Troubleshooting on-the-ground communication problems

[Matt Ward]

My COM2 is my 430W.  The comm antenna for that radio is on the bottom of the plane.  When I'm on the ground making radio calls, that radio is basically unusable.  From multiple places on my home field and other fields, I get the old "that transmission was garbled" from the other party.  I'm trying to troubleshoot it but honestly don't know where to start.  Seems like the problem could be in the Garmin, the coax, the antenna, or any of those installations.  In the air, the communications with this radio are fine.  And on the ground or in the air, the nav functions are fine.

Any tips on what I should try?

Thanks!

 

[carusoam]

Any issues while in the air?

Bottomside antennas are known to be limited for ground communications... 
Best regards,
 

Inspection and cleaning of all the connections to antenna cable, and grounding planes, can be helpful...

Best regards,

-a-

[Matt Ward]

In the air is fine. That’s what I figured. Just hoping it was something I could address. It’s not worth moving the antenna!

[bfreelove]

I've had the fun of troubleshooting a comm antenna mounted on the bottom of the airplane (in my case an airplane with carbon fiber fuselage skins that make it even more challenging).

 It's unlikely to be the radio (although it could be) since it seems fine in the air.  The first step is to check the antenna SWR (most avionics shops will have equipment to do this).  If you get anything more than 1.5:1 on the antenna (1.2:1 is ideal) then you need to check the coax, and also make sure the antenna is getting a good ground to the skins.  Having the antenna on the underside will make it more sensitive to any issues and this will show up on the ground.  In our case we kept making larger ground planes until it worked okay, with the aluminum skins you have the best ground plane possible, but it needs to have good continuity to the skin to work.  A bad coax, or bad connector can also affect this. If all that checks okay, then it's the radio.