A Four-pole Bandpass Filter with Wide Bandwidth and Sharp Skirt Using a Single dielectric-loaded Cavity Resonator

This paper presents a novel four-pole-two-zero bandpass filter using a multi-mode dielectric-loaded cylindrical cavity resonator. The resonator is constructed by a hollowed cylindrical dielectric disk loaded at the center of a cylindrical cavity, and fed by a pair of probes with 180 degrees placement. Four resonant modes
$\text{(TM}_{010,} \text{TE}_{111}{}^{\mathrm{e}}, \text{TE}_{211}{}^{\mathrm{e}}, \text{TM}_{110}{}^{\mathrm{e}})$
can be gathered by properly tuning the lengths of probes and the radius of the dielectric disk. A transmission zero can be generated at both the upper and lower stopband. In addition, a wide spurious window can be achieved by embedding a metallic cylinder within the hollowed region of the dielectric disk. For demonstration, a four-pole filter operating at 3.5 GHz was designed, fabricated, and measured. A good agreement between the simulation and measurement can be observed. The measured result suggests a low insertion loss of 0.32 dB with a wide fractional bandwidth of 45.6%. Two transmission zeros can be observed at the upper and lower stopband for good selectivity.