Band-Splitting Filter

T h e long-distance waveguide communication system will handle an extremely broad band extending perhaps from 40 to 80 kmc. 1 For regeneration and amplification this band must be divided into channels around 400 mc apart. Promising filters capable of performing this channel separation have been described elsewhere, 2 ' 3 ' 4 b u t it is improbable t h a t satisfactory filtering can be obtained if approximately 100 channel-dropping filters are to be stacked one after another. T h e main reasons for possible trouble are: ( a ) Resonance of unwanted modes. This occurs because some of the filters are required to operate over a range of frequencies covering more t h a n one octave. ( b ) Multiple reflections. Although the reflection from each filter is small, the combined reflection of as m a n y as 100 m a y become prohibitively large at discrete frequencies. Troubles from these sources can be reduced by dividing the broad 40-kmc band into several subbands. T h e width of t h e subbands can be adjusted to accommodate a suitable number of channel-dropping filters. This paper describes a filter capable of dividing a band in two parts. It can easily handle one octave in the millimeter region with low insertion loss because it operates with low loss mode TE 0 i° mostly in 2 inch diameter waveguide. T h e splitting process can be repeated as m a n y times as necessary by cascading similar filters. 197