Circuit model for compact wideband combline resonators using interresonator taps and capacitive loading

In this paper, design of wideband microwave filters in combline configuration using interresonator taps and interdigitally-coupled capacitive loading is described. The design is based on increasing the loading capacitance at the open end of each combline resonator, thus reducing the resonant frequency and hence it is required to reduce the physical length to get the desired frequency. The filter is physically compact and its spurious response is pushed away from the edge of passband. In this work, a simple circuit model comprised only of lumped elements is developed to model the interresonator tapped-in coupling mechanism between symmetric resonators that are capacitively loaded with interdigital capacitors. This circuit model is used to explain the behavior of coupling phenomena and its relation to the physical parameters of coupling section.