Out-of-band improvement by BPFS with multiple attenuation poles using a condition of variable coupling length of a parallel partially coupled-line section

A combination of two microstrip-line resonators, which are partially coupled, are considered in a parallel partially coupled-line bandpass filter (BPF), where one resonator is one half-wavelength (λ/2) long, and the other (whose one end is grounded) is only one quarter-wavelength (λ/4) long. Therefore, the length of a coupled-line section can be varied based on the position of the grounded end. Five conditions of the movable coupling length are simulated, which greatly influences the spurious responses of the BPF. This property is numerically investigated in this paper. The analysis shows that, based on the grounding position, this method is capable of realizing improved out-of-band characteristics by locating the multiple attenuation poles in the stopband and improved spurious responses up to five times of the pass-band frequency (5f₀) of a parallel partially coupled-line BPF. An empirical model of the BPF is fabricated, and the numerical results are verified via comparison with the experiment.