Gain Improvement of a Low-Profile Horizontally Polarized Omnidirectional Antenna Using a Parasitic Loop

Conventional horizontally polarized omnidirectional (HPO) antennas typically achieve gain enhancement by employing vertically stacked array configurations, which compress the beamwidth at the cost of increased height. To address this limitation, this article presents a low-profile HPO antenna that achieves gain enhancement without increasing its height. The key innovation lies in introducing a parasitic large loop around the primary loop antenna. By carefully controlling the amplitude and phase relationship of the currents in both loops, their radiation patterns cancel each other in undesired directions while maintaining the maximum radiation direction in the azimuth plane, effectively narrowing the main beam and improving directivity. To implement this concept, series-fed dipole subarrays based on pin-loaded double-sided parallel stripline (DSPSL) are employed to realize the loop structures and establish the required current distribution. Ultimately, a complete HPO antenna, configured as a ring array of four such subarrays, is designed and experimentally validated. Experimental results demonstrate that, compared to a conventional HPO antenna with no parasitic loop, the proposed antenna achieves a 60∘ beamwidth reduction in the elevation plane and an average gain improvement of 2.2 dB in the azimuth plane, while maintaining excellent omnidirectional radiation and a compact low-profile structure.