Compact box-section bandpass filter with enhanced stopband characteristics

A compact bandpass filter (BPF) with enhanced stopband characteristics is presented in this paper. The proposed BPF consists of four improved stepped impedance resonators (SIRs), which implements into a box-section structure. To improve the frequency selectivity and the stopband characteristics, additional transmission zero (TZ) was introduced by mixed electric and magnetic (EM) coupling. A new filter with 31.2% fractional bandwidth (FBW) has been designed and fabricated to verify the validity of the proposed method. The filter has two transmission zeros, where TZ₁ is located in 0.4 GHz with 78.7 dB rejection and TZ₂ is located in 2.7 GHz with 50.2 dB rejection. Measured results are in good agreement with the electromagnetic simulation.