The new issue of MicroComputer Journal_ (now published by _Midnight
Engineering's_ Bill Gates (no, the _other_ one!)) has a construction
article that details a 19,200 bps RF modem operating full duplex at
~400 and 900 MHz. The modem uses a single chip xmitter and a single
chip rcvr, and gets around 80 ft on the author's prototype. The whole
thing with a MAX 232 fits on a board that is ~ 3x4 inches.