Compact and highly selective dual/tri-band BPFs using folded T-shaped stub-loaded resonators for WLAN and WiMAX applications

In this article, a series of dual-/tri-band bandpass filters (BPFs) using folded T-shaped stub-loaded resonators (TSSLRs) are presented for WLAN and WiMAX applications. The proposed TSSLR is constructed using two λ/4 resonators and a T-shaped folded stub. A controllable transmission zero will be generated at the desired transition band leading to sharp roll-off skirts and excellent out-of-band suppression by adjusting the length of the folded stub. After combining two or three TSSLRs with flexible feed-lines, very compact overall sizes of 0.29 λ_g × 0.16 λ_g, 0.30 λ_g × 0.22 λ_g for dual-band BPFs and 0.28 λ_g × 0.19 λ_g, 0.30 λ_g × 0.22 λ_g for tri-band BPFs can be obtained. The measured results of the two dual-band BPFs (I and II) show excellent characteristics of low insertion loss of 1.2/0.87 and 0.7/0.77 dB and wide fractional bandwidth (FBW) of 15.3%/24.2% and 28.7%/16.3%, respectively. Insertion losses of the tri-band BPFs (III and IV) based on simulation results are approximately 0.2 dB and FBWs range from 10.34% to 15.02%, which prove excellent performance and further verify the design method. Furthermore, ultra-high roll-off factors of 54 dB and 67.5 dB per 100 MHz have been achieved in simulation results of the tri-band BPF IV's 1st and 2nd up transition bands, respectively.