Accurate Synthesis of Input-Reflectionless Dual-Passband Filter

This article presents a rigorous synthesis method for input-reflectionless dual-passband filters. As this work provides closed-form formulas for the normalized coupling values of both the filtering and matching sections for producing zero reflection at all frequencies, they can be obtained instantly without using time-consuming trial-and-error methods. As the formulas are generalized for a case with arbitrary cutoff frequencies in the normalized frequency domain, an input-reflectionless filter with two passbands separated arbitrarily can readily be synthesized and designed. For demonstration, the normalized coupling values obtained by using the proposed synthesis method have been applied to the design of a <inline-formula> <tex-math notation="LaTeX">$Ka$</tex-math> </inline-formula>-band waveguide input-reflectionless dual-passband filter. The measurement shows a reflectionless feature over the entire WR-42 band which verifies our synthesis and design approaches.