Compact Balanced Bandpass Filter Design Using Asymmetric SIR Pairs and Spoof Surface Plasmon Polariton Feeding Structure

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This letter demonstrates a design of a compact balanced bandpass filter (BPF) with stopband extension and common-mode (CM) suppression using asymmetric stepped-impedance resonator (SIR) pairs and a spoof surface plasmon polariton (SPP) feeding structure. The configuration of balanced SIR pairs is analyzed theoretically to obtain a wide stopband response and intrinsic CM suppression within differential-mode passband. Moreover, a novel SPP feeding structure is adopted as a low-pass filter and impedance matching section for enhancing filter transmission responses. Finally, a balanced BPF operating at 0.85 GHz with the SPP feeding structure is designed and fabricated to verify the design method.