A 50Ω Microstrip Line Fed Shorted Hexagonal Microstrip Antennas with Reduced Cross-Polarization

Abstract A Hexagonal Microstrip Antenna (HMSA) is an approximation to a Circular MSA (CMSA), which is not symmetrical over two principal planes, unlike a CMSA. Consequently, two types of feed positions are feasible for each X and Y-axes. When a quarter-wave transformer or inset feed is used to match the edge impedance of the HMSA, it becomes asymmetrical over the feed axis (E-plane), which leads to increase in the crosspolarization level in the H-plane. In this paper, to reduce the crosspolarization level of HMSA along H-plane and to match the edge impedance of the HMSA on each X and Y-axes with the 50Ω-Microstrip (MS)-line-feed, a single and a pair of shorting posts have been used. The current distribution of the shorted HMSA is changed due to the loading of shorting posts into the patch, which in-turns help to match the edge input impedance. A detailed investigation on the characteristics of the shorted HMSA with modified fundamental mode frequencies has been presented. The co-polarization to crosspolarization ratios (CTCR) in H-plane of the HMSA with a pair of shorting posts has been reduced substantially by more than 20 dB at the broadsight direction in comparison with that of an HMSA with a single shorting post or with an inset. The simulated results of the HMSA with direct 50Ω-MS-line-feed are in good agreement with measured ones.摘要 六角形微带天线（Hexagonal Microstrip Antenna, HMSA）是圆形微带天线（Circular MSA, CMSA）的近似结构，与CMSA不同，其并不在两个主平面内保持对称。因此，针对X轴与Y轴各存在两种可行的馈电位置。当采用四分之一波长变换器（quarter-wave transformer）或嵌入式馈电（inset feed）匹配HMSA的边缘阻抗时，天线会在馈电轴（E平面，E-plane）上呈现非对称性，进而导致H平面（H-plane，H平面）内的交叉极化水平升高。本文为降低HMSA在H平面内的交叉极化水平，并使HMSA在X、Y轴方向的边缘阻抗均可与50Ω微带线（50Ω-Microstrip, MS）馈电相匹配，采用了单个及成对短路柱（shorting posts）结构。将短路柱加载至微带贴片（patch）后，会改变带短路柱的HMSA的电流分布，进而有助于匹配边缘输入阻抗。本文对修改了基模频率的带短路柱HMSA的特性展开了详细研究。相较于采用单个短路柱或嵌入式馈电的HMSA，采用成对短路柱的HMSA在视轴方向的H平面内的同极化与交叉极化比（co-polarization to crosspolarization ratios, CTCR）显著提升了20 dB以上。采用直接50Ω微带线馈电的HMSA仿真结果与实测结果吻合良好。