A compact broadband microstrip bandpass filter using folded multiple-mode resonator

On the basis of coupledline sections and impedance steps as inverter circuits, novel wideband and compact filters are presents. The compact broadband microstrip filter using a folded multiple-mode resonator contains only one resonator, which operates as not only a resonant element, but also an open stub. In addition, the filter owns a compact size, a broad passband, sharp skirts and two transmission zeros either below or above the passband. For the prototype filters, center frequency around 20GHz was selected. Bandwidth is between 10.6 to 28 GHz, and insertion loss amounts to around 0.5 dB. For the selected center frequency and on a substrate with a dielectric constant of 2.2, the smallest microstrip filter is only 15*5 mm² in size.