700 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



SOME LOOSELY COUPLED TRANSDUCERS IN MULTIMODE WAVEGTODE 



In connection with research on low-loss circular-electric-wave trans- 

 mission/ there developed a need for means with which to measure the 

 power present in any one of the modes of a multi-mode round waveguide. 

 In particular, it was known that the circular electric wave in round 

 waveguide converts readily to the TMn wave due to curvature of the 

 line,^ and a direct measurement of the effect was needed. The TMn wave 

 will not exist in the round waveguide without the presence of at least 

 four other modes, and in the waveguide size used for the experiments 

 five other modes could propagate. In designing a transducer for this ap- 

 plication, therefore, it was necessary to evaluate the discrimination 

 function, equation (4), with regard to mode discrimination between five 

 different pairs of modes as well as to insure directivity. Moreover, the 

 TMu wave is degenerate with the circular electric wave TEoi , i.e., they 

 have the same phase constant. Therefore, mode discrimination against 

 TEoi could not be obtained through the phase difTerence effects described 

 by (4). This discrimination was obtained using geometric balance in the 

 individual coupling orifices, which were narrow slits on the center line 

 of the wide side of the rectangular guide, as sho-wTi in Fig. 39. The shape 

 of the coupling distribution employed was that described in connection 

 with Fig. 14 except that 80 point couplings were used to simulate the 

 raised-cosine coupling distribution (instead of 40 as in Fig. 14) in order 

 to assure good directivity for the very long coupling length that was 

 required. The round guide diameter was two inches, the rectangular guide 

 width 0.820 inches, calculated to produce the same cut-off frequency in 

 the rectangular guide as exists for the TMu wave in the round wave- 

 guide. The coupling length was about 17 inches. 



One simple method for evaluating the mode content of such a trans- 

 ducer is to measure the azimuthal distribution of electric field at the 

 round guide wall using the radial probe technique described by M. 

 Aronoff.^ If the power in a single mode is a great deal larger than the 



Fig. 39 — A TEio° to TMuO coupled wave transducer. 



