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 六边形微带天线（Hexagonal Microstrip Antenna, HMSA）是圆形微带天线（Circular MSA, CMSA）的近似结构，但其与CMSA不同，并不在两个主平面内保持对称。因此，针对X轴与Y轴方向，各存在两种可行的馈电位置。当采用四分之一波长变换器或嵌入式馈电来匹配HMSA的边缘阻抗时，天线将在馈电轴（E面）上呈现非对称特性，进而导致H面内的交叉极化电平升高。为抑制HMSA在H面的交叉极化电平，并实现其X、Y轴边缘阻抗与50Ω微带线馈电（50Ω-Microstrip (MS)-line-feed）的匹配，本文分别采用了单根短路柱与成对短路柱的方案。将短路柱加载至辐射贴片后，会改变带短路柱的HMSA的电流分布，进而辅助实现边缘输入阻抗的匹配。本文针对加载短路柱且基模频率经过修正的HMSA的电磁特性展开了详细研究。与采用单根短路柱或嵌入式馈电的HMSA相比，采用成对短路柱的HMSA在H面的同极化与交叉极化比（Co-polarization to Crosspolarization Ratios, CTCR）在正射方向上显著提升，幅度超过20dB。采用直接50Ω微带线馈电的HMSA的仿真结果与实测结果吻合良好。