Crazy Radio Issue, Help Needed.
RF co-ax should be grounded at both ends.
Screened audio wiring should be grounded at one end only.
I will go for a bonding issue with the radio boxes.
It is probably some sort of prop modulation or screening that is always there but does not show up with a good installation.
What is the range of the radios when airborne? Is it a lot less than it used to be?
Do a bonding check with a bonding tester, (they used to have a double probe and a single probe). I think the readings should be 0.02 ohms or less.
Do a check of all bonding including engine to airframe and all components.
Remove all earth connections, clean the metal and re-make the connections if there's any doubt about the bonding.
I'm a bit rusty on this stuff. It's over 30 years since I did installation and maintenance on these things.
Good luck.
Screened audio wiring should be grounded at one end only.
I will go for a bonding issue with the radio boxes.
It is probably some sort of prop modulation or screening that is always there but does not show up with a good installation.
What is the range of the radios when airborne? Is it a lot less than it used to be?
Do a bonding check with a bonding tester, (they used to have a double probe and a single probe). I think the readings should be 0.02 ohms or less.
Do a check of all bonding including engine to airframe and all components.
Remove all earth connections, clean the metal and re-make the connections if there's any doubt about the bonding.
I'm a bit rusty on this stuff. It's over 30 years since I did installation and maintenance on these things.
Good luck.
Thread Starter
Join Date: Mar 2009
Location: Perth.AU
Age: 52
Posts: 15
Likes: 0
Received 0 Likes
on
0 Posts
I can add some more information and deal with some suggestions.
The interference is highly directional and also linked to the RPM somehow. When it initially started it cut in at 1900RPM , after we changed the plugs for new plugs it started at 2100RPM, since the Mags were rebuilt it moved to 2200RPM. As we have totally eliminated anything in the ignition system as the cause of the noise I think it is reasonable to assume that it is linked to the vibration from the engine, the engine has run smoother as the ignition system was modified and improved.
The prop is well balanced and the idea that the prop is somehow out of balance is unlikely.
Yesterday the aircraft had the problem for most of the day whilst it was working, and then I dropped of the last passengers and headed out to the run-up bay to do some testing and the problem was gone. I made 4 attempts over a couple of hours to get the problem to re-appear so that I could do some tests and it never did.
This morning for the first flight of the day the problem came back immediately and was present for every flight during the day. I finished late and did not have a chance to do any testing, tomorrow morning I am planning to do more tests.
The radio guys tested the VSWR of both radios and they were all fine. The other end always gets the transmissions as 5's which would seem to rule out shadowing as the cause. Also there are 2 antennas in different locations on different radio sets that both behave the same way. The only difference being that since we replaced the antenna on com1 with a better larger and higher gain antenna we pick up the interference more strongly with the result that the transmitting station is much harder to understand.
The noise can only be described as a white noise, no distinct pitch or tone, no relation to engine RPM except that it appears and disappears within 50rpm of 2200rpm every time. There is a chop to it, but it does not change with RPMs.
I have run the aircraft with everything except com1 de-energised, including the alternator, with no change.
I think we have ruled out the ignition system as the cause.
We have ruled out the alternator.
We have ruled out every other electrical device.
I have more time tomorrow between flights and I am hoping the problem is still there and I can try some troubleshooting.
Specifically I want to try the radio with the antenna disconnected, with an ungrounded antenna connected, and the hand-held connected to the fixed antenna to try and determine where it might be coming from.
Thanks for all the suggestions so far, I'll keep you updated on my progress.
The interference is highly directional and also linked to the RPM somehow. When it initially started it cut in at 1900RPM , after we changed the plugs for new plugs it started at 2100RPM, since the Mags were rebuilt it moved to 2200RPM. As we have totally eliminated anything in the ignition system as the cause of the noise I think it is reasonable to assume that it is linked to the vibration from the engine, the engine has run smoother as the ignition system was modified and improved.
The prop is well balanced and the idea that the prop is somehow out of balance is unlikely.
Yesterday the aircraft had the problem for most of the day whilst it was working, and then I dropped of the last passengers and headed out to the run-up bay to do some testing and the problem was gone. I made 4 attempts over a couple of hours to get the problem to re-appear so that I could do some tests and it never did.
