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The sensitivities of square-law detectors and superheterodyne receivers presently available at the shorter millimeter wavelengths are reviewed and extrapolated to the submillimeter region. It is concluded that, for applications which permit wide reception bandwidths, significantly more sensitivity will result from use of a direct-detection system consisting of a chopper, square-law detector and narrow-band amplifier than will be attainable with superheterodynes in the foreseeable future. The expected sensitivity of a direct-detection system at wavelengths of the order of 1 mm is compared with that of the most sensitive centimeter radiometers.
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Long et al. (1961) studied this question.
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