On Monday, February 2, 2015 at 9:00:07 PM UTC-8, George Haeh wrote:
A single coax may be easier to fish
through than at least 6 wires between a
Trig head and remote plus + & - to the
remote plus optional gear position and
GPS.

You would normally use about half of the pins on the controller head B-9 connector used and yes its often going to be easier to fish those, than a RF coax cable, but more importantly a coax, even a semi-flexible one does not enjoy being flexed in use. And the Trig transponders are designed to have relatively short cable runs between the RF box and the antenna. With a TT-21 main box mounted in a glider panel and a long coax run to the tail, you are increasing the possibility that install will fail a RF output power test.

You may have issues with the length of your coax run the tail battery box. I don't know how you would secure your tube in place and you don't want it flexing and telegraphing any mechanical movement into the cable. also be careful of things like shielding of the transponder signal by the tailplane or other carbon fiber parts. The good old boring downward pointing 1/4 wave stub antenna placed somewhere roughly behind the main gear with a proper ground plane works *very well*. Most L2 antennas are installed inside the tail boom of fiberglass fuselage gliders, not way back in the fin. More modern fin/hinge line mounted transponder installs down by glider manufacturers are very careful about using premium low-loss cable.