This morning for the first flight of the day the problem came back immediately and was present for every flight during the day. I finished late and did not have a chance to do any testing, tomorrow morning I am planning to do more tests.
The radio guys tested the VSWR of both radios and they were all fine. The other end always gets the transmissions as 5's which would seem to rule out shadowing as the cause. Also there are 2 antennas in different locations on different radio sets that both behave the same way. The only difference being that since we replaced the antenna on com1 with a better larger and higher gain antenna we pick up the interference more strongly with the result that the transmitting station is much harder to understand.
The noise can only be described as a white noise, no distinct pitch or tone, no relation to engine RPM except that it appears and disappears within 50rpm of 2200rpm every time. There is a chop to it, but it does not change with RPMs.
I have run the aircraft with everything except com1 de-energised, including the alternator, with no change.
I think we have ruled out the ignition system as the cause.
We have ruled out the alternator.
We have ruled out every other electrical device.
I have more time tomorrow between flights and I am hoping the problem is still there and I can try some troubleshooting.
Specifically I want to try the radio with the antenna disconnected, with an ungrounded antenna connected, and the hand-held connected to the fixed antenna to try and determine where it might be coming from.
Thanks for all the suggestions so far, I'll keep you updated on my progress.
That's strange, forget - I was going to ask the very same question re humidity! Reason being the HT leads. Have they been checked with a lead tester? Humidity has an effect when they're on the way out.
Join Date: Jul 2008
Location: Skating away on the thin ice of a new day.
Posts: 1,116
Received 13 Likes
on
8 Posts
forget . stevef re humidity , sorry but he answered that in post one.
Join Date: Feb 2001
Location: UK
Posts: 243
Likes: 0
Received 0 Likes
on
0 Posts
Sorry, off - thread, but why has the word "issue" replaced the perfectly suitable word "problem" ? We don't have "problems" now, we have "issues".... I can't STAND IT !!!!!! The two words mean two completely different things... but the word "issue" is now used to mean "problem" ! WHY ???? Apologies for going off - thread ...... but no apologies for making my point!
Guest
Posts: n/a
Originally Posted by hangflyer
The noise can only be described as a white noise, no distinct pitch or tone, no relation to engine RPM except that it appears and disappears within 50rpm of 2200rpm every time. There is a chop to it, but it does not change with RPMs.
Join Date: Aug 2005
Location: Birchington, Kent, England
Age: 82
Posts: 176
Likes: 0
Received 0 Likes
on
0 Posts
Static?
Hangflyer
If all else fails, there is a possibility that you may have an odd static problem. Static can do odd things so I would check that your aileron, flap, elevator and rudder bonding is good. Also check that any static wicks that you have are present and in good condition.
Just an outside chance but who knows?
If all else fails, there is a possibility that you may have an odd static problem. Static can do odd things so I would check that your aileron, flap, elevator and rudder bonding is good. Also check that any static wicks that you have are present and in good condition.
Just an outside chance but who knows?
Join Date: Jun 2009
Location: NNW of Antipodes
Age: 81
Posts: 1,330
Received 0 Likes
on
0 Posts
Having had a close look at the suggestions previously offered, I suspect that the "white noise" is just that. The problem probably relates to a diode or zener diode being triggered into white noise mode by a particular range of circumstances. Humidity playing a part, could also be indicative of some capacitive coupling that changes marginally and aids to trigger the problem.
Now you'll need a "portable" white noise detector to home in on the probable source.
The fact that reception is poor from on the nose, doesn't mean that the source of the interference is forward, in fact the maximum pickup of the noise will be on the side of the antenna, which gives you a huge scope. So, look closely at the airframe wiring schematics and identify the location of any possible culprits. They don't necessarily need to be diodes, a corroded connection could also exhibit similar symptoms.
Your problem, to reproduce the effect on the ground!
Now you'll need a "portable" white noise detector to home in on the probable source.
The fact that reception is poor from on the nose, doesn't mean that the source of the interference is forward, in fact the maximum pickup of the noise will be on the side of the antenna, which gives you a huge scope. So, look closely at the airframe wiring schematics and identify the location of any possible culprits. They don't necessarily need to be diodes, a corroded connection could also exhibit similar symptoms.
