Analysis, Design, and Implementation of Miniaturized Multimode Waveguide Filters Based on Epsilon-Near-Zero Channel Concept

This article presents two miniaturized multimode waveguide bandpass filters (BPFs) based on epsilon-near-zero (ENZ) channel concept. By connecting parallel shorting stubs on the ENZ channel, multiresonance modes and one transmission zero (TZ) are introduced, broadening the bandwidth of the passband and upper stopband, respectively. The effect of arbitrary waveguide discontinuity on this resonant mechanism is analyzed, and the effective size reduction of the third-order filter is achieved with the folded ENZ channel. The design procedure and miniaturization of the proposed BPFs are presented in detail. The proposed second-order filter shows a 6.8% 3-dB fractional bandwidth (FBW) with a center frequency of 9.6 GHz, an insertion loss of 0.42 dB at 9.6 GHz, and an out-of-band rejection of 30 dB up to 12 GHz. Especially, with the filter thickness of 2.3 mm ( 0.245 λ
_g
at 32 GHz), the fabricated third-order filter shows a 6.2% 3-dB FBW with a center frequency of 32 GHz, an insertion loss of 0.6 dB at 32 GHz, and an out-of-band rejection of 30 dB up to 36 GHz. Simulated and experimental results are of good agreement.