High-Selective Bandpass Filters Based on New Dual-Mode Rectangular Strip Patch Resonators

The resonant characteristic of rectangular strip patch resonator (RSPR) is studied in detail, and a new dual-mode resonator based on TM
₁₀₀
and TM
₀₂₀
modes is proposed to design bandpass filters (BPFs). A two-pole BPF with two finite transmission zeros (FTZs) based on one nondegenerate dual-mode RSPR is presented and analyzed. And then, a four-pole BPF with four FTZs based on two cascaded nondegenerate dual-mode RSPRs is proposed. The two dual-mode RSPRs are connected directly by a quarter wavelength strip-line. Finally, a four-pole BPF with the center frequency of 5.8 GHz is designed, fabricated, and measured. The measured insertion loss in center frequency, 1-dB bandwidth, and return loss are 1.45 dB, 206 MHz, and 12.85 dB, respectively. Measured
$Q$
-factor is about 315.