COUPLED WAVE THEOKY AND WAVEGUIDE APPLICATIONS 



"01 



) 120 160 200 240 280 320 360 



AZIMUTHAL ANGLE IN DEGREES 



Fig. 40 — Distribution of radial electric field at the guide wall for the forward 

 and backward waves of the transducer of Fig. 39. 



power in any other mode of the multi-mode guide, the radial probe 

 technique narrows down the possible mode types to a very few. Measure- 

 ments of this type, recorded in Fig. 40, indicate that the forward wave 

 has the radial electric field distribution to be expected for the TMn wa^'e. 

 PTowever, the forward wave might have the same radial field distribution 

 at the wall and actually be the TEn wave instead of TMn . The TEu 

 wave is very simply generated from a dominant mode rectangular guide, 

 by means of a long taper transition along the axis of propagation from 

 the rectangular cross section to the circular cross section. Such a trans- 

 ducer was used to measure the output wave of the TMu transducer and 

 it was found that the TEn component was down on the order of 30 db 

 below the value which would be present if the radial field intensity ob- 

 served at the top of Fig. 40 had been due to TEn • By a process of elim- 

 ination, therefore, and by virtue of the fact that we have a pure pattern 

 suggesting the presence of a single mode, we have established that the 

 mode generated is actually T~Sln • Other checks can of course be made, 

 such as measurement of the phase constant of the output wave. 



The backward wave shown at the l)ottom of Fig. 40 has a maximum 

 field more than 20 db below the maximum field of the forward wave and 



