Design of Magnetic Dipole Enhanced Antenna Arrays with High Aperture Efficiency

Abstract

Millimeter-wave antennas experience significant attenuation in the atmosphere, which limits their communication range. To improve the reliability of long-distance communications, various approaches aim to enhance the gain performance. However, these approaches tend to increase the volume of the communication system. For instance, using large reflectarray antenna or transmitarray to boost gain requires more space, while employing Fabry-Perot cavity or lens antennas raises the system profile. These solutions conflict with trends toward miniaturization, integration, and lightweight designs. Planar antenna arrays are particularly favored for their high aperture efficiency and low-profile characteristics. Instead of simply increasing the number or aperture size of antenna element to improve the array gain, focusing on increasing the aperture efficiency is more advantageous due to the efficient utilization of space.

This thesis proposed a novel differential feeding loop to generate magnetic dipole radiation, exhibiting a uniform electric field distribution around the aperture and a wide impedance matching bandwidth. Depend on this magnetic dipole loop, a series of high aperture efficiency antenna arrays are developed to enhance the gain while maintaining good performance such as wide bandwidth and stable radiation patterns. By rationally arranging the spatial configuration, radiation magnitude, and phase difference of these magnetic dipoles, linear-polarized (LP), dual-polarized (DP) and circular-polarized (CP) antenna arrays were successively designed, demonstrating excellent performance characteristics.

First, a novel LP antenna named stacked magnetic dipole antenna is proposed. This element consists of a rectangular loop and a radiating aperture, which together generate two magnetic dipole radiations in a stacked configuration. By carefully optimizing the separation and radiation magnitude of the two magnetic dipoles, the design achieves a directive, broadside radiation pattern with a phase difference of nearly 90°. This antenna offers several advantages, including wide bandwidth of 42%, high directivity, compact size, high polarization purity, stable gain (1.3-dB variation) and consistent radiation patterns. In an 8 × 8 antenna array, a maximum gain of 27 dBi and peak aperture efficiency of 97% are demonstrated.

Secondly, building on the previous LP antenna design, a DP antenna array with high aperture efficiency and excellent port isolation is proposed. This antenna element is constructed by two cross bowtie loops and four parasitic shorting patches. The magnetic dipole radiations, generated from the differential feeding loops, function as both radiators and excitation sources for the patches, ensuring a uniform electric field distribution and high aperture efficiency. The symmetric structure, differential exciting pins, and separated feeding network contribute to high simulated port isolation of -50 dB. Based on this DP element, a 4 × 4 antenna array is designed and fabricated. The measurement results reveal a wide bandwidth of 33%, with a peak gain of 21.4 dBi and an impressive aperture efficiency of 84%.

Thirdly, extending from the above DP antenna design, a high-gain CP antenna is proposed to mitigate the transmission loss and multipath fading. This CP antenna is designed using two orthogonal bowtie loops and four corner-truncated patches. By adjusting the lengths of the two orthogonal loops, two magnetic dipoles with orthogonal polarization are generated with a stable 90⁰ phase difference. The four parasitic corner-truncated patches are carefully designed to enhance aperture efficiency and balance the radiation magnitude for two polarizations. As a result, a wide overlapped bandwidth (S₁₁ < -10 dB and AR < 3 dB) of 24% is achieved. Building on this element, a 2 × 2 subarray with a surrounding strip and an 8 × 8 CP antenna array are developed and fabricated. Measurement results closely align with the simulations, demonstrating an overlapped bandwidth of 20.6%, ranging from 27.4 GHz to 33.7 GHz, with a measured peak gain of 27.4 dBi and an aperture efficiency of 85% at 30 GHz.

At last, several extended works are introduced, including a reconfigurable dual LP and dual CP antenna, a wideband CP antenna array utilizing sequentially rotated ME/EM elements, and a wideband 2-bit transmitarray with stable phase difference.

Date of Award	15 Jan 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hang WONG (Supervisor)