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Components and techniques have been developed which substantially increase the frequency range over which superheterodyne-type radiometers are practical. High-sensitivity radiometers have been built to operate at frequencies within the 2.1-mm and 1.3-mm atmospheric absorption windows. Through the use of harmonic mixing reasonably priced and readily available 4-mm reflex klystrons were used as local oscillator sources for each of these radiometers. The LO noise problem, which is particularly critical at the shorter mm wavelengths, was circumvented through the use of a microwave intermediate frequency. The use of such a high IF eliminated the need for a balanced mixer and made it feasible to obtain a large IF bandwidth (1.5 Gc) which substantially further improved the thermal sensitivity of these radiometers. With a 10- second integration time, minimum detectable temperature differences of 0.3°C and 1.0°C were obtained with the 140-Gc and 225-Gc radiometers, respectively. Preliminary atmospheric absorption measurements made with these radiometers indicate a total vertical attenuation of 1.8 db at 138 Gc and 4.5 db at 225 Gc.