A 50 OHM MICROSTRIP LINE FED SHORTED HEXAGONAL MICROSTRIP ANTENNAS WITH REDUCED CROSS-POLARIZATION
Keywords:Cross-polarization, Fundamental mode, Inset feed, Edge fed hexagonal antenna, Shorting post
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 feeding technique like a quarter-wave transformer or inset fed is used to match the edge impedance, the HMSA structure becomes asymmetrical over the feed axis ( E-plane), which leads to increase in the cross-polar level in the H-plane. In this paper, to reduce the cross-polarisation level of HMSA along H-plane, a method of loading shorting posts into the patch have been used. To match the edge impedance of the HMSA on each X and Y-axes with the 50- Ohm Microstrip (MS)-line-fed, initially a single shorting post has been used. Later on, a pair of shorting post is used along both the axes of HMSA for operation at around 2.4 GHz. The current distribution of the shorted HMSA has been changed due to the loading of shorting posts into the patch, which in-turns help to match the edge input impedance. Therefore, without changing the patch geometry, a wide extent of impedance matching can be realised. A detailed investigation on the characteristics of the shorted HMSA with modified fundamental mode frequencies has been presented. The co-polarization to cross-polarization ratios (CTCR) in H-plane of the HMSA with a pair of shorting posts has been reduced substantially with 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 50 Ohm-MS-line-feed are in good agreement with measured ones.
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