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A sensitive microwave radiometer system operating at short centimeter wavelengths has been developed which will allow large extensions of the known spectra of a large number of radio sources; facilitate the measurement of source polarization; give information on galactic structure and the sources of galactic radio emission; provide new data on the physical structure of planetary nebulas; and provide a means of measuring more accurately planetary temperatures, and the precise position of the brighter radio sources. A traveling-wave tube radiometer operating at 8000 mc with a bandwidth of 1000 mc and sensitivities of the order of 0.01Â°K is described. The radiometer is more than one order of magnitude more sensitive than other existing radiometers operating at 8000 mc. The very serious effects of gain fluctuations, acting on small residual signals, when trying to achieve very high sensitivities, are discussed. A means of eliminating such effects by introducing compensating noise has been found successful. Radio observations with this radiometer in conjunction with a 28-foot parabolic reflector have shown that: 1) The predicted sensitivity is achieved. 2) Zero-level stability is extremely high. 3) It has been possible to detect in detail the distribution of radio brightness at this wavelength in the vicinity of the galactic plane. 4) Radiation from the planets Jupiter and Saturn has been detected, this being the first detection of Saturn as a radio source. 5) Radiation from two planetary nebulas has been detected, this being the first detection of these objects as radio sources.