Rigorous design method for distributed-element bandpass filter with reflectionless response at two ports

This letter reports an accurate and rigorous design method for a distributed-element reflectionless bandpass filter capable of producing a reflectionless response at two ports simultaneously. The design method relies on novel transmission-line reflectionless filter prototypes. This work presents a stub-based reflectionless filter prototype and its design formulas in terms of a target transmission response. It also provides a stub-less coupled-line reflectionless filter prototype equivalent to the stub-based prototype in case where stubs are not feasible. A second-order bandpass filter has been designed and fabricated to verify the design method. Reasonable agreement between the measured performance and the simulated one has been obtained, confirming that the proposed design is valid.