Abstract: Microwave warming apparatus includes a housing defining a heating waveguide with a longitudinal ridge and a heating cavity. A slot extends through the ridge into the heating cavity for receiving a cartridge containing a looped tube so that the tube extends into the heating cavity where the tube contents are heated by energy coupled into the waveguide. Receiving waveguides adjacent to the slot sense the thermal radiation emanating from the tube and deliver corresponding signals to a radiometer which produces a temperature indication. The cartridge includes a support member which maintains the shape of the tube loop. A pair of notches in the support member have walls enabling the notches to complete the receiving waveguides in the housing.