Your problem, to reproduce the effect on the ground!
Join Date: May 2009
Location: UK
Age: 41
Posts: 39
Likes: 0
Received 0 Likes
on
0 Posts
Spectrum analyser on audio?!?
@mike-wsm
How can he do spectrum analysis on recorded audio of the noise? VHF is in the MHz, audio capture in the kHz. If its RF noise he should have to connect spectrum analyser on antenna coax. After the radio 'box' everything is 'translated' (modulated?) in audio spectrum. Frequency of noise would be lost, or would be missleading after modulation in audio freq.
Maybe I'm wrong or misunderstood (in that case delete the post)...
If I'm not, alternative solution:
Get an osciloscope from someone having acess to a univ./school lab, they have 50ohm coax input, have it do an FFT on the antenna input of the radio. No idea how you power the osciloscope in the plane though.
Spectrum analysers are usually bulkier than osciloscopes. at least the ones I have access to. Some osciloscopes save the data also in USB sticks for later process.
How can he do spectrum analysis on recorded audio of the noise? VHF is in the MHz, audio capture in the kHz. If its RF noise he should have to connect spectrum analyser on antenna coax. After the radio 'box' everything is 'translated' (modulated?) in audio spectrum. Frequency of noise would be lost, or would be missleading after modulation in audio freq.
Maybe I'm wrong or misunderstood (in that case delete the post)...
If I'm not, alternative solution:
Get an osciloscope from someone having acess to a univ./school lab, they have 50ohm coax input, have it do an FFT on the antenna input of the radio. No idea how you power the osciloscope in the plane though.
Spectrum analysers are usually bulkier than osciloscopes. at least the ones I have access to. Some osciloscopes save the data also in USB sticks for later process.
Join Date: Jun 2009
Location: NNW of Antipodes
Age: 81
Posts: 1,330
Received 0 Likes
on
0 Posts
DERG
The basic detector is nothing more than a full wave or half wave vhf/uhf pin diode rectifier with an antenna on one side and high impedance meter to measure the output voltage on the otherside. No rocket science, the detection method is looking for a noise source within a meter or two and not miles away. Commonly known as Field Strength Meter.
Simplicity, is an existing VHF airband portable with some means of measuring the signal strength of the input noise on any vacant frequency. Access to the AGC voltage is a means of doing so.
There will be heaps of details on RF rectifier devices on the www.
The basic detector is nothing more than a full wave or half wave vhf/uhf pin diode rectifier with an antenna on one side and high impedance meter to measure the output voltage on the otherside. No rocket science, the detection method is looking for a noise source within a meter or two and not miles away. Commonly known as Field Strength Meter.
Simplicity, is an existing VHF airband portable with some means of measuring the signal strength of the input noise on any vacant frequency. Access to the AGC voltage is a means of doing so.
There will be heaps of details on RF rectifier devices on the www.
Join Date: Feb 2008
Location: USA
Posts: 93
Likes: 0
Received 0 Likes
on
0 Posts
@Dimitris, I think the point of @mike-wsm was that spectrum analysis/scope of the audio might give an important clue to the noise source....you got a piece of rotating machinery with dodgy brushes/a spark etc, you're going to see some big spikes proportional to its RPM. Also remember that RF doesn't start in the MHz...radio 4 is still on LW @ 192kHz! It can go MUCH lower than that...
Sure it would be really nice to sniff the actual RF spectrum near the antenna, or the power leads etc too. I have just the portable 5GHz analyzer for the job..but it's not the kind of gear most people have to hand.
It's also important to realize that a great many noise problems are not "in band"..that's is in the frequency range the radio is tuned to receive signals at. The RFI finds it's way into internal circuits that are tuned to work at other typically much lower frequencies and get's mixed in with the real signal there. This noise MIGHT be VHF wideband (i.e throughout the 118-136MHz bands) but very likely it's not (handheld not hearing it was a big clue), it's probably something much narrower band and of much lower frequency.
Hopefully the next batch of experiments disconnecting the coax etc will tell us what path the noise takes into the radios and well be able to eliminate some of the many good suggestions we have.
