I recieved this when I asked the same question last year to Futaba. This is if you want to replace the whole thing.

flyer495th from RC Universe Discussion Forums - Ask questions and forwarded a message to you at 10/6/2004 12:28:27 PM:

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RE: Busted Antenna wire on Rx....advise needed - by Krysta
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Hi Bob,

While we can not recommend that anyone change their receiver wire, as this can cause serious problems such as loss of reception and major interference issues, and also void any warranty that you may have on your radio system.

But, if you are very good with a soldering iron, you should replace the COMPLETE antenna at the board, not just solder at the cut. Also after doing so, do a thorough ground check, you would want to look for 22 gauge wire, and the length will be:

approximately 19 1/2" for surface
approximately 39 1/2" for Air

Sincerely,
Krysta
Lead Futaba Customer Service and Programming Technician