Weird Radio Problem

[SuperDave]

I've been troubleshooting a transmitting problem for a while now and still haven't come up with an answer. I have a Terra radio that has good reception and transmits well on the ground (loud and clear) but when in the air everybody says it sounds bad, like 2/5 or 3/5 or just "unreadable". I've tried different headsets and all the usual troubleshooting steps and today even plugged in my handheld to the antenna coax and had the same results. I powered the handheld from the plane and then by its battery and people said it sounded a little better with the battery.
So, transmits and receives well on the ground but only receives well in the air. It seems to be pointing to the antenna but why only in the air?
Any input from the radio gurus would be greatly appreciated.

 

[PCHunt]

My (Limited) experience is that on the ground, there isn't very much vibration from the engine. [Particularly if the engine isn't running!!] Once you are airborne, the engine/prop combination is vibrating everything in the plane, including any bad antenna connections.

I would suggest that you check all antenna connections, especially the grounding of the antenna through the mounting screws.

 

[Walt]

Start with the basics, double check power and grounds, be sure you have 14v going to the radio during transmit (low voltage will cause distortion and the radio draws a lot more current when transmitting compared to receive mode).

 

[SuperDave]

Keep'em Coming

During the troubleshooting process I ohmed the cables and grounds and didn't find anything so I am also thinking that vibration has something to do with it. I originally suspected a bad connection at the radio that was affected by vibration but hooking up the handheld took the panel mount radio out of the equation. I'm going to pull the antenna and clean the mount holes real well and see if that makes any difference.
Thanks for the ideas!

 

[TylerII]

Radio emit garbled in air

Start checking antenna by checking the BNC connectors first. RF energy is a bit of black magic and just loves poor semi-conducting connections. One or both the BNC connectors has a center pin connection that is questionable. It the center conductor was even slightly crimped when the pin was installed it will fatigue and you have a semi-connection. The result will be bad transmit audio. Aviation VHF is AM transmission and the modulator can get real sensitive to RF. It will happen on any transmitter that is hooked to it as well as you observed.
Tyler
A&P
N1174MT