Design of a patch dual-mode bandpass filter with second harmonic suppression using open stubs

In this article, a harmonic suppressed patch dual-mode bandpass filter (BPF) with open stubs is presented. For harmonic suppression, equivalent T-shaped transmission lines replace connecting transmission lines. The added shunt open stubs in the T-shaped lines work as band-stop filters (BSFs) at the harmonics. A dual-mode filter is designed and fabricated with fractional bandwidth of 5 % at 2 GHz with an insertion loss of less than 2.5 dB and a return loss of better than 10 dB, while the second passband is located around 2.3f₀ (4.61 GHz). Compared with the conventional square patch filter, the proposed filter also shows a size reduction.