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Garmin Radio Noise Issue

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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
 
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.
 
Do you have the transmit interlock connections wired between your two radios?
 
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 directlybelowtheantennashouldbeaflatplaneoveraslargeanareaaspossible(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."
 
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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.
 
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?
 
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
 
Check your DIP switches

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.
 
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.

DIP Switch Settings.png

Steve
 
I flipped DIP switch #3 to on last night and in a quick test, it seemed to help, but not totally eliminate the issue. I am planning on a flight this afternoon and will report back if this eliminated the issue. If not, I will likely move one of the COM antennas from the belly to the top of the airplane during my annual in the next month or two.
 
Jason,

I’m guessing the problem is noise on the GTR-20 when transmitting on the GTN-650, and no noise on the GTN-650 when transmitting on the GTR-20.

Is this correct?
Carl
 
Jason, I'm chasing a very similar issue. Did you figure out the cause?

I haven't gotten as far as checking the dip switch position mentioned above (I have a GMA 240, not a GMA 245) but regardless, that sounds like it would simply mask the problem. The problem is that #2 is showing RX on certain frequencies when Com 1 is transmitting on certain frequencies.

I always have Com 2 set to Guard with the volume turned up. So when my Com 1 transmission is picked up by Com 2 (RX indicated on Com 2 with Com 2 set to a different frequency than Com 1), I hear my own very scratchy transmission in my headset on Com 2. This is verified by turning the Com 2 volume all the way down. I then hear my Com 1 transmission clearly.

Com #1 = GTN650Xi.
Com #2 = GTR200
Audio Panel = GMA240

Com antennas are Comant bent whip, belly mounted under each seat.
 
What EFIS display do you have?
The reason I ask is because I was reading the Dynon Skyview installation manual and came across the following note about audio humming and a solution for it on page 4-34. Maybe it'll help you.
 

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  • Skyview Audio Humming.jpg
    Skyview Audio Humming.jpg
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I am experiencing the same issue. I have GMA240 with IFD440 on com 1 and dynon com on 2.
Does the gma240 hame the dip switch to mute ?
 
What EFIS display do you have?
The reason I ask is because I was reading the Dynon Skyview installation manual and came across the following note about audio humming and a solution for it on page 4-34. Maybe it'll help you.

Thanks, but it's Garmin. Right now it's two legacy Garmin GDU-370's (non-Touch) but will soon be swapped to GDU-470's (Touch).

I am experiencing the same issue. I have GMA240 with IFD440 on com 1 and Dynon com on 2.
Does the gma240 have the dip switch to mute ?

I don't know if the GMA-240 has the dip switch. I'll have to do some reading. But even if it does, that's not a proper solution to the problem. It will simply mask the problem. The problem is that Com 2 is receiving bleed through from Com 1 and it shouldn't be. That's what needs to be corrected.

The harness was originally built by Stein and I see on the schematic that one side of the GTR-200's P2001 Pin 5 Xmit Interlock (input?) is tied into what was the GNS430W's P4002 Pin 4 Com Mic Key (input?) and also the GMA-240's J1 Pin 12 Com 1 Mic Key (output?).

The GNS430W has been swapped by a proper radio shop for a GTN650Xi. I don't know what changes, if any, they made to that wiring.

Carl Froelich sent me some very helpful information regarding the Interlock requirements, so that's what I'm going to start looking into.

Interlock.jpg
 
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Thanks, but it's Garmin. Right now it's two legacy Garmin GDU-370's (non-Touch) but will soon be swapped to GDU-470's (Touch).



I don't know if the GMA-240 has the dip switch. I'll have to do some reading. But even if it does, that's not a proper solution to the problem. It will simply mask the problem. The problem is that Com 2 is receiving bleed through from Com 1 and it shouldn't be. That's what needs to be corrected.

The harness was originally built by Stein and I see on the schematic that one side of the GTR-200's P2001 Pin 5 Xmit Interlock is tied into what was the GNS430W's P4002 Pin 4 Com Mic Key and also the GMA-240's J1 Pin 12 Com 1 Mic Key.

The GNS430W has been swapped by a proper radio shop for a GTN650Xi. I don't know what changes, if any, they made to that wiring.

Carl Froelich sent me some very helpful information regarding the Interlock requirements, so that's what I'm going to start looking into.

