New Dual-Mode Ring Bandpass Filter Using Symmetrical Left-Handed Transmission Line

This paper presents a new dual-mode microstrip ring bandpass filter (BPF) based on the left-handed transmission line (LH-TL). To achieve dual-mode response, a high impedance LH-TL can easily be realized to perturb a pair of degenerate modes, which difficulty is realized in the conventional dual-mode ring filter. The proposed BPF is designed and fabricated on an FR-4 substrate to suit the 2.45 GHz ISM band application with a fractional bandwidth of 16.2 percentages. Across the bandwidth, the measured insertion loss is less than 1.5 dB, whereas the input and output return losses are better than 20 dB from 2.37 to 2.59 GHz. The measured results are in good agreement with the simulated results.