Compact Hairpin-Line Bandpass Filter with Harmonic Suppression by Periodic Grooves

The present paper highlights the design of a third-order double-folded hairpin line Chebyshev bandpass filter centered at 2.5 GHz with improved second harmonic suppression performance applicable for Wireless Local Area Network (WLAN). The conventional hairpin-line filter has been folded twice at the open end arms and accordingly, a size reduction of 19% has been obtained. The skirt characteristics have been improved and the second harmonic has been shifted to higher frequency due to the folding mechanism. However, the harmonic attenuation level remains almost unaffected. Subsequently, square grooves with optimum dimensions and periodicity have been employed at the outer edges of the coupled resonators to achieve phase velocity compensation between even- and odd-modes. As a result, more than 40 dB of harmonic suppression along with size reduction of 25% has been achieved with three periodic square grooves.