Compact CHMSIW filters with TZs

Novel capacitively loaded half-mode substrate integrated waveguide (CHMSIW) filters with transmission zeros (TZs) are presented in this study. The CHMSIW is proposed by embedding a metallic stripline and inductive post in the HMSIW, which introduces a series inductor-capacitor (LC) branch circuit in waveguide thereby generating TZ above the passband. Furthermore, as the embedded stripline introduces additional capacitance, a significant size reduction can be achieved for the CHMSIW in comparison with conventional HMSIW. Two types of CHMSIW resonator topologies are developed by varying the number of LC branches, with multiple TZs achieved by inserting multiple LC branches in waveguide. On the basis of the CHMSIW resonators, compact bandpass filters are designed and fabricated, where TZs are obtained in the upper stopband and a sharp transition is achieved from the passband to the stopband. To further improve the selectivity, mixed coupling is introduced in CHMSIW filters to generate additional TZs in the lower stopband. A prototype of the mixed-coupled CHMSIW filter is designed and tested, and high selectivity as well as compact size and good out-of-band rejection have been achieved for the proposed mixed-coupled CHMSIW filter.