begin to guess what is going on inside the receiver.
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If I park 200M away from the Tower the problem is unchanged.
This is interesting because the VHF antenna has a clear line of sight to the VHF antenna on top of the tower.
At this range I can taxi in circles with the engine RPMs high enough for the problem to be there and the noise starts at +-20 degrees of the nose pointing towards the tower. The rest of the 320 degree rotation the noise is not there at all. When the noise starts coms go from 5s (everywhere else) to 2s for those 20 degrees.
I initially felt the at the signal was going "getting chopped" as it went through the prop, however I can put the aircraft in positions where there is an absolutely clear LOS between the transmitting and receiving antennae and it still happens if the station is aligned with the nose of the aircraft.
One of the major problems I have is that I am not a LAME, as this is a Charter category aircraft I am not free to take things apart myself, I need to bring ideas to the engineers or avionics guys who then charge me a fortune to see if they can fix the problem.
At the risk of sounding unkind, it does seem to me that in most cases today the standard procedure is to replace boxes until the problem goes away, often the problem is determined to be "the box" because a new one fixed it (sometimes because they broke the old one removing it). I am at the point now where they have replaced pretty much all the replaceable components in the system and I still have the issue and seem to be running out of ideas.
There also seems to be a lot of contrary theory amongst the techs.
There are 2 competing companies that have both had a go.
One says that the problem must be shadowing of the antenna by the fuselage in front which would cause a weaker signal and hence the directional nature of the problem.
The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.
Does anyone actually know which one of them is correct?
This is interesting because the VHF antenna has a clear line of sight to the VHF antenna on top of the tower.
At this range I can taxi in circles with the engine RPMs high enough for the problem to be there and the noise starts at +-20 degrees of the nose pointing towards the tower. The rest of the 320 degree rotation the noise is not there at all. When the noise starts coms go from 5s (everywhere else) to 2s for those 20 degrees.
I initially felt the at the signal was going "getting chopped" as it went through the prop, however I can put the aircraft in positions where there is an absolutely clear LOS between the transmitting and receiving antennae and it still happens if the station is aligned with the nose of the aircraft.
One of the major problems I have is that I am not a LAME, as this is a Charter category aircraft I am not free to take things apart myself, I need to bring ideas to the engineers or avionics guys who then charge me a fortune to see if they can fix the problem.
At the risk of sounding unkind, it does seem to me that in most cases today the standard procedure is to replace boxes until the problem goes away, often the problem is determined to be "the box" because a new one fixed it (sometimes because they broke the old one removing it). I am at the point now where they have replaced pretty much all the replaceable components in the system and I still have the issue and seem to be running out of ideas.
There also seems to be a lot of contrary theory amongst the techs.
There are 2 competing companies that have both had a go.
One says that the problem must be shadowing of the antenna by the fuselage in front which would cause a weaker signal and hence the directional nature of the problem.
The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.
Does anyone actually know which one of them is correct?
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Also we have 2 different brands of radio on 2 different length coax cables on 2 different types of VHF antenna located on 2 different parts of the fuselage that both have the identical problem! Harmonics?
Both radios have been swapped with other units. Not specific to just one unit.
Both radios have been swapped with other units. Not specific to just one unit.
Thanks HF for the additional information. Some other contributions have been very useful, one or two not quite so IMHO (that's all I want to say on that 
)
There is a small amount of truth in each of them but neither would get a satisfactory pass in the real world with my old radio theory tutor.
The first statement might be a consideration if we were dealing in micro-watts over a considerable distance. Unless we have a seriously desensitised reciever (two on them in fact) I suggest that the shielding effect of the fuselage would be minimal given the transmit power of the radio equipment used in the tower ground installations which I would expect to be 'significant'. This theory is further eroded by the fact (if I understand correctly) that even with the aircraft on the ground in close proximity to the tower such that line of sight conditions between the transmitting and receiving antennas, the problem is still evident.
The second statement has some credibility in that if the shield of coax cable to the antenna is bonded to the skin of the aircraft (metal is assumed here) and thus to the airframe (grounded to use the more common terminology) at the antenna feedpoint the entire aircraft becomes the ground (some would say counterpoise) against which the whip of the antenna either radiates or receives a signal. This is the sort of system I would expect in this aircraft (the turtle deck is metal (aluminium) isn't it ?????) .
There is usually some directivity in that the radiation patten of the antenna would normally be the strongest in the direction of the largest area of ground plane. In this instance I think the directivity effect would probably favour signals in front of the aircraft if at all. Others might argue for signals from the rear of the aircraft being stronger in both transmit and receive. A beer or several are needed here to arrive at the correct answer by concensus. But I would be astonished beyond measure if either possibility were serious enough to produce the effect we are seeing here.
