WAVi:(;rinK as \ coMMrxic \ ri(»\ midk m 



1213 



25 



t '0 



0.1 0.2 0.4 0.6 0.8 1.0 2 4 6 8 10 20 40 60 100 



LINE LENGTH BETWEEN MODE FILTERS -TEqi HEAT LOSS IN DECIBELS 



Fig. 22 — Improvement in Pi/P„ due to addition of mode filters. Total line 

 length equals 20 db TEoi heat loss. (Plotted for an = 0.5 Ou). 



the spacing of mode filters at less than 0.1 db to the signal wave or use of 

 a continuous form of transmission line having a ratio of heat loss in the 

 unused mode to the heat loss in the used mode on the order of 500. 



DIRECT EVALUATION OF MODE CONVERSION MAGNITUDES 



The important influence that mode conversion effects are expected to 

 exert on signal fidelity lead us to make direct evaluations of the con- 

 version coefficients. The direct evaluation consisted of transmitting 

 (actually or in imagmation) a pure circular-electric wave into one end 

 of a w^aveguide section and, by measurement or by calculation on the 

 basis of known geometry, determining the relative magnitude of the 

 power converted to the unused modes. 



The simplest experimental technicjue for analyzing mode impurities 

 consists of a short radial probe at the guide wall. The radial probe re- 

 sponds to energy in any mode of propagation except the circular electric 

 family, and serves as a versatile instrument for measuring the order of 

 magnitude of mode conversion effects. The limitations of the technicjue 

 stem from (1) the fact that the probe responds to the vector sum of the 

 amplitude of the radial electric field compon(Mits of about 35 modes (in 

 the 5" line case), and this sum varies with circumferential and longitudi- 

 nal position of the probe even though the power present in the modes is 

 constant; and (2) the fact that the sensitivity of the probe response to a 

 given magnitude of power in the guide is variable from mode to mode, 



