Measurement of discontinuities in waveguides

Discontinuities in long wave-guide runs carrying a frequency-modulated (FM) radio relay signal cause signal reflections (echoes), which in turn cause intermodulation distortion. Previous papers1,2 have discussed an FM radar method for determining the location and magnitude of these discontinuities. This paper discusses the application of this method using commercially available equipment. The two most important characteristics of the measurement method are its sensitivity, i.e., how small a reflection can be measured with a given accuracy, and its resolution, the smallest distance between two separately measurable discontinuities of approximately the same magnitude. The equipment discussed has a signal-to-noise ratio of 6 db (decibels) for a 1% reflection 12 feet away with a 1-foot distance between separately resolvable discontinuities anywhere within a 100-foot run. The theory of the measurement method, the use of the equipment, and methods for maximizing the sensitivity and resolution are discussed.