Impedance matching bandpass filter with a controllable spurious frequency based on λ/2 stepped impedance resonator

An impedance matching bandpass filter (BPF) with arbitrary image impedance (Z₂) of parallel/antiparallel coupled lines SIR is presented in this study. The proposed structures consist of conventional SIRs and additional transmission lines with an electrical length of π/2-θ₂ at the input and output ports. Moreover, the coupling coefficients of the coupled line can be controlled by the image impedance and it does not affect the bandpass responses. Since the first spurious frequency of the conventional SIR BPF can be suppressed with a transmission zero by using antiparallel coupled lines, the proposed BPF can provide wide stopband and high attenuation characteristics. For the experimental validation of the proposed filter, three types of BPFs with 20-50 and 50-100 Ω termination impedances were designed and fabricated at the operating centre frequency (f₀) of 2.6 GHz. By using the antiparallel coupled line instead of the parallel coupled line, the first spurious frequency is occurred at 3.71f₀ and the stopband rejection considerably improved.