On Measurements of Noise in Low-Noise Receiving Systems

A detailed theory of noise generation in waveguides reveals that the effect of standing waves may not in general be neglected, as has commonly been done in the past. Some special cases in which this standing-wave effect may be neglected are pointed out. The theory of a simple, cheap, and very accurate method for measuring the noise contribution of the input waveguide in very low-noise receiving systems is also presented. In this method, no hot-cold body is used, but only a room temperature load, a fixed short circuit, and a fixed open circuit. Measurements on a 3-GHz traveling-wave maser system support the theory and also show that the simple method yields a result at least as accurate as can be obtained using a commercial hot-cold body.