Microwave heating apparatus for heating fluid or tissue includes an elongated catheter for placement adjacent to high/dielectric high loss organic fluid or tissue in a patient. The catheter has a distal end and a proximal end and includes an antenna adjacent to the distal end and a cable having one end connected to the antenna and a second end. A transmitter provides a transmitter signal capable of heating the fluid or tissue and a first receiver receives a first signal indicative of thermal radiation from a first depth in the fluid or tissue, producing a first output signal in response thereto. There is also a second receiver for receiving a second signal indicative of thermal radiation from a second depth in the fluid or tissue, producing a second output signal in and response thereto. The apparatus also includes active and passive diplexers connected in series between the transmitter and the second end of the cable. A controller controls the active diplexer so that when the transmitter is transmitting, the first signal is coupled to the passive diplexer but not to the second receiver and when the transmitter is not transmitting, a second signal from the passive diplexer is coupled to the second receiver but not to the transmitter. During operation of the apparatus, the passive diplexer couples the transmitter signal only to the antenna while coupling a first signal from the antenna only to the first receiver so that the apparatus can heat the fluid or tissue and determine the actual temperatures of the fluid or tissue at two different depths.