Compact LHM-based band-stop filter

In this paper, a compact, low-insertion-loss, high selective band-stop filter is proposed. The filter is composed of four unit cells, two of which are anti-symmetrically connected left-handed metamaterial (LHM) unit cells in combination with their mirror images to obtain the symmetric return losses at both ports. The design of LHM unit cell is based on direct connection of spiral resonator and thin wire to decrease the band-stop frequency further than the conventional split ring resonator designs. The dimensions of the proposed structure are as small as 9.7 mm × 20 mm, while high frequency selectivity could be achieved due to the transmission zeros. The filter has an insertion loss of better than 2 dB even with highly lossy FR4 substrate, and a return loss of larger than 10 dB in the passbands of 2.41-2.65 GHz and 4.26-5.72 GHz. The band rejection of greater than 30 dB is numerically calculated within 3.16-3.54 GHz with high frequency selectivity of 69 dB/GHz.