Precision Measurement of Impedance Mismatches 

 in Waveguide 



By ALLEN F. POMEROY 



A method is described for determining accurately the magnitude of the reflection 

 coeflicient caused by an inipe-iance mismatch in waveguide by measuring the 

 ratio between incident and reflected voltages. Reflection coeflicients of any 

 value less than 0.05 (0.86 db standing wave ratio) can be measured to an accuracy 

 of ± 2.5%. 



TONG waveguide runs installed in microwave systems are usually 

 -*— ' composed of a number of short sections coupled together. Although 

 the reflection at each coupling may be small, the effect of a large number in 

 tandem may be serious. Therefore, it is desirable to measure accurately 

 the very small reflection coefficients due to the individual couplings. 



A commonly adopted method for determining reflection coefficients in 

 phase and magnitude in transmission lines has been to measure the standing 

 wave ratio by means of a traveling detector. Such a system when carefully 

 engineered, calibrated and used is capable of good results, especially for 

 standing waves greater than about 0.3 db. 



Traveling detectors were in use in the Bell Telephone Laboratories in 

 1934 to show the reactive nature of an impedance discontinuity in a wave- 

 guide. A traveling detector was pictured in a paper^ in the April 1936 

 Bell System Technical Journal. Demonstrations and measurements using 

 a traveling detector were included as part of a lecture on waveguides by 

 G. C. Southworth given before the Institute of Radio Engineers in New 

 York on February 1, 1939 and before the American Institute of Electrical 

 Engineers in Philadelphia on March 2, 1939. 



Methods for determining the magnitude only of a reflection coefficient 

 by measuring incident and reflected power have been developed by the Bell 

 Telephone Laboratories. A method used during World War II incorporated 

 a directional coupler^. The method described in this paper is a refinement 

 of this directional coupler method and is capable of greatly increased accu- 

 racy. It uses a hybrid junction^ to separate the voltage reflected by the 

 mismatch being measured from the voltage incident to the mismatch. 

 Each is measured separately and their ratio is the reflection coefficient. 



The problem to be considered is the measurement of the impedance 

 mismatch introduced by a coupling between two pieces of waveguide due to 

 differences in internal dimensions of the two waveguides and to imperfec- 

 tions in the flanges. The basic setup might be considered to be as shown 

 in Fig. 1. The setup comprises a signal oscillator, a hybrid junction, a 



446 



