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 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{0}$</tex-math> </inline-formula>). In addition, the simulated results showed that the modified resonator can achieve more compact volumes, wider spurious stopband, and higher unloaded quality (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{u}$</tex-math> </inline-formula>) factors compared to the published works. A four-pole bandpass filter was designed for validation, which presents the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2216~Q_{u}$</tex-math> </inline-formula>, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.4~f_{0}$</tex-math> </inline-formula> 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.