The prop modulation possibility as presented and argued by others earlier today is very interesting and seemingly credible. However I am troubled by the intermittent nature of the problem, and that the onset of the problem has been lifted by quite a few hundred RPM as changes were made to engine equipment which improved the performance. Prop modulation is possible perhaps, but surely there would have to be other contributing factors.
Finally, one especially puzzling aspect is that, as I understand it, the noise is stronger on the radio which has the newer, higher gain antenna. That again really suggests some sort of RF interference which is being received by that antenna at a higher received sig strength than the other one. (No it doesn't help a lot - at least I don't think it does - but it's just another question without logical answer)
What is the answer then? I don't know. I'm baffled like almost everyone else but somehow, someway, the cause will be identified and the solution found.
FOR
(all written in hast - with apologies to grammatical and/or technical purists - it's been a long hard day, this might be better attempted after a good sleep)
)
One says that the problem must be shadowing of the antenna by the fuselage in front which would cause a weaker signal and hence the directional nature of the problem.
The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.
Does anyone actually know which one of them is correct?
The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.
Does anyone actually know which one of them is correct?
The first statement might be a consideration if we were dealing in micro-watts over a considerable distance. Unless we have a seriously desensitised reciever (two on them in fact) I suggest that the shielding effect of the fuselage would be minimal given the transmit power of the radio equipment used in the tower ground installations which I would expect to be 'significant'. This theory is further eroded by the fact (if I understand correctly) that even with the aircraft on the ground in close proximity to the tower such that line of sight conditions between the transmitting and receiving antennas, the problem is still evident.
The second statement has some credibility in that if the shield of coax cable to the antenna is bonded to the skin of the aircraft (metal is assumed here) and thus to the airframe (grounded to use the more common terminology) at the antenna feedpoint the entire aircraft becomes the ground (some would say counterpoise) against which the whip of the antenna either radiates or receives a signal. This is the sort of system I would expect in this aircraft (the turtle deck is metal (aluminium) isn't it ?????) .
There is usually some directivity in that the radiation patten of the antenna would normally be the strongest in the direction of the largest area of ground plane. In this instance I think the directivity effect would probably favour signals in front of the aircraft if at all. Others might argue for signals from the rear of the aircraft being stronger in both transmit and receive. A beer or several are needed here to arrive at the correct answer by concensus. But I would be astonished beyond measure if either possibility were serious enough to produce the effect we are seeing here.
The prop modulation possibility as presented and argued by others earlier today is very interesting and seemingly credible. However I am troubled by the intermittent nature of the problem, and that the onset of the problem has been lifted by quite a few hundred RPM as changes were made to engine equipment which improved the performance. Prop modulation is possible perhaps, but surely there would have to be other contributing factors.
Finally, one especially puzzling aspect is that, as I understand it, the noise is stronger on the radio which has the newer, higher gain antenna. That again really suggests some sort of RF interference which is being received by that antenna at a higher received sig strength than the other one. (No it doesn't help a lot - at least I don't think it does - but it's just another question without logical answer)
What is the answer then? I don't know. I'm baffled like almost everyone else but somehow, someway, the cause will be identified and the solution found.
FOR
(all written in hast - with apologies to grammatical and/or technical purists - it's been a long hard day, this might be better attempted after a good sleep)
Last edited by FullOppositeRudder; 23rd Feb 2011 at 11:22. Reason: clarification needed in one statement
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Does anyone here remember Fred the Wheel Tapper from an RAF poster of the ‘60s. Fred’s job was to find cracks in the wheels of railway carriages. He’d whack them with his hammer and if they didn’t sound right he’d have them changed. This cost the railway company a fortune until someone discovered that Fred’s hammer was cracked.
So, of all the radios you’ve tried, have they been through the same shop with the same test equipment and same tech? Just curious.
So, of all the radios you’ve tried, have they been through the same shop with the same test equipment and same tech? Just curious.
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full opp rudder
Prop mod or wavelength is interesting but does not explain, as you say, why it just appeared out of the blue or intermittant.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.
Lots of clues there.
Need to work with what we know, use good radio theory and experience.
When I have had weird radio problems more often than not it is something earthed that should not be, or something that is poorly grounded / bonded/ shielded that should be.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.
Lots of clues there.
Need to work with what we know, use good radio theory and experience.
When I have had weird radio problems more often than not it is something earthed that should not be, or something that is poorly grounded / bonded/ shielded that should be.
I'm still suspicious of the ignition lead shielding (which prevents wave energy emissions). It's a simple test procedure for your AMO and an inexpensive fix.
ampclamp
Also this:
More clues here, which have perhaps been glossed over...
