An Ultra-Wideband Bandpass Frequency Selective Structure Based on Hybrid Multimode Resonators

In this paper, an ultra-wideband (UWB) bandpass frequency selective structure (FSS) based on hybrid multimode slotline-microstrip-slotline resonators is presented. The proposed FSS consists of a periodic array of stacked slotline-microstrip-slotline structures, which can achieve mode conversion between the incident spatial waves and the guided waves. Then, by constructing multimode resonators in the hybrid slotline-microstrip-slotline structures, a UWB bandpass response can be achieved with multiple in-band transmission poles. Furthermore, by introducing cross-couplings between the first and last slotline resonators, a pair of transmission zeros are generated at both sides of the desired passband to enhance the roll-off-slopes and the out-of-band performance. In order to verify the concept, a proposed FSS example is designed and fabricated. The measured results exhibit UWB bandpass filtering responses with seven inband transmission poles and a pair of transmission zeros close to the passband, which indicates 117% fractional bandwidth.