Technique for the measurement of standing-wave ratios at low power level

Abstract

This paper describes a technique for the measurement of large standing-wave ratios which is closely related to the standard method of Roberts and von Hippel. The latter method gives rise to errors of uncertain magnitude when the minimum field strength of the standing wave, approaching the general noise level of the detector system, can no longer be accurately established. The new method entails making two width-ofminimum measurements, close to the minimum but away from the noise level. Computer-drawn curves and nomograms have been produced to facilitate the determination of standing-wave ratio from these measurements. The curves may also be used to give an indication of the magnitude of the measurement errors which can be tolerated, and some possible sources of these errors are discussed. It is concluded that, under some experimental conditions, the new method is entirely preferable to that of Roberts and von Hippel, and that, in any experimental arrangement, the range of measurement of standing-wave ratio could be extended by applying this new technique, when the limitations of the standard method are exceeded.List of symbols D = detector-system meter reading E,E U E 2 -electric-field strengths in a transmission line £" = electric-field strength corresponding to noise level E o = electric-field strength of incident travelling wave E min = minimum E in standing wave E max = maximum E in standing wave G = detector-system gain k = ratio of field strengths in standard method K = ratio of field strengths in new method /,/i,/ 2 = distances along transmission line S = v.s.w.r. j8 = phase constant, 2TT/A A£",AA:,A/(,A/,AW> = errors of measurement A = wavelength in transmission line p = modulus of reflection coefficient w,w u w 2 = distances between points of equal £ a t standing-wave minimum 1