We present the design of a novel bolometer intended for measurement of radiation within a waveguide. The thermal link in the bolometer is a low-thermal conductance micromachined beam. Power is coupled from an antenna to a resistive termination at the center of the beam. The optimization of the design for an astronomical measurement leads to an operating temperature of 0.48 K, accessible with aHe3refrigerator. Measurements on the antenna absorber in a waveguide show an average absorption of 94&percent; from 33 to 50 GHz. ©1997 American Institute of Physics.