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  #1  
Old 10-10-2019, 09:28 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
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Default Garmin GTR 200 cross talk

For you Garmin experts out there.

Helping an RV-14 builder with a flying plane. He has a Garmin radio stack (GTN-650, GTR 200 and GMA 240 Audio Panel).

The panel was wired and tested by an avionics shop. The two comm antennas are mounted in per plans locations. The antennas and feed lines have been checked with an antenna analyzer.

Both radios work fine by themselves. When transmitting on the GTR 200 and monitoring another frequency on the GTN-650 all is fine. When transmitting on the GTN-650 and monitoring another frequency on the GTR 200, the GTR 200 squelch is broken and feeds noise into the audio panel.

After all the checks I'm convinced the problem is with the GTR 200.

Is this a known issue with the GTR 200? I assume I can just crank the squelch setting up a lot higher but hoping there is another fix.

Carl
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  #2  
Old 10-10-2019, 11:40 AM
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The GTR200 requires the transmit interlock be wired in a dual com installation when the antennas are close to each other to help prevent overloading the receiver input.
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Last edited by Walt : 10-10-2019 at 11:44 AM.
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  #3  
Old 10-10-2019, 03:28 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
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Quote:
Originally Posted by Walt View Post
The GTR200 requires the transmit interlock be wired in a dual com installation when the antennas are close to each other to help prevent overloading the receiver input.
Sorry to hear this - never had this issue when running the GTN-650 and the Dynon radio.

Carl
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  #4  
Old 10-10-2019, 03:40 PM
12vaitor 12vaitor is offline
 
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From the Garmin GTR 200 Installation manual, note the caution on damaging the non-transmitting radio:

2.5.1 COM Antenna Location
The GTR 200/200B COM antenna should be well removed from all projections, engines and propellers.
The ground plane surface directly below the antenna should be a flat plane over as large an area as possible
(18 inch square, minimum). The antenna should be mounted a minimum of six feet from any DME or
other COM antennas, and four feet from any ADF sense antennas. The COM antenna should also be
mounted as far as practical from the ELT antenna. Some ELTs have exhibited re-radiation problems that
cause interference with other radios, including GPS. This can happen when the COM (GTR 200/200B 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-comm), the COM antennas
should be spaced for maximum isolation. A configuration of one topside antenna and one bottom side
antenna is recommended. The GTR 200/200B requires a transmit interlock.

Simultaneous COM performance varies significantly across installations and is affected by both the
isolation between the COM antennas and the separation of the tuned frequencies. Each installation should
be individually examined to determine the expected performance of simultaneous COM.

CAUTION
If the COM antennas are mounted less than six feet apart with a direct line of sight
between the antennas the non-transmitting radio could be damaged when the other radio
transmits. An example of direct line of sight is both antennas mounted on the bottom or
top surface of the aircraft. For metallic aircraft one antenna should be mounted on the
bottom close to the front and the other on the top of the aircraft close to the tail such that
the aircraft structure is between the two antennas. For composite aircraft additional
shielding may be needed between top and bottom mounted COM antennas.

John Salak
RV-12 N896HS
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  #5  
Old 10-10-2019, 03:49 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
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Yep - just read this.

So for most installs that translates to no capability for the pilot talking on Comm #1 and the Co-Pilot talking on Comm #2. Most people do not care, but as I found out monitoring on Comm #2 is a problem.

I cranked up the squelch today and it seems to help. Iíll find out on next flight.

Carl
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  #6  
Old 10-10-2019, 04:26 PM
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Hello,

Page 4-5 of the Rev. E GMA 240 installation manual explains that if you add a ground jumper to connector J2402 , pin 30, you enable the "MUTE ON COM TX" feature which mutes the receive input for all monitored radios while the PTT is active for the selected COM radio.

The GMA 245 audio panel has a similar feature which is enabled with DIP switch 3.

This is a great way to make sure that no feedback is received on any of the monitored radios when transmitting on powerful 10W COM radios such as the GTR 200 and GTN 650.

We mounted one COM/GPS antenna on top of our RV-7A (for the GTN) and a second COM antenna on the bottom (for the GTR 200) to provide the recommended separation for high power dual COM installations. Of course the GTR 200 has a transmit interlock to protect the receiver from high energy received from the other COM radio during transmission. The GTN 650 doesn't use a physical transmit interlock, but performs this protection function internally.
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  #7  
Old 10-10-2019, 05:19 PM
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Post Split COM

Quote:
So for most installs that translates to no capability for the pilot talking on Comm #1 and the Co-Pilot talking on Comm #2.
The GMA 240 audio panel installed in this aircraft does not support what is known as split COM operation where the pilot PTT operates COM1 and the copilot PTT operates COM2, and they can both operate/transmit independently at the same time.

The GMA 245 audio panel does support split COM operation. Just push the COM1 MIC and COM2 MIC buttons together to enable, but you would need excellent antenna separation (at least separated by the fuselage) for this to be a useful feature.

Thanks,
Steve
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