Contents I. Introduction
Microstrip antenna arrays are commonly used in mobile telecommunication, electronic toll collection (ETC), and airborne telecommunication systems, where size, weight, manufacturing cost, and conformability are the main requirements. Common approaches for implementing feeding networks include parallel feeding [1], series feeding [2]–[5], and parallel-series feeding [6], [7]. A parallel feeding network can provide in-phase excitation to all elements and achieve good match and directional radiating characteristics in wide frequency range. A series feeding network has relatively shorter transmission line, therefore the network has lower dissipation loss and smaller size. Parallel-series feeding employs both benefits, i.e., broadband performance and low dissipation loss. A parallel-series feeding network can be implemented by connecting a series-fed subarray with a parallel feeding network [6], or by using combined parallel/series feeding structure to excite radiating elements [7]. However, the combined structure of X-shape or T-shape can neither provide good match at all ports nor achieve good isolation between split ports. This not only limits the impedance bandwidth, but also makes it difficult to achieve low sidelobe levels considering the mutual coupling between radiating elements.
