Design of Miniaturized Microstrip LPF and Wideband BPF With Ultra-Wide Stopband

This letter presents a novel lowpass filter (LPF) structure which consists of a section of high-impedance microstrip line (HIML) with a pair of radial stubs (RSs) loaded at its center and a pair of stepped-impedance open stubs (SIOSs) loaded on both ends of HIML. The RSs loaded HIML exhibits a lowpass property and has a wide stopband. The loaded SIOSs not only can improve the roll-off rate of LPF significantly but also is able to extend the stopband. Based on such a LPF structure, a pair of highpass shorted HIMLs are paralleled on both sides of LPF to construct a wideband bandpass filter (BPF) with wide stopband. To validate the proposed method, two filters, i.e., a LPF with cutoff frequency f_c at 1.07 GHz and a BPF centered at f₀ = 0.89 GHz with -3 dB fractional bandwidth of 87%, are designed and fabricated. The measured results show the fabricated LPF has a -20 dB isolation bandwidth from 1.27 f_c to 13.7f_c while the fabricated BPF has a -20 dB isolation bandwidth from 1.56 f₀ to 15 f0. Moreover, the fabricated LPF and BPF also have the compact size of 0.107 λ_gc ×0.083 λ_gc and 0.123 λ_g0 ×0.073 λ_g0, respectively. Good agreement can be observed between the simulation and measurement.