1. An internally cooled catheter apparatus comprising:an elongated flexible coaxial cable having proximal and distal ends, said cable including a hollow center conductor, an outer conductor and an electrically insulating layer in an annular space between said conductors substantially along the length of the cable;
an antenna at the distal end of the cable; and
a diplexer including a branched transmission line comprised of a common path with a tubular inner conductor segment one end of which is connected to the center conductor of the cable at the proximal end of the cable, a transmit path configured for connecting the common path to a transmitter which transmits signals of a first frequency range, and a receive path configured for connecting the common path to a receiver which detects signals of a second frequency range, the receive path having the tubular inner conductor segment of the common path extending therethrough and adapted for connection to a coolant source, the center conductor and the tubular inner conductor forming a continuous coolant pathway,
wherein said diplexer is for isolating the signals on the transmit path from the signals on the receive path.