Sure it would be really nice to sniff the actual RF spectrum near the antenna, or the power leads etc too. I have just the portable 5GHz analyzer for the job..but it's not the kind of gear most people have to hand.
It's also important to realize that a great many noise problems are not "in band"..that's is in the frequency range the radio is tuned to receive signals at. The RFI finds it's way into internal circuits that are tuned to work at other typically much lower frequencies and get's mixed in with the real signal there. This noise MIGHT be VHF wideband (i.e throughout the 118-136MHz bands) but very likely it's not (handheld not hearing it was a big clue), it's probably something much narrower band and of much lower frequency.
Hopefully the next batch of experiments disconnecting the coax etc will tell us what path the noise takes into the radios and well be able to eliminate some of the many good suggestions we have.
Join Date: Jul 2008
Location: Skating away on the thin ice of a new day.
Posts: 1,116
Received 13 Likes
on
8 Posts
stevef
"Sometimes us over-fifties have the attention span of a geriatric goldfish"
Don't worry mate, wrong side of 50 here too. I had to double check the first post myself.The trouble is as threads get longer less newcomers read all that has gone before.End up going in circles.Very easy to forget with so may inputs.To prove this we have the following...
The fact hangflyer has actually moved the noise from 1900 by changing the plugs then from 2100 to 2200 with a mag rebuild must be telling us something.
In the same post hangflyer says we have ruled out the ignition system but so far that is the only thing that has positively done something tangible to the noise apart from a higher gain antenna which made it worse.So whatever it is, it appears to be coming down the antenna and has some relation to the engine ignition system.Maybe not THE cause but it sure looks to be affecting it.
Occham's razor?
Don't worry mate, wrong side of 50 here too. I had to double check the first post myself.The trouble is as threads get longer less newcomers read all that has gone before.End up going in circles.Very easy to forget with so may inputs.To prove this we have the following...
The fact hangflyer has actually moved the noise from 1900 by changing the plugs then from 2100 to 2200 with a mag rebuild must be telling us something.
In the same post hangflyer says we have ruled out the ignition system but so far that is the only thing that has positively done something tangible to the noise apart from a higher gain antenna which made it worse.So whatever it is, it appears to be coming down the antenna and has some relation to the engine ignition system.Maybe not THE cause but it sure looks to be affecting it.
Occham's razor?
How about changing out the plug harness? I did not see anything in the posts about that one, and a shifting ground may explain the 1900 to 2200 difference. Or with the repaired/rebuilt mags the energy has changed some how.
Changing the Coax to Triax might help too. We used them on the medical helicopters. Ground the outside cover at the transmitter, and the inside as normal.
Changing the Coax to Triax might help too. We used them on the medical helicopters. Ground the outside cover at the transmitter, and the inside as normal.
Join Date: Nov 2006
Location: Somewhere in England
Posts: 105
Likes: 0
Received 0 Likes
on
0 Posts
Hi chaps, another radio amateur here to just chip in a bit, for what it's worth. The directionality seems to me to be related to the signal strength of the ground station at that particular heading, compared with the on-board noise generator. Weaker in my opinion. OP, have you listened with the squelch wide open on the frequency(ies?) of interest when the ground station ceases transmitting? Do you hear the white noise now? Do you hear it on channels one up and one down with the squelch open? Trying to ascertain this would give us all a clue and prove my signal-to-noise ratio. If the noise is still present it certainly would appear to be on-board as there is a slight doubt in my mind that it is for sure. If it is, I strongly suspect a voltage regulator somewhere and after doing the usual load shedding try switching off each VHF one at a time in case one is affecting the other. I do hope the above can be of some use and hope you find and publish the cause soon! Good luck!
Hangflyer: PM me if you can get an audio recording of the noise
Analyzing the audio could certainly be useful - that is exactly what the problem is! Something nasty ending up in the audio - so looking at it in the frequency domain may provide some very helpful clues. If there is any periodicity to the noise then that information may just lead us to the source.
I have a bunch of very nice analysis tools available (my main work is flight simulator audio) and would be happy to see what I can find.
- GY
I have a bunch of very nice analysis tools available (my main work is flight simulator audio) and would be happy to see what I can find.
- GY
Does the noise disappear if the cabin speaker is selected, rather than listening in the headset ?