You might be able to refill a butane-type container with helium for a ballon type device. You'd need proper adapters to decant from a higher pressure supply. Then you could either fly a ballon proper or an aerial for radio signals.
A round ballon would make a lousy radar target so you are on the right track with some sort of tetrahedron. You might inquire at a competitive kite shop about making something that could be deployed at low altitude and then stay aloft in a light breeze with enough lift to suspend (X? gms).
The NEMA input is the easy part
(KD7SCR in the amateur radio world)