Wideband coaxial filters with impedance matching for VHF/UHF diplexer design

This paper presents a novel synthesis process for wideband coaxial filter terminated with capacitors. At very high frequency and ultra-high frequency, we make use of helix as series inductor, dielectric disk as series capacitor, gap between inner metal disk and outer metal tube as shunt capacitor. By using the proposed synthesis technology and the extracted element formulas, two wideband quasilumped coaxial filters within different frequency ranges are designed and fabricated. Then they are directly connected to form a diplexer by using coaxial lines as impedance transformers. The diplexer can be used in a transceiver system combining two signals at 235 similar to 280 MHz (VHF) and 315 similar to 360 MHz (UHF) into one signal before the transmitted antenna.