COUPLED WAVE THEORY AND WAVEGUIDE APPLICATIONS 70.') 



than 3.45 radians per meter. If the coupling coefficient for TEio° to 

 TEoi° is equal to that for TEio° to TEsi^ it follows that the total coupling 

 length must be greater than 0.455 meters, because complete power 

 transfer requires that the product of coupling-length times coupling- 

 coefficient be exactly 7r/2 (see Fig. 17). Actually, the TEio° - TEsi^ 

 coupling may be greater than the TEio° — TEsi^ coupling Avhich leads 

 to the requirement for longer coupling intervals. It is evident that the 

 shorter coupling intervals may be employed at the sacrifice of greater 

 mode impurities. The preceding calculations were made for the TEio° — 

 TEsi^ and TEio° — TEoi° transfer ratios as though only one mode of the 

 multi-mode waveguide were present at a time, i.e., using a theory based 

 on coupling between two waves instead of a theory for the simultaneous 

 coupling between a plurality of waves. It is felt that this is probably 



Fig. 42 — An experimental circular electric wave (TEioi^ to TEoiO) transducer 

 for 24,000 mc. 



justified provided that the coupling per unit length is weak and only one 

 mode in each guide carries an appreciable amount of power. 



Fig. 42 shows a photograph of one of the models used to obtain experi- 

 mental data. The coupling holes w^ere located in the narrow wall of the 

 rectangular waveguide, thus avoiding coupling to all of the TM modes 

 of the round waveguide. The total coupling length was 0.55 meters. The 

 coupling orifices were spaced about 0.3 wavelengths in the dominant- 

 mode rectangular waveguide, which assured reasonable directivity in 

 the transfer of power between waveguides, provided that two or more 

 coupling elements were employed. 



The transfer loss between the rectangular wa\'eguide and the circular 

 electric mode of the round waveguide was measured as a function of the 

 number of coupling elements, using the structure of Fig. 42 with the 

 addition of a mo^•able thin-walled metallic cylinder. The latter could be 

 moved inside the transducer in such a way as to cover up a varial)le 

 number of couphng holes, and contained a long wooden termination so 

 that all the power entering the mo\'able cylinder was absorbed. The inner 

 diameter of the movable eyhnder was large enough to propagate the 



