3D-Printed Waveguide Bandpass Filter with Locally Resonant Cross-Shaped Metamaterial for Improved Out-of-Band Rejection

This paper presents a waveguide bandpass filter (BPF) consisting of locally resonant cross-shaped metamaterial for improved out-of-band rejection. The proposed metamaterial unit cell, which is included in the waveguide’s filter section, induces increased capacitive stored electric energy density, thereby achieving sharp out-of-band (OOB) rejection. A cross-shaped metamaterial unit cell is positioned in the filter section, while the impedance matching section, termed meta-ports, is placed adjacent to the filter section. The length of the proposed waveguide BPF is 2.49λ at the lowest operating frequency, which is shorter than the other waveguide BPF filters proposed in the literature. A prototype of the proposed design is built using 3D printing technology, along with copper plating, to reduce the relatively heavy weight of waveguide filters. The measured results show that the proposed filter achieved a reflection coefficient of less than -10 dB and a transmission coefficient fluctuation of less than -1 dB in the 7.25–7.75 GHz passband. Moreover, an OOB rejection of more than -40 dB was achieved—higher than that attained by previously reported 3D-printed waveguide BPF filters.