Preliminary design details and laboratory test results of a prototype microwave radiometer operating near 20 GHz are presented. Radiometric stability to 10 millikelvin on time scales from 100 to 10,000 s is the goal for the final system. The use of noise diodes as receiver gain calibrators is examined, and test results showing 30 parts per million stability per day in the intercomparison of noise diodes are presented. Temperature control of the radiometer is also examined, and the design of a solid state temperature control system capable of 1 mK stability near room temperature is presented. A new design for a mechanically actuated Dicke switch is also presented. The results of these laboratory developments are applied to a prototype radiometer. Early indications are that design goals are being met in the prototype. Future confirmation of the system's theoretical performance will only be possible with the completion of the final radiometer units.