COUPLED WAVE TlfKOltV VXD AYAVEOriDF, AIM'TJCATIOXS 



099 



Table I 



be inapplicability of the theory. Loose coupling theory predicts better 

 than 35 db directivity over a broad frequency band at a coupling length 

 of about 2\g ; therefore one might expect to obtain better overall results 

 b}^ using two cascaded arrays each about 2X9 long, and each having a 

 transfer loss of 8.4 db to get the 3 db net transfer loss. Observed directivi- 

 ties for such a coupling array are also given in the top of Fig. 38; in this 

 case values in the 32-37 db region were obtained. The destructive inter- 

 ference associated with addition of backward wave components is more 

 nearh^ of the form computed b}^ loose coupling theory because the ex- 

 citing wa^'e is more nearly constant over the length of one of the arrays. 

 The obser^'ed return loss at any one of the four waveguide entries, when 

 the others are terminated, is gi^-en for the o.5\g and cascaded 2X3 cou- 

 pling arrays at the bottom of Fig. 38. The cascaded 2Xg combination is 

 again superior to the single long taper. The characteristic of being in- 

 herently matched at all terminals makes the coupled-line tjq^e of 3 db 

 hybrid attractive at the very high fre(|uencies where lumped element 

 matching becomes difficult if not impracticable. 



Where space is at a premium, or where more constant transfer loss 

 values are to be desired a shorter array composed of larger holes is at- 

 tractive. A single linear taper of the shape outlined above and 2\y long 

 was observed to have better than 22 db directivity and better than 25 db 

 return loss over the 3.1 to 3.5 cm band. The observed loss values of the 

 three coupling arrays discussed above are given in Table I. The coupling 

 arrays composed of larger holes have less slope in the loss versus fre- 

 quency characteristic for side-wall coupling. 



