Investigation of a new form of micro-wave interferometer for determining the velocity of electromagnetic waves

Abstract

A very sensitive symmetrical micro-wave interferometer utilizing four electromagnetic horns is described. Operating at 24 005 Mc/s (λ ≏1.25 cm), its basic function is to measure wave-length in air, and the resulting value when multiplied by frequency and refractive index gives phase velocity in vacuo . Because of diffraction, the velocity thus obtained is higher than the true free-space value by an amount dependent upon the size of the horns and the spacing between them. The investigation has been primarily concerned with the use of the instrument under conditions of large diffraction correction (up to the equivalent of 100 km/s), so that the accuracy obtainable in eliminating this effect could be assessed. The effect of deliberate distortion of the transmitted and received wave-fronts upon the diffraction correction has also been ascertained. In addition, errors arising from random and systematic scattering of the micro-wave energy between transmitting and receiving horns have been investigated. The ten observations made under optimum conditions have been used to derive a preliminary value for the free-space velocity of electromagnetic waves in vacuo , namely, c 0 = 299 793.0 ± 0.3 km/s. The error limit given represents the standard deviation of single observations in statistical combination with estimated systematic errors.