Medical catheter apparatus for minimally invasive applications includes a probe for radiating electromagnetic waves of a first frequency capable of heating tissue and detecting thermal emissions from that tissue of a second frequency indicative of tissue temperature. The probe has an inner conductor extending along the probe to a conductive probe tip and a concentric tubular outer conductor having a leading end spaced rearwardly from the probe tip and a closed trailing end shorted to the inner conductor. A dielectric sheath surrounds the outer conductor which sheath has a leading end spaced rearwardly from the leading end of the outer conductor. A diplexer integrated into the probe includes a filter circuit positioned inside the outer conductor and a coupling capacitor connected between a point on the inner conductor and the input of the filter circuit such that the conductors between that point and the shorted trailing end of the outer conductor constitute a quarter wave stub at the second frequency.