0018-926X (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TAP.2018.2851616, IEEE Transactions on Antennas and Propagation > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Coupled λ/4 resonator Coupled λ/2 resonator T-junction without resistor Patch ∠90° ∠0° A f A f Fig. 1. Topology of the proposed wideband circularly polarized patch antenna via co-design of patch radiator and feeding network. Abstract—A wideband circularly polarized (CP) patch antenna with enhanced axial ratio (AR) bandwidth via co-design of feeding network is proposed in this paper. One λ/4 resonator and one λ/2 resonator are introduced and coupled in proximity to two orthogonal points of a square patch radiator to excite two orthogonal modes. These two resonators not only function as impedance matching elements to achieve equal magnitude of two orthogonal modes in a wide frequency band, but also provide the required 90°phase difference. In this way, two minima in the AR response appear, resulting in extended AR bandwidth. The wideband design of the antenna is carried out based on the equivalent circuit model, where the patch radiator is treated as a GLC parallel resonator. In particular, the feeding network and the patch are jointly considered as a three-port network for synthesis design. By mapping the radiated far field to the magnitude and phase on the radiation conductance in the circuit model, the AR response of the antenna becomes predictable in a straightforward manner. Measured results experimentally demonstrate that the 3-dB AR bandwidth of our proposed CP patch antenna is 2.7 times that of the traditional one fed with two 90°offset lines. Index Terms — Synthesis design, circular polarization, axial ratio bandwidth, wideband, filtering antenna network. I. INTRODUCTION UE to the attractive features of circularly polarized (CP) waves in transmission and reception, CP antennas have been becoming critical components in modern wireless communication systems. CP patch antennas [1], [2] are usually more preferred because of their low profile, light weight, and Manuscript received by October 8, 2017. This work was supported by the National Natural Science Foundation of China under General Program (61571468), the University of Macau under Research Grants (MYRG2017-00007-FST & CPG2017-00028-FST), and the Macao Science and Technology Development Fund under FDCT Research Grant (091/2016/A2). (Corresponding author: Lei Zhu) Q.-S. Wu is with the School of Information Engineering, Guangdong University of Technology, Guangzhou 510006, China, and also with the Department of Electrical and Computer Engineering, Faculty of Science and Technology, University of Macau, Macau SAR, China. X. Zhang is with the ATR National Key Laboratory of Defense Technology, College of Information Engineering, Shenzhen University, Shenzhen 518060, China, and also with the Department of Electrical and Computer Engineering, Faculty of Science and Technology, University of Macau, Macau. L. Zhu is with the Department of Electrical and Computer Engineering, Faculty of Science and Technology, University of Macau, Macau SAR, China (e-mails: leizhu@umac.mo). low cost. However, their inherent narrow axial ratio (AR) bandwidth has become an annoying concern. As one class of CP patch antennas, the single-fed type is only fed by one probe or microstrip line [3]. By introducing perturbation elements such as slots, stubs, or other truncated elements, the two degenerate modes of a single-fed patch antenna could be slightly and appropriately split into equal amplitude and quadrature phase at the operating frequency to produce CP radiation [4], [5]. Alternatively, patch antennas with cross slots [6]–[8], U-slot [9], or fed by an asymmetric Y-shaped stub [10], [11], L-shaped strip [12] have been proposed for CP radiation. Nevertheless, the aforementioned single-fed CP antennas suffer from narrow AR bandwidth, since good CP property with AR = 1 can only be achieved at a single frequency point. As another class of CP patch antennas, the dual- or multi-fed types is fed with equal magnitude and 90°phase difference by virtue of an external polarizer [3], [13], [14], such as the 3-dB hybrid or offset feeding lines. Offset feeding lines, with one branch a quarter wavelength longer than the other, are usually set at the edges of the patch. This kind of CP patch antennas inherently suffers from narrow AR bandwidth because the frequency dependence of offset feeding line is greater than that of the usual hybrid. Stacked patch is also employed to design the wideband CP antenna utilizing two radiation modes, and sometimes, extra feeding techniques are needed to provide the required magnitude and phase [15]–[21]. Unfortunately, the structures of this kind of CP antennas are usually cumbersome, and there is no synthesis method presented to effectively design A Wideband Circularly Polarized Patch Antenna with Enhanced Axial Ratio Bandwidth via Co-design of Feeding Network Qiong-Sen Wu, Member, IEEE, Xiao Zhang, Member, IEEE, and Lei Zhu, Fellow, IEEE D