Looks like the GMA240 doesn’t have the dip switches
 
If I understand correctly what I'm reading here, I'll have check to see if the radio shop that swapped the GNS to the GTN connected the GTR-200's P2001 Pin 4 output (which isn't even depicted on my schematic) to the GTN-650Xi's P1003 Pin 9 Input.
 
I don't know if the GMA-240 has the dip switch. I'll have to do some reading. But even if it does, that's not a proper solution to the problem. It will simply mask the problem. The problem is that Com 2 is receiving bleed through from Com 1 and it shouldn't be. That's what needs to be corrected.

To enable the MUTE On COM TX feature on the GMA 240, you just need to permanently ground pin 30 on connector J2402.

GMA 240 Mute On COM TX.png

Personally, I feel this needs to be enabled on all systems so you never hear anything but your voice when transmitting on the selected COM radio.

It sounds like your 2 COM antennas are side-by-side on the bottom of the plane, which isn't ideal spacing (too close). Much better to have one antenna on top and one on bottom.

Of course the first thing to check is that the transmit interlocks are properly wired for your updated installation as Justin explained in his posting.

Steve
 
To enable the MUTE On COM TX feature on the GMA 240, you just need to permanently ground pin 30 on connector J2402.

View attachment 50292

Personally, I feel this needs to be enabled on all systems so you never hear anything but your voice when transmitting on the selected COM radio.

It sounds like your 2 COM antennas are side-by-side on the bottom of the plane, which isn't ideal spacing (too close). Much better to have one antenna on top and one on bottom.

Of course the first thing to check is that the transmit interlocks are properly wired for your updated installation as Justin explained in his posting.

Steve

Thanks, Steve. The Stein schematic doesn't depict Pin 30 on J2 (J2402), so it seems it wasn't wired that way originally. Regardless of potential workarounds, I want to correct the problem.

I'm aware the side-by-side antenna placement isn't ideal per Garmin's specs. Unfortunately, I never flew the plane before the GNS-430W was removed, so I don't know if this problem predates the GNS-650Xi installation.

The plane has been flying for 10 years but it's clear to me that the previous owner/builder didn't delve too deeply into the EFIS and avionics.
 
If I understand correctly what I'm reading here, I'll have check to see if the radio shop that swapped the GNS to the GTN connected the GTR-200's P2001 Pin 4 output (which isn't even depicted on my schematic) to the GTN-650Xi's P1003 Pin 9 Input.

To enable the MUTE On COM TX feature on the GMA 240, you just need to permanently ground pin 30 on connector J2402.

View attachment 50292

Personally, I feel this needs to be enabled on all systems so you never hear anything but your voice when transmitting on the selected COM radio.

It sounds like your 2 COM antennas are side-by-side on the bottom of the plane, which isn't ideal spacing (too close). Much better to have one antenna on top and one on bottom.

Of course the first thing to check is that the transmit interlocks are properly wired for your updated installation as Justin explained in his posting.

I brought the plane back to the shop that installed the GTN-650Xi. They wired the Transmit Interlock as called-out in this post (GTR-200 P2001-4 Transmit Interlock Output to GTN-650Xi P1003-9 Transmit Interlock Input).

Unfortunately, the bleed through still persisted. We tried to disconnect the ELT antenna as a trouble shooting measure, to no avail.

Ultimately, we did as Steve suggested and grounded J2402-30 on the GMA-240. This works as advertised. Com #2 is muted when transmitting on Com #1. It's not a solution but a work around. I can still see "RX" displayed on Com #2 when transmitting on Com #1 but I don't hear the bleed-through static.

It's possible the problem could be the antenna placement, though I would have expected wiring the Transmit Interlock would have eliminated the issue. When the floors are removed at the next Condition Inspection, I will disconnect the Com #2 antenna and check to see if "RX" is still displayed on the #2 radio when transmitting on #1.
 
Note that the GTN650 and the GNC255 does NOT have a interlock input or output. But the GTN650Xi does have a interlock terminal.
It appears to me that there is a different "interlock" design.
The GTN650 and GNC255 has an internal "interlock" that has raised questions.
https://www.doi.gov/sites/doi.gov/files/uploads/IATB_2016-01_0.pdf

I have one antenna on top and one on the bottom and the RG400 has a
tinned copper braid on the outside grounded in both ends.
This works fine.

Good luck
 
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