A compact elliptical substrate integrated waveguide balanced bandpass filter with high-selectivity

A novel quarter-mode elliptical substrate integrated waveguide resonator (QMESIWR) is proposed to design a compact balanced bandpass filter (BPF) with high selectivity. Metal columns are integrated to enhance common-mode suppression. By leveraging source-load coupling (SLC), spurlines, and coupled bandgaps, five transmission zeros (TZs) are generated outside the passband, improving frequency selectivity. C-slot lines and gap coupling strengthen dual-resonator coupling, further optimizing performance. Measurements align with simulations, demonstrating operation at 4.2 GHz (3 dB bandwidth: 180 MHz) with >27 dB in-band common-mode suppression and >70.8 dB out-of-band suppression. This design proves suitable for complex modern electromagnetic environments.