Design of Compact Unidirectional Dielectric Resonator Antenna

Abstract

Unidirectional antennas are required in applications that need the radiation to be concentrated in a desired direction. One way of achieving this is by the superimposition of complementary sources. The resulting antennas can have attractive characteristics of high front-to-back ratios (FTBRs), broad beamwidths, and stable radiation patterns. In some earlier embodiments of the use of complementary sources, large ground planes or cavities are used for maintaining the unidirectional radiation, and this results in bulky structures. As a method of providing a basic source, the dielectric resonator (DR) antenna (DRA) is known to provide inherent advantages of compactness, low loss, a variety of radiation patterns, and high degree of design flexibility. In this thesis, DRAs and complementary sources are studied for the purpose of providing compact antennas that radiate unidirectionally either at a broadside or in a lateral direction.

The brief introduction, general description of the DRA, and basic theory of complementary sources are provided as background in the first three chapters. This is followed by Chapter 4, which introduces a broadside unidirectional DRA on a compact ground plane. The DRA HEM_11δ mode is combined with the directed surface electric current on the small ground plane, to provide a horizontal magnetic and electric dipole. Compared with a traditional DRA with a ground plane approximately 1λ₀ × 1λ₀ in size (λ₀ is the wavelength in the air), the proposed antenna achieves more than twice the FTBR with only a ground plane of about 16% of the usual size. The proposed DRA achieves a FTBR > 15 dB over the frequency range 2.30-2.55 GHz (10.1%). In a second example, a circularly polarized (CP) unidirectional DRA is developed for the same compact ground plane. Four rectangular etched-outs on the ground plane are employed for a broader axial ratio (AR) bandwidth. In addition, the CP antenna uses a ϕ-shaped slot in order to achieve a better FTBR bandwidth. The overlap of the impedance bandwidth, AR bandwidth, and 15dB FTBR bandwidth (FTBR > 15 dB) covers the frequency band 2.40-2.55 GHz (6.1%), making the antenna attractive for some CP applications.

Unidirectional ring DRAs with lateral radiation patterns were investigated in Chapter 5. The DR HEM_11δ+1 mode and an axial monopole are excited simultaneously to provide a horizontal magnetic dipole and a vertical monopole, respectively. The measured 10-dB impedance bandwidth is 12.1%, but the 15-dB FTBR bandwidth is ~4.0%. In order to widen the FTBR bandwidth, the DR HEM_11δ+2 mode combines a slot mode to provide a wideband horizontal magnetic dipole, while a cone monopole mode is used as a wideband vertical electric dipole. The wideband version has much wider measured impedance and 15-dB FTBR bandwidths of 43.6% and ~14%, respectively.

Finally, a compact CP uni-lateral DRA was proposed in Chapter 6. Two sets of complementary CP field patterns are utilized to produce CP uni-lateral field. The first set is obtained using the DR TE_01δ+1 and TM_01δ mode. For the second set, it is provided by the CP DR-loaded slot antenna which makes use of the cross shaped slot on the ground plane. As a result of combining the two sets of complementary patterns properly, unidirectional left-hand circular polarization (LHCP) field is generated in -y direction. The measured overlap of impedance and AR bandwidths verifies the operation over a frequency band of 2.39-2.51 GHz (4.9%). It shows a measured FTBR of 15.5 dB between the LHCP and right hand circular polarization (RHCP) at 2.44 GHz.

The proposed unidirectional DRAs show potential use in modern wireless communication when compactness and unidirectional properties are needed. In addition it is suggested that the DRA can be constructed from transparent material such as glass, other practical functions can be realized by integration in decorations, socket panels, and switches.

Date of Award	16 Feb 2016
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwok Wa LEUNG (Supervisor)