An Improved Design of Waveguide Band-Rejection Filters

T h e m i c r o w a v e w a v e g u i d e b a n d - r e j e c t i o n filter ( B R F ) n o w used i n m a n y r a d i o s y s t e m s h a s m a n y u n d e s i r a b l e f e a t u r e s . T h e designer finds t h a t t h e a c t u a l b a n d w i d t h i s c o n s i s t e n t ^ n a r r o w e r t h a n t h e designed value, a n d t h a t t h e filter h a s a u n i q u e p a s s b a n d V S W R b e h a v i o r which b e c o m e s worse a s t h e f r e q u e n c y goes f a r t h e r awaj^ f r o m t h e l 2 T H E BELL SYSTEM T E C H N I C A L JOURNAL, JANUARY 1008 stopband. This becomes quite a severe problem in filters designed for the high frequency or low frequency channels of the band. (For example, in the 4 GHz band,* the return loss of a filter with the stopband at 3710 MHz becomes progressively poorer as the frequency approaches 4190 MHz, and vice versa). In addition, the midband frequency of the VSWR curve and of the corresponding delay curve is found to differ from the midband frequency of the insertion loss curve at which the filter is tuned. The offset of a typical 3-cavity maximally flat B R F at the 4 GHz band with 3 dB points at ± 1 7 MHz can be as much as 2 MHz. Such an uncontrolled shift causes extreme difficulties in the delay equalization of a radio system. The present B R F design is based on the lumped-element low-pass prototype filter design technique. 1 After making a proper frequency transformation, the loaded Q of each cavity can be computed. The cavities are then separated by waveguide lengths which give an effective spacing of an odd multiple of quarter wavelengths at the midband frequency, which is a standard technique to realize ladder filters in waveguide.