Design of 3-D Printed Coaxial Filter Based on Modified Two-Section Stepped Impedance Resonator

This letter proposed a kind of modified two-section coaxial stepped impedance resonator. The analysis expression was given for the first time to obtain the fundamental resonant frequency ( $f_{0}$ ). In addition, the simulated results showed that the modified resonator can achieve more compact volumes, wider spurious stopband, and higher unloaded quality ( $Q_{u}$ ) factors compared to the published works. A four-pole bandpass filter was designed for validation, which presents the $2216~Q_{u}$ , $8.4~f_{0}$ stopband range with an electrical length (EL) of ~1/15 wavelength. Direct 3-D metal printing technique was adopted to manufacture the filter for a rapid experimental demonstration. As a result, the measured results agreed well with the simulated ones.