I. Introduction
Microstrip filters have been widely used in a variety of RF/microwave circuits and systems for their attractive features of low cost, easy fabrication, small sizes and so on. With the rapid development of modern communication systems, more and more miniature filters with good performances are required. Now, patch resonators [1]–[3] are more attractive in planar filters applications due to the patch resonator filters [2]–[5] have simpler structures reducing the fabrication uncertainty, more compact sizes, lower loss and higher power handing features compared with the line-based ones. The basic principle for designing a patch resonator filter is the selectivity and application of all sorts of resonant modes.