A method is proposed for a half-mode substrate integrated waveguide (HMSIW) bandpass filter (BPF) to obtain dual-band below the cutoff frequency. Complementary split-ring resonators (CSRRs) and microstrip open-stubs are integrated on the top of an HMSIW cavity. The structure produces two center frequencies both below the cutoff frequency of the originalHMSIW without increasing the cavity size. Results indicate that the center frequencies are 2.95 GHz and 3.99 GHz, and 3 dBfractional bandwidths (FBWs) are 9.1% and 4.6%, respectively. There is a transmission zero between the two frequencies, which enhances the out-of-band suppression performance. The measured results are in good agreement with the simulated ones. This new combination not only obtains two usable passbands below the cutoff frequency, but also makes the filter more compact. The filter has some practical and application significance.
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