Yes, the GTX45R has only one antenna; there is an option for a GPS antenna as well but I won't need that. I read the manual and can't find any limitations about antenna cable length. It does say to keep the antenna away from gear, other antenna's, etc.
So I guess if I wanted I could use the cable run to the tail for the transponder back there; not sure what antenna I would want. I was planning a wingtip Nav antenna; maybe I could just put it under as well rather than just plugging the hole with a plate of some kind.... How well would a cat-whisker antenna work back under the tail?
Wish I'd known this at the beginning!!!!