Band-pass filters are used to attenuate frequencies outside a narrow pass-band. They are commonly used as components in the design of combiners, for the attenuation of noisy side bands, to achieve additional decoupling between transmitters installed in close proximity in the same antenna system, and to combine digital channels with existing analogue channels.
The BF4-3Cppp band-pass filter consists of three magnetic coupled coaxial resonators with adjustable coupling and BF4-4Cppp with four and BF4-6Cppp with six. One end of the resonator is open-circuited and the other short-circuited, and tuning is accomplished by rotating the circular central conductor at the lower end of each resonator. The coaxial input and output lines are coupled to the first and last resonator by magnetic coupling loops.
The band-pass filter is factory tuned to the desired operating channels. All filters are retunable on whole frequency bands IV and V.
All these filters are suitable to operate as an output filter of the transmitter, most commonly are used as components of combiners (starpoint or constant impedance combiners).