MA XI MA LL Y-FLA T FIL TERS IN WA VEGUIDE 705 



from stock of a given diameter. The expression for the normahzed sus- 

 ceptance of off-center posts is given by the relation^^ • ^^ 



B'-'^ „r ;.. ■... m 



ITS 



sec 





where s is the distance off center. 



Experimental Data 



The principles of waveguide filter design as outlined in the foregoing have 

 been used in several applications. For example the channel branching fil- 

 ters in the New York-Boston microwave radio relay link consist of two reso- 

 nant cavities separated by the equivalent of f wavelength sections of 

 waveguide. The transmitting modulators in this relay system also use 

 two-chamber filters to separate the wanted sideband from the unwanted 

 sideband. The transmission band in each of these applications was 10 mc 



Fig. 18— Diagram of a transmission line filter consisting of four resonant cavities and 

 three connecting lengths of line. 



and the image frequency or the unwanted sideband which was to be re- 

 flected was 130 mc away. 



In another case the requirements were that the standing wave ratio should 

 be less than 0.64 db over a band of 20 mc and more than 28 db 30 mc on 

 each side of the midband frequency. The design formulae indicated that a 

 filter consisting of four cavities would be needed. These, then, would take 

 the general configuration shown in Fig. 18, where the first and last cavities 

 are formed by the obstacles jBi and the length of fine A , while the two mid- 

 dle cavities are formed by the obstacles JB2 and the length 4 . The lengths 

 /12 and ^2 correspond to the transforming sections of transmission line which 

 connect the cavities together. The loaded Q's required to meet the speci- 

 fications turned out to be ^1 = 12.25 and Q2 = 30.0, after allowance had 

 been made for the selectivities of the f wavelength connecting sections. 

 Assuming that the cavities would be formed with inductive obstacles, as 

 shown schematically in Fig. 19, the susceptances to obtain these selec- 

 tivities were obtained from Fig. 11 based on equation 32. This gave 



Bi = -4.08 



B2 = -6.36 



