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 strcture. 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 TZ(1) is located in 0.4GHz with 78.7dB rejection and TZ(2) is located in 2.7GHz with 50.2 dB rejection. Measured results are in good agreement with the electromagnetic simulation.