The noise only occurs on an active frequency. Note the effect of squelch off on an unused frequency...
Brain working overtime but not coming up with much
- GY
Also this:
Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.
If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.
If you change to an unused frequency and turn off the squelch there is none of the noise.
The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.
If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.
If you change to an unused frequency and turn off the squelch there is none of the noise.
The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.
The noise only occurs on an active frequency. Note the effect of squelch off on an unused frequency...
Brain working overtime but not coming up with much
- GY
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Ampcamp said
One othe big clue is that it happens at -every- distance to base station so almost has to be some modulation effect as opposed to an independant (fixed level) rfi source.
Both the intemitant and RPM sensitivity can be explained by prop vibration, which if I understood a previouse post correctly, is basically a "in the direction of spin" vibration (torque load/unload) that occurs above a certain speed. This onset speed was raised slightly when the ignition system was improved.
My theory is that this vibration is intermitantly grounding/ungrounding one (or both) of the blades, or less likely some mechanically related structure.
Only when ungrounded does the prop blade create a reflection/multipath/interfernce situation.
The signal does not have to go directly "through" the prop for this to happen, think tall buildings and ghosty (analog) TV reception. It does have to be mostly in line, hence the 20 degree range of the effect.
The handheld not seeing the noise can be explained by the effect being larger at "magic" ( ~ 1/4 wavelength multiples) distances from the prop.
Worse with new antenna could be that a slight difference in (electrical/effective) distance to prop.
One interesting test would be to take the handeld outside and slowly move toward the plane when noise is happening.
Not sure one could (safely!!!) get close enough to get the effect or not though. Results of handheld connected to antenna would also be usefull.
Prop mod or wavelength is interesting but does not explain, as you say, why it just appeared out of the blue or intermittant.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.
Lots of clues there.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.
Lots of clues there.
Both the intemitant and RPM sensitivity can be explained by prop vibration, which if I understood a previouse post correctly, is basically a "in the direction of spin" vibration (torque load/unload) that occurs above a certain speed. This onset speed was raised slightly when the ignition system was improved.
My theory is that this vibration is intermitantly grounding/ungrounding one (or both) of the blades, or less likely some mechanically related structure.
Only when ungrounded does the prop blade create a reflection/multipath/interfernce situation.
The signal does not have to go directly "through" the prop for this to happen, think tall buildings and ghosty (analog) TV reception. It does have to be mostly in line, hence the 20 degree range of the effect.
The handheld not seeing the noise can be explained by the effect being larger at "magic" ( ~ 1/4 wavelength multiples) distances from the prop.
Worse with new antenna could be that a slight difference in (electrical/effective) distance to prop.
One interesting test would be to take the handeld outside and slowly move toward the plane when noise is happening.
Not sure one could (safely!!!) get close enough to get the effect or not though. Results of handheld connected to antenna would also be usefull.
multi-path
Not an a/c man (did you guess ?) but is the prop metal ? Reason I ask is that if the spacing between the blades is right then it might be giving you multipath which would modulate the signal. Far enough from the tower and the fraction of the received signal intercepted by the blades would decrease so the effect would vanish. The rpm dependency would have to be explained by agc timeconstant or something. The directionality of a hand-held (unless you want to risk life and limb would prevent that happening to it.
Key clues to my mind:
- Non-airframe grounded receiver does NOT 'hear' noise => Noise is not an RF transmission
- Opening squelch on the a/c radios on an open frequency does not 'hear' noise => Infers something is happening to the transmitted signal BEFORE reception by the a/c radios (and reinforces this is not an additional RF signal)
- Only occurs to stations when within +/-20 degrees of being on-the-nose => Something in the front of the a/c is responsible! (Thinks big whirling thing out front...) What angle does the prop diameter subtend to the tower station at 200m. Does the angle of interference appear to change?
- Noise is not always present => See post above by MurphyWasRight! I'm with you on that idea.
Back to thinking some more
- GY
- Non-airframe grounded receiver does NOT 'hear' noise => Noise is not an RF transmission
- Opening squelch on the a/c radios on an open frequency does not 'hear' noise => Infers something is happening to the transmitted signal BEFORE reception by the a/c radios (and reinforces this is not an additional RF signal)
- Only occurs to stations when within +/-20 degrees of being on-the-nose => Something in the front of the a/c is responsible! (Thinks big whirling thing out front...) What angle does the prop diameter subtend to the tower station at 200m. Does the angle of interference appear to change?
- Noise is not always present => See post above by MurphyWasRight! I'm with you on that idea.
Back to thinking some more
- GY
Last edited by GarageYears; 23rd Feb 2011 at 14:50. Reason: Read MRW post!