Design of Quasic-Elliptic Bandpass Filter and Diplexer Using Inverted-L-Shape Coaxial Resonator for Realization of Electromagnetic Mixed Coupling

Electromagnetic (EM) mixed coupling creates transmission zero (TZ) either at lower or upper stopband of bandpass filter (BPF) with inline configuration of resonators. This is a very attractive feature for designing quasic-elliptic BPF. However, traditional mixed coupling needs extra complicated coupling structure among two resonators which increase the design complexity, larger the circuit size and higher the tooling cost. This study proposes a very simple design approach of mixed coupling without the need for additional dedicated coupling components. An inverted-L-shape resonator (ILSR) is proposed for this purpose. By only properly arranging the orientation of two ILSRs, mixed coupling can be effectively controlled and thus arbitrarily places TZ either at lower or upper stopband. This feature facilitates the realization of quasic-elliptic BPF response with high selectivity and inline filter configuration. It is noteworthy that for an Nth-order filter, it is possible to attain a maximum of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N -1$ </tex-math></inline-formula> finite TZs. For rapid prototyping and experimental demonstration, a fourth-order BPF and a third-order diplexer were designed and fabricated using additive manufacturing 3-D printing technology. Results indicate excellent agreement between all test outcomes and simulation results, thereby validating the proposed methodology.