Absorptive Bandpass Filter with a Pair of Transmission Zeros

In this paper, the design approach for an absorptive bandpass filter with a pair of transmission zeros is introduced. The design is based on using dangling resonators coupled to the output port. One of the unique features of the proposed design approach is that the resonant frequencies of the dangling resonators determine the locations of the transmission zeros. In addition, the design method allows us to relocate the transmission zeros without degrading a reflectionless response at the input port. For verification, we have designed three third-order microstrip line filters centered at 3 GHz having 10 % bandwidth. The filters have been designed to have one transmission zero in the lower stopband and one in the upper stopband. For demonstrating the relocation facility, the filters have been designed to have different sets of transmission zeros. The filters have been fabricated and measured for verifying the presented design method.