More information. The below was taken from the Microaire M760 manual.

"For certified aircraft the M760 should only be operated with a TSO
DO-160D compliant antenna. The antenna may be ¼ wave whip or ½ wave
dipole, using 50ohm coaxial cable and a BNC connector for connection.
For non-certified aircraft using a non-TSO compliant antenna, the VSWR
must be checked to ensure the ratio does NOT exceed 1:3:1 across
118.000 - 136.975Mhz range. A VSWR of 1:5:1 can be tolerated by the
M760 without injury to the transceiver, but transmission performance
starts to become impaired."

So I guess the next questions are;

1) What is a TSO compliant antenna? - It appears that this is an FAA
"Technical Standard Order" (http://en.wikipedia.org/wiki/
Technical_Standard_Order). Looking at the Aircraft Spruce web site I
find lots of references to TSO transceivers but not to TSO antennas.
I was unable to bring up the http://airweb.faa.gov web site on TSOs.

2) Does my glider have a TSO compliant antenna? Like most glass
ships, my antenna is imbedded in the vertical stabilizer. I guess I
will have to contact the manufacturer.

3) Does a TSO compliant antenna mean that I don't have to worry about
coax length? If so, why not?

Thanks, John