I hope you'll forgive me for butting in, Gary, but I want to correct a couple of things that seem to be a common misconceptions around transponder installations.
978, 1030 and 1090MHz (ADS-B and transponder frequencies) are all in the middle of the ultra-high frequency (UHF) band -- from 300MHz to 3GHz -- and have a wavelength of around 30cm. Not low frequency or long wavelength.
The installation documents from
Dynon (on pg. 11-12),
Garmin (on pg. 2-4) and
Appareo (on pg. 6) call for a
maximum coax length that equates to 1.5 or 2.0 dB signal loss. None of them make any mention of a minimum length and Dynon has
said explicitly on their forum that there is no minimum antenna coax length.
Quoting from the Garmin installation manual: "Note that
any 50 ohm, double shielded coaxial cable assembly that meets airworthiness requirements and the 1.5 dB
maximum loss figure (including connectors) may be used." [emphasis added]
I think there's some confusion between the guidance for transponder and GPS antennas. It's often the case that passive GPS antennas (those that connect to the receiver via coax and do not contain any powered, active circuitry) require a
minimum signal loss of around 2dB, driving a specification for some minimum cable length. This is
not the case for transponders.
Many GPS antennas (Dynon is a good example) contain a receiver and decoder in the antenna puck. They are powered by the EIFS network and transmit serial position data via the same wire bundle, so the only length concerns are those of the data network.
Finally, wavelength match is important with respect to the antenna, not the coax. It's impedance match that matters with coax. Impedance doesn't change with coax length; just insertion loss.