KX155 Squeal & loud Side tone on Transmit

[MyassisDragon]

When I monitor a Freq on COM1 and transmit on COM2 (KX155) I get loud feed back transmitting and it sounds like I am screaming in my own ear.

This does not always happen (I regularly transmit on ground on COM2), and I only noticed it on the last flight. This was a night flight below freezing if it matters and my wife was constantly twiddling with the squelch (but that didnt seem to fix anything).


Setup is a Garmin 340 audio panel with GNS530 COM1 and KX155 COM2 and Bose A20 headsets (battery power).

Any ideas what to check next?

I have not tried to cycle panel lights off yet but I am pretty sure I have used COM2 on ground at night with out issue since this is my normal procedure (Awos/Ground on COM2 - Approach/Tower on COM1) and would have noticed.

 

[hiflier]

I have a 430 com 1 and an old collins com 2 with the same problem. The avionics guy told me that the old one isn't shielded to protect it from output of the Garmin. Doesn't matter to me; I just use com 2 for atis unless there should be a problem with com 1.

 

[bnt83]

When I monitor a Freq on COM1 and transmit on COM2 (KX155) I get loud feed back transmitting and it sounds like I am screaming in my own ear.

This does not always happen (I regularly transmit on ground on COM2), and I only noticed it on the last flight. This was a night flight below freezing if it matters and my wife was constantly twiddling with the squelch (but that didnt seem to fix anything).


Setup is a Garmin 340 audio panel with GNS530 COM1 and KX155 COM2 and Bose A20 headsets (battery power).

Any ideas what to check next?

I have not tried to cycle panel lights off yet but I am pretty sure I have used COM2 on ground at night with out issue since this is my normal procedure (Awos/Ground on COM2 - Approach/Tower on COM1) and would have noticed.

Unless you have

Com 1 - Modern well shielded radio with antenna at least ____ feet (6 per garmin STC ) from com 2 antenna

Com 2 - Modern well shielded radio with antenna at least ____ feet (6 per garmin STC ) from com 1 antenna

You'll have problems.

The best spacing for this type of operation is one com antenna on top of the airplane and one on the bottom. Even then it may not work if one of the coms is old and spews RF out of its own chassis or the coaxes/RF connectors are bad.

There are suppression circuits on many com radios for this very reason. When one com is transmitting it tells the other to be quite, which would prevent operation in "split mode" where one crew member uses com 1 and another crew member uses com 2, simultaneously.

 

[bnt83]

I've been reading up on antenna spacing a bit and each manufacturer and the FAA has their own opinion. I'm not an RF engineer and have forgotten most of what I learned getting a ham radio license 15 years ago.

Looking at the tops of even the newest high wing Cessna, I doubt there is 4 feet between the whip com antennas. The 1968 C177 has a little under 3.5 between the factory installed com antennas.

AC-43.13 says “at least 1/2 wavelength (of the antenna operating frequency) distance available between antennas.” “ wavelength in feet = 984 / frequency range mhz” 4.15 to feet 3.9 feet

MicroAir FAQ says “The shortest aerial should be spaced at least 1.5 times its length away from a longer aerial.”

Garmin GTN650 Install Manual “At least 6 feet from other Com Antennas and two feet from GPS antenna

 

[bluerooster]

I don't know about the GNS series, but the GNC also requires a notch filter.