HIS‐based bandpass filter with improved upper‐stopband performance

A bandpass filter (BPF) with improved upper‐stopband performance has been studied and implemented with the aid of a ground plane based on high impedance structures (HIS). The microstrip structure of the filter on the top layer consists of three‐order cascaded cross‐shaped resonators loaded by folded split ring slots. The ground plane contains two groups of 2 × 5 lattice metallic mushroom array, showing high impedance at the spurious band so that the upper‐stopband bandwidth of the BPF can be effectively broadened. The HIS‐based ground can independently control the stopband without affecting passband performance. Experimental results demonstrate that the BPF exhibits an extremely broad stopband region from 15.5 to 49.2 GHz with a rejection level above 20 dB.