Design of a compact ultra-narrow band dual band filter for WiMAX application

A novel narrow-band bandpass filter with two fully controllable passbands is proposed in this letter. The filter is designed through a systematic, yet simple approach, in which two independently-controllable passbands are realized using two different coupling paths. Each passband can be tuned independently only by changing one parameter of the filter. The measured and simulated results are in good agreement, showing, two passbands cantered at 3.5 and 5.8 GHz with fractional bandwidths of 0.8% and 1%, respectively. The return loss in the first and second passbands are better than 19 dB. The filter exhibits a very good out of band rejection performance, suppressing spurious bands up 16 GHz. The demonstrated filter has a compact size of 0.21λg × 0.13λg, where λg is the guided wavelength at 3.5 GHz.