A centimetre-wave parallel-plate spectrometer

A spectrometer has been designed for the measurement at a wave-length of 1.25 cm of the refractive index of samples formed into prisms. The radiation is totally enclosed by two parallel plates approximately 4ft in diameter, 3/16 in apart, and enters the instrument by a sectoral horn free to move over a 90° sector of the circumference. The receiving aerial can be either a similar horn or an open-ended waveguide, and may be positioned anywhere on the circumference. The principles on which the instrument is designed are discussed, and details are given of the mechanical construction and of the tests made upon its accuracy. The electrical performance has been examined in a variety of ways, and it has been established that beam directions can be located to within 0.25°. Variations in the received signal power occur and are shown to arise from a variety of effects, such as plate spacing, aerial penetration into the plate region and interference by unwanted signals such as the transmitter side-lobes. None of these causes a significant change of the direction of the main beam. Measurements on polystyrene prisms show a 3% error in refractive index attributed to imperfect contact between the dielectric of the prism and the spectrometer plates. This error can be reduced to less than 0.5% by coating the prism surfaces with tin foil.