Rainier - Sorry if I am hijacking this thread, hope I can ask a follow up question on your first point.
I have been having transmit problems with a V6 radio and discovered the phone cables are shielded 2 wire cable using the shield to conduct the ground in violation of your direction. The mic cables are 3 wire shielded cable, no problem there.
The symptom is high levels of static (to the point the transmission is unintelligible) when transmitting on ~125MHz to 127MHz. The transmission is clean on other frequencies. Reception is clean on all frequencies.
Could the phone cables be causing this transmit problem?
thanks - lyle
The headphone side is not sensitive to interference and very tolerant so likely not the issue (bit not completely impossible).
If things vary with frequency that usually is a solid hint that something is resonating as antenna which is connected to the radio somewhere in a way it should not be.
In most cases in my experience such issues are caused by one or more of the following:
a) Bad antenna tune (bad SWR). This tends to be very frequency dependent. Reflected energy from the antenna tends to contaminate the radio grounds with RF and this can leak into the audio circuits - in bad cases causing havoc.
This can be caused by the antenna itself, the cable (damage, sharp tuns (kinks) in the cable, incorrect cable) or bad terminations in connectors.
b) Direct radiation from antenna into headsets, active parts of ANR headsets or the microphone amplifiers inside headsets or into the headset cables (some types tend to have poor or even no screening at all). This is affected by distance to the antenna and the antenna radiation pattern.
c) RF coupling into the aircraft ground. In this case we have an innocent ground cable or structure connection somewhere that happens to form a loop (both ends connected to ground and there is considerable physical area inside the loop). If the loop happens to have the right dimensions (related to the wavelength of the transmitted signal) it acts pretty much as a secondary winding of a transformer and induces a low impedance RF power source into the ground. Now, depending on how that power travels and where it goes - it can again affect your radio. This one also often causes problems with other electronics (a typical one would be for example changing the altitude readout on an EFIS). Bad SWR can do similar though.
Radios are becoming more powerful and manufacturers are getting good at controlling signal modulation so the carrier wave tends to be well modulated. While this is good it unfortunately reveals any radio installation issues much easier.
Finding the causes usually is done by trail and error (using educated guesses and a SWR meter). Ferrites made to be effective at VHF frequencies can be used to prevent or at least suppress RF signals traveling on wires. While I don't like to use them (it feels like one is not fixing the root cause) - they can be helpful.
Rainier
CEO MGL Avionics