Garmin Radio Noise Issue

[UnPossible]

Hey - I have been trying to trouble shoot an issue I am having with my Garmin COM radios. FYI - I have a 650 as COM1, a GTR-20 as COM2 and a GMA-245 Audio panel controlling everything.

I have been having an intermittent issue with noise when I hold the push to talk. Digging a little deeper, I noticed that the RX indicator for COM 2 was coming on with the noise. What appears to be happening is that the COM2 is picking up when COM1 is transmitting and creating some sort of feedback noise. This is happening when COM2 is tuned to within 2MHz of COM 1 (i.e. if COM1 is on 122.0 and COM2 is on 123.0 I hear the noise. if COM1 is on 122.0 and COM2 is on 127.0, no noise.) If I shut down COM2 on the audio panel, no noise either.

Logically it seems like I am getting some sort of bleed over into COM2 when COM1 transmits. Both my COM antennas are on the belly of my RV-10, under the rear seats (which I believe is a common location for RV-10). The RG-400 antenna cabling runs up different sides of the fuselage (i.e. COM1 runs along the left side and COM2 runs along the right).

I'll call Stein (they built harness) and Garmin on Monday morning, but thought I'd check to see if anyone else has run across a similar issue, or have any suggestions on how to fix this issue.

Thanks,
Jason

[Pat Hatch]

Typically there will be some sort of transmit interlock connections between the two COMs, which will prevent one receiver from receiving the opposing transmitter's transmissions. In the case of two GTR 20's for example, look at pins 4 and 5 on the schematic. Not sure with a 650 and GTR 20 what the pinouts for this would be but worth checking.

[mburch]

Do you have the transmit interlock connections wired between your two radios?

[UnPossible]

Originally Posted by mburch

Do you have the transmit interlock connections wired between your two radios?

Looking at my diagram from Stein, the Com Mic Key on the 650 is wired into the Transmit Interlock In on the GTR-20.

[Richard Connell]

two antennae on the belly next to each other is not compliant with any Garmin Radio install requirements. Thats 6' from any other COM.

I know "everyone does it" but I reckon its a poor setup.
I had two on the belly in my 7 and it was a continual PITA. The 10 has an antenna on the roof and one on the belly. works perfectly.

Garmin:
"The GTN COM antenna should be well removed from all projections, engines and propellers. The ground plane surface directlybelowtheantennashouldbeaflatplaneoveraslar geanareaaspossible(18" square,minimum).Theantenna should be mounted a minimum of six feet from any DME or other COM antennas, four feet from any ADF sense antennas, and two feet from the GTN and its GPS antenna. The COM antenna should also be mounted as far apart as practical from the ELT antenna. Some ELTs have exhibited re-radiation problems generating harmonics that may interfere with GPS signals. This can happen when the COM (GTN or any other COM) is transmitting on certain frequencies such as 121.15 or 121.175 MHz, which may cause the ELT output circuit to oscillate from the signal coming in on the ELT antenna coax.
If simultaneous use of two COM transceivers is desired (split-COM or simul-COM), the COM antennas should be spaced for maximum isolation. A configuration of one topside antenna and one bottom side antenna is recommended. The GTN does not require a transmit interlock, but other COM radios such as the GNS 430W may require it for split-COM operations."

[BobTurner]

As others have noted, there is a transmit interlock pin to reduce/prevent this. Wire the PTT from #1 to the interlock for #2, and vice versa (ground the interlock pin to mute the receiver not being used). Of course, that means you can’t hear anything from #2 while transmitting on #1. I don’t know that mounting two com antennas so close together is ‘standard’, for the reasons that you’ve discovered. Also bottom mounted antennas may have some problems while on the ground at certain airports.

[BobTurner]

Double check the interlock wiring. This doesn’t seem right, although some radios just reduce the gain, not actually a complete mute. Why do you want #2 selected to receive at all, when transmitting on #1?

[Pman]

Originally Posted by UnPossible

Hey - I have been trying to trouble shoot an issue I am having with my Garmin COM radios. FYI - I have a 650 as COM1, a GTR-20 as COM2 and a GMA-245 Audio panel controlling everything.

I have been having an intermittent issue with noise when I hold the push to talk. Digging a little deeper, I noticed that the RX indicator for COM 2 was coming on with the noise. What appears to be happening is that the COM2 is picking up when COM1 is transmitting and creating some sort of feedback noise. This is happening when COM2 is tuned to within 2MHz of COM 1 (i.e. if COM1 is on 122.0 and COM2 is on 123.0 I hear the noise. if COM1 is on 122.0 and COM2 is on 127.0, no noise.) If I shut down COM2 on the audio panel, no noise either.

Logically it seems like I am getting some sort of bleed over into COM2 when COM1 transmits. Both my COM antennas are on the belly of my RV-10, under the rear seats (which I believe is a common location for RV-10). The RG-400 antenna cabling runs up different sides of the fuselage (i.e. COM1 runs along the left side and COM2 runs along the right).

I'll call Stein (they built harness) and Garmin on Monday morning, but thought I'd check to see if anyone else has run across a similar issue, or have any suggestions on how to fix this issue.

Thanks,
Jason

Jason,

Interesting post, I have the exact same issues with my Garmin radios, also wired from the same vendor.
Please keep us informed on what you may learn to remedy the problem.

Thanks for your post!

Mike

[JTJones]

I had a similar problem and changed the position of the #3 DIP switch on the GMA245 to ON and it stopped. I think it was explained to me as a “feedback loop.” The one variable is I also found an ignition wire on one of my PMags that wasn’t completely seated so was arcing a bit possibly causing interference? I corrected that at the same time as I set the DIP switch to ON. So I’m not completely sure if the DIP switch fixed it or seating the mag wire did.

[Longez]

As Jeffrey stated, you will want to make sure that DIP switch #3 is in the Down (ON) position on the GMA 245 audio panel to prevent the crew from hearing feedback from any monitored radio when transmitting on another radio.



Steve