Superheterodyne radiometers for short millimetre wavelengths

Millimetre-wave radiometers suitable for making temperature measurements on plasmas are discussed. After a brief review of radiometer theory, special attention is paid to the design of the low-frequency radiometer circuits, the performance of mixer and harmonic-mixer crystals at short millimetre wavelengths, the local-oscillator power needed and the suppression of local-oscillator noise and spurious signals. An early 140-Gc/s radiometer, with second-harmonic mixing and an overall noise factor of 25dB, is mentioned, and a detailed description is then given of a recent 140Gc/s radiometer which contains: a fundamental local oscillator, VX3352 mixer crystals and a 408Mc/s i.f. amplifier commencing with an Adler tube. An overall noise factor of 19dB has been achieved with this equipment and its performance is fully discussed. Some typical recordings of the 2mm radiation from an argon noise tube and Zeta are included. Various ways of obtaining improved radiometer performance at millimetre wavelengths are mentioned. The appendixes are devoted to radiometer theory and an analysis of the halfwave commutator detector.