Harmonic Suppression by Using T-shaped Spur-Line in a Compact Hairpin-Line Bandpass Filter

This article exhibits the design of a fourth-order compact hairpin-line filter centered at 2.5 GHz and 3 dB fractional bandwidth of 5% along with a sharp roll-off factor and wide stopband characteristics required for Wireless Local Area Network (WLAN). Miniaturization of the conventional hairpin-line filter has been achieved by folding the open end arms twice towards the inward direction and accordingly, a size reduction of 42% has been obtained. Subsequently, T-shaped spur-lines with optimum dimensions have been incorporated at both the inner and outer edges of the coupled arms of the folded hairpin-line cell. Accordingly, two fourth-order folded filters with outer T-shaped and double T-shaped spur-lines have been designed and verified experimentally. An extended stopband with a rejection level of 39 dB up to 3.2f_0 along with a size reduction of 46% has been achieved