Unidirectional antenna with loaded dielectric substrate

Abstract

This thesis presents a study on the enhancement of the performance of a recently developed wideband planar antenna with unidirectional radiation pattern. The antenna, designated as the magneto-electric dipole, was designed based on the complementary antenna concept for achieving wide bandwidth, low cross polarizations, low back lobes, symmetrical radiation patterns and stable gain. The first objective of this thesis is to reduce the thickness and size of the antenna while maintaining its excellent electrical characteristics. The approach of dielectric loading is employed. A unidirectional antenna with partially-loaded dielectric substrate, εr=2.65, is studied. The height of the proposed antenna is only 18mm, which is reduced by 40% comparing with the original design. Results show that the proposed antenna can provide wide bandwidth of 48.7% (SWR≤2). The average gain is 8.1dBi across the entire operating bandwidth. However, relatively high back-lobe radiation around 12dB is found at the lower frequency. Another design of the unidirectional antenna with fully-loaded dielectric substrate, εr=2.65 is investigated. Its occupied area is even smaller that that of a partially-loaded unidirectional antenna. In order to improve the performance of the previous design, a back-lobe suppression technique is also proposed. A box-shaped metallic wall is mounted on the ground plane. Results point out those antenna performances can be enhanced by utilizing a metallic wall. The front-to-back ratio is reduced by 9dB at the lower frequency. A more stable radiation pattern with relatively low cross-polarization and back-lobe radiation, nearly identical E-and H-plane patterns are also be found across the operating bandwidth comparing with the case without metallic wall. Finally, the second objective is to modify the design for dual-polarization radiation, which is popular in modern wireless communications for enhancing channel capacity. Unidirectional antenna with partially-loaded dielectric substrate for dual polarization is examined. A dual-polarized magneto-electric dipole with loaded dielectric substrate contains two pairs of short-circuited patches, which excites by four Г-shaped probes for two orthogonal polarization, +45∘.A box-shaped metallic wall is also applied in the dual polarization antenna designs to improve the antenna performances. The antenna yields reasonably wide bandwidth with an isolation less than -29dB across the operating frequency. The far field radiation patterns are stable across the operating bandwidth.