Compact lowpass filter with wide stopband using coupled rhombic stubs

A compact and high selective lowpass filter (LPF) with wide stopband is presented using coupled rhombic stubs. The proposed LPF unit is constructed by loading a rhombic stub on a transmission line. Then, a folded three-element LPF is proposed to achieve compact size and high selectivity by introducing coupling between adjacent rhombic stubs. The operating mechanism of the filter is investigated and explained, by adopting the physically-based LC-equivalent circuit model. The implemented final LPF exhibits a wide stopband of up to 11.5 f_c with a rejection level of better than 35 dB. Furthermore, the final LPF features a compact size of 0.12λ_g × 0.10λ_g, where λ_g is the guide wavelength of f_c, and a very high figure-of-merit (FOM) of 27 142.