906 



THE BELL SYSTEM TECHNICAL JOUENAL, JULY 1956 



-102 

 8 



UJ 



u 



UJ 



O 



to 

 to 

 o 



-10 



z 

 o 



z 



> 



< 



_l 



cr 



-t 



\ 



I 



1 



-10" 



6 8-, 



-10 2 



-10 



■1 



ATTENUATION OF SPURIOUS MODE IN DECIBELS 



Fig. 6 Theoretical and measured relative insertion loss in the TEoi trans- 

 mission system of Fig. 4. 



several times the length of the tapers, the losses in the transitions are 

 fairly small compared to the losses in the multimode guide and this 

 justifies assumption (5). The resonance due to the other modes is too 

 small to be appreciable. This is understandable since, according to (10), 

 the value of the mode conversion for the TE03 (Fig. 5) and the attenua- 

 tion for the shortest length of pipe tested, the calculated relative inser- 

 tion loss is less than —0.1 db. 



CONCLUSIONS 



The resonance of spurious modes in a closed environment can produce 

 a large insertion loss of the transmitting mode. In a fairly narrow band 

 device it is possible to avoid this problem by selecting a proper wave- 

 guide size for the closed environment. In a broad-band system the losses 

 can be minimized by providing a high attenuation and a low mode con- 

 version for the spurious mode. For example, it may be noted, by refer- 

 ring to Fig. 3, that mode conversion as high as —20 db with a spurious 

 mode loss of — 8 db results in only an —0.1 db insertion loss for the 

 transmitting mode. 



