706 



BELL SYSTEM TECHNICAL JOURNAL 



These susceptances were realized with centrally located round posts, for 

 which the data of Fig. 17 has been plotted, and this filter was constructed 

 according to the calculated dimensions which are shown in Fig. 20. Each 

 of the four cavities was tuned separately to resonance near midband by ad- 

 justing a capacitive plug located in the center of each. The characteristic 

 then obtained is plotted in Fig. 21, which shows that the standing wave 



|-£i--|*-£i2-*|*-£2'*|*-e22-*j*-£2-*|*-£i2-^£i-^ 



Ll 1-2: 



L2 L2 



:l2 l, 



Fig. 19 — A four-cavity filter which utilizes inductive obstacles. 



-J 1.980 L J 2.090 L- J2.O90L J 1.960 L- 



I »-3.191-^ |*-3.246-i| |*-3.191-*j | 



|?T T?| |9| »^ 



0.112" D 



■TUNING PLUGS 



0.112" D 



Fig. 20— The calculated dimensions for a four-cavity maximally-flat filter in 0.872" x 

 1.872'' rectangular waveguide. 



"4010 40 30 4050 4070 4090 



FREQUENCY IN MC 



Fig. 21 — Measured characteristic of four-cavity filter of Figure 20. 



ratio met the design points quite well. These are shown as circles in the 

 figure. The insertion loss of this filter was less than 0.7 db over a 25-mc 

 band and less than 0.3-db at midband. 



Another maximally-flat waveguide filter consisting of fifteen resonant 

 cavities gave an insertion loss of two decibels at midband, 4-db loss at 20- 

 mc bandwidth and 40-db loss at 30-mc bandwidth. The input standing 

 wave ratio was less than 1.0 db over a 20-mc band. Its characteristics are 

 plotted in Figs. 22 and 23. This excellent performance is remarkable in 



