A Balanced Filtering Antenna Array With High Gain, Steep Selectivity, and Multiradiation Nulls Parallel-Fed by Differential Broadband Network

In this communication, a
$2\times2$
balanced filtering antenna array parallel-fed by a differential broadband network is presented. The array has four identical stacked filtering antenna elements, each of which consists of a main patch etched with a folded U-shaped slot, a pair of ear-shaped parasitic patches, a stacked patch, and a folded T-shaped strip. The conceived antenna topology can enhance the gain flatness, improve selectivity, and increase the number of radiation nulls. Meanwhile, by introducing a novel ring-shaped transition structure (RSTS) to the balanced-to-single-ended (BTSE) four-way feeding network, a broad bandwidth, low insertion losses, low phase difference errors, and high common-mode (CM) suppression are realized. To validate the method, an integrated antenna array with a center frequency of 2.5 GHz is fabricated and measured. Experimental results exhibit a boresight gain of 13.7 dBi, four radiation nulls, and a square factor (
$SF_{10}$
) of 1.21.