Miniaturised single-layer bandpass filters using triple and quad-mode patch resonators

The authors propose miniaturised single-layer bandpass filters (BPFs) by utilising a kind of new triple- and quad-mode patch resonators. The triple-mode resonator consists of a shortened square patch and a loaded metallic via at the centre. Characteristics of the new resonator are studied based on the mode analysis, and the first three resonant modes can be controlled well by changing the length of four via arms and the width of the open slot. Thus, a three-pole BPF can be designed with the new triple-mode patch resonator. To realise higher-order filter, two triple-mode patch resonators are cascaded to design a six-pole BPF. Moreover, an inter-digital slot-line is further etched on the triple-mode patch resonator to achieve a new resonant mode, and a quad-mode patch resonator is achieved without increasing its size. A miniaturised four-pole BPF is implemented using the quad-mode patch resonator. For the demonstration, three-pole triple-mode, six-pole triple-mode and four-pole quad-mode patch filters are designed, fabricated and measured. The measured results are in good agreement with the simulated ones.