Microair FL760 Transmission Problem Champ AGM Battery Only

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lynngarner

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I'm training in a Champ 7DC (no generator or alternator) with battery start and Microair Panel Mount Radio (5 Watts I belive). Using Odessey AGM battery with 170 CCA. Batteries Plus load checked it today and said it is within spec at about 12.8 volts (1 year old). Radio receives well but I cannot transmit more that about 300 feet by line of sight. Cleaned the D Sub connectors, and antenna is well connected and has functioned fine in the past. Electrical/ground connections are very good. I charge the battery between every flight. Problem started a month ago. Any ideas, workarounds, suggestions are appreciated. lg
 
The connectors are BNC both ends. The cable appears to be solidly connected on both ends. Could not locate an ID on the cable, but it is black and thinner than typical antenna cable. Still appears to be coax. Antenna is mounted on the metal wing gap cover. It is slanted at about 45 degrees back and is about 18" long.
 
We found an aircraft with RG-59 antenna cable - probably Radio Shack, 75 ohm.
We replaced it with RG-58 and dramatically improved transmission performance. RG-400 is the current aircraft standard, and is 50 ohm cable.

From AI Google:

The main difference is that RG58 has a 50-ohm impedance and is best for radio and amateur radio applications, while RG59 has a 75-ohm impedance and is typically used for video applications like CCTV and cable TV. This means they are not interchangeable; using the wrong cable will result in poor signal and interference because the impedance must match the equipment.
 
I believe you are recommending RG-400 cable, correct? Sorry for not completely understanding. I'll try this once I'm sure which cable to try. Thanks...lg
 
RG-400 and RG-142 are both acceptable but -400 is a stranded core where -142 is solid so the flexibility, bend radius, etc. are different.

SteinAir.com has a library of video tutorials for doing all sorts of electronic/avionics tasks. Here is the one for crimping coax which isn't as hard as you might think and allows you to make antenna cables that are exactly the right length. Ask your A&P/IA about legality issues and check equipment manuals for minimum length requirements that may apply.

 
RG-58 or RG-400. You won't notice the difference.
Stein is a good guy. Bought most of our GTR-200s from him. His wire is a lot less expensive, but the same stuff.
 
Embarrassing to say the least..the antenna connection under the wing gap cover was loose. Bob, I did check and the cable is 50 ohm resistance. I could even transmit inside the hanger and get a response. Thanks for your help. lg
 
Not embarrassing - that happened to me once. Could be expensive, though - if your antenna comes completely disconnected you can damage the transmitter.
 
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