Wide-stopband and high selectivity step impedance resonator bandpass filter using T-network and antiparallel coupled line

A wide-stopband attenuation with high-selectivity step impedance resonator (SIR) bandpass filter (BPF) is presented in this study. The T-network stubs are used to produce transmission zeros (TZs) close to the centre frequency (f₀) with a high-selectivity. Moreover, the antiparallel coupled lines can produce a TZ at spurious frequency of conventional SIR BPF for all step impedance ratio (K). For the validation, a half-wavelength (λ/2) SIR BPF with T-network stubs at the input/output ports was designed at f₀ of 2.6 GHz. From the experiment, the insertion and return losses at f₀ were determined to be 0.91 and 21.25 dB, respectively. The insertion and return losses were better than 1.1 and 16.5 dB, respectively, of the 180 MHz bandwidth. The TZ frequencies (f_Z1, f_Z2, and f_Z3) were located at 1.69 GHz (0.63f₀), 2.98 GHz (1.12f₀), and 3.17 GHz (1.19f₀), respectively, with a high selectivity. The stopband attenuation was better than 25 dB from DC to 2.23 GHz and from 2.89 to 9.82 GHz.