Bandstop filter based on composite right/left handed transmission line at dual WLAN bands

Abstract

A compact dual-band bandstop filter (BSF) based on composite right/left handed transmission line (CRLH TL)is proposed and designed without any external impedance matching block. The filter is to suppress the interference of the signal at 5.2 and 5.8 GHz, which is the Unlicensed National Information Infrastructure, radio following the IEEE 802.11a standard and system. The filter is realized by using interdigital capacitor coupled with shorted stub periodic structure based on the characteristic of CRLH TLs. Single and dual-band BSFs are designed, fabricated and measured to validate the design principle. The filter is designed in single layer and the length of the resonators is 3.48 and 4.08 mm, corresponding to the filters operating at 5.8 and 5.2 GHz, respectively. The total length of the filter is 28.89 mm. Designed topologies of dual-band BSFs with centre frequencies of 5.2 GHz and 5.8 GHz exhibit an rejection better than 15 dB with a 10 dB bandwidth of 2.5% (5.125-5.255 GHz) and 3.2% (5.726-5.907 GHz), respectively. The measured largest signal rejections are 15.98 dB at 5.23 GHz and 15.98 dB at 5.6 GHz. The 10-dB fractional band-width (FBW) is 3% from 5.14 GHz to 5.30 GHz and 1.25% from 5.57 GHz to 5.64 GHz, and the measured insertion loss is 2.74 dB at 5.4 GHz.