High isolation diplexer using quarter-wavelength resonator filter

A microstrip diplexer with very high isolation is presented. The diplexer consists of two compact filters with quarter-wavelength resonators coupled by the metallised vias. Each metallised via is inductive and behaves as a K-inverter. This type of filter has two adjustable transmission zeros on each side of the passband, which can be shifted by changing the coupling coefficient between the source and load. By locating the transmission zero of one channel at the passband of the other channel, very high isolation of the diplexer can be achieved. Simulated results of the fabricated diplexer agree well with measured results.