Dual-plane direct-coupled bandpass filters with open-ended stubs based on CRLH zeroth-order resonators

This study presents a direct-coupled bandpass filter with open-ended stubs based on composite right-left handed zeroth-order resonator (CRLH-ZOR). The design approach starts from the conventional bandpass filter on the foundation of admittance inverters and shunt open-ended stubs. The inverters which provide couplings between adjacent resonators are accomplished by direct-connected transmission lines equivalent to quarter-wave lines. The coupled-resonator method can be utilized for designing the interstage couplings by introducing a simple conversion. A balanced dual-plane CRLH TL open-ended on one side, serves as ZOR stub with shunt resonance is adopted as resonating components. By appropriately combing the direct-connected TLs and the ZOR open-ended stubs, a new dual-plane direct-coupled filter can be constructed. Specifically, a filter with center frequency of 1.5GHz and bandwidth of 32.05% is validated by theory and experiment. The experimental results are well-predicted by full-wave simulations.