A Dual-Band Cavity Bandpass Filter using Interdigital Technique

This paper proposes a dual band cavity bandpass filter (BPF) using interdigital technique on irises. The proposed cavity filter was designed at the fundamental frequency of 3.9 GHz (C-Band) and the second resonance frequency could be shifted to the frequency of 5.2 GHz (WLAN) by the effect of interdigital irises. The capacitive values of the interdigital iris can be varied with the finger gap and its impedance, resulting in the second resonance frequency shifting. Moreover, using the interdigital technique can reduce the filter size form lambda/2 to lambda/4 caused by the slow wave effect on each cavity order. This dual band cavity BPF was made from brass material with the thickness of 1.5 mm for high power handling. A good filter performance is obtained with insertion loss (S-21) of 0.73 dB and return loss (S-11) of 15 dB.