High-frequency filter using coplanar line resonator

A high-frequency filter comprising:

• a substrate;
• a ground conductor disposed on one main surface of the substrate and having an opening;
• a center conductor disposed on one main surface of the substrate in the opening and making up a coplanar line resonator of a coplanar structure together with the substrate and the ground conductor;
• an input line of a microstrip line structure, disposed on the other main surface of the substrate and electromagnetically coupling with one end of the center conductor;
• and
• an output line of a microstrip line structure, disposed on the other main surface of the substrate and electromagnetically coupling with the other end of the center conductor:
• wherein at least one of the input line and the output line has a closed loop line portion surrounding a corresponding end of the center conductor and crossing the center conductor transversely through the substrate;
• and
• wherein on the other main surface of the substrate, a stub overlapping with the center conductor is arranged from a transverse portion where the closed loop line portion crosses the center conductor;
• wherein the closed loop line portion forms a first attenuation pole in a band characteristic of the coplanar line resonator and the stub forms a second attenuation pole in the band characteristic,
• wherein the first attenuation pole and the second attenuation pole are positioned at both sides around a center frequency of the band characteristic of the coplanar line resonator.