Single- and Differential-Fed Patch Filters in Antenna (FiAs) for Modern Wireless Communication Systems

In this article, two filter-in-antenna (FiA) designs based on single- and differential-fed patch antennas are proposed. A shorted vertical cavity is first created in the center of the rectangular patch for the integration of filters. Since the patch antenna operates at the TM10 mode, the center-located shorted vertical cavity does not affect the operation mode of the patch antenna. A multicoupled miniaturized filter is then designed and inserted into the vertical cavity. A feeding structure from the patch down to the ground is created to excite the radiating patch to realize the single-fed patch FiA (SFiA). The fabricated SFiA features a compact size of $0.42\lambda _{\mathrm {g}} \times 0.4\lambda _{\mathrm {g}} \times 0.1\lambda _{\mathrm {g}}$ , excellent filtering performance (return loss of 15 dB, insertion loss of 0.6 dB, wide stopband width of $1.6{f} _{0}$ , high selectivity, and so on), and good antenna performance (unidirectional radiation, stable gain of 6.0 dBi, low back radiation of below -20 dB, and so on). Furthermore, a differential-fed patch FiA (DFiA) with a size of $0.44\lambda _{\mathrm {g}} \times 0.42\lambda _{\mathrm {g}} \times 0.1\lambda _{\mathrm {g}}$ is developed to enhance radiation performance, with two filters incorporated into the vertical cavity. Apart from demonstrating almost the same excellent filtering performance as the SFiA, the DFiA can provide a higher gain of 6.4 dBi, and stable and symmetrical E- and H-plane radiation patterns, along with lower cross-polarization levels of less than -20 dB. With the excellent filtering and antenna performance in such a compact structure, the proposed patch FiAs would be attractive candidates for modern wireless communication systems, such as sixth-generation (6G) mobile communications.