Transparent Dielectric Resonator Antennas for Wireless Communications

Abstract

Transparent antennas are attractive since they can be integrated with devices for visible light applications. Systems so obtained can be aesthetic, compact, and low cost. Traditionally, there are mainly three methods to achieve transparency, namely TCO (transparent conducting oxide) film method, mesh grid method, and liquid method (e.g., water). These methods, however, have their inherit problems. TCO film sacrifices conductivity, whereas mesh grid structure is not completely transparent. For the liquid approach, it is sensitive to temperature and suffers from high loss. In contrast, glass dielectric resonator (DR) antenna (DRA) combines the transparency of glass and high efficiency of DRA. In this thesis, the glass DRA is investigated for transparent applications.

The existing transparent antennas are reviewed in Chapter 1. It is followed by the basic theory of cylindrical DRA in Chapter 2. Next, two designs of mirror-integrated DRAs (MIDRAs) are studied in Chapter 3. The mirror consists of a glass layer with a very thin light-reflective film coated at its back. It is overlaid on top of a dielectric resonator antenna (DRA), forming a two-layer DRA when the thin light-reflective film is neglected. To allow the wave to penetrate from the DRA to the glass layer, the film is made of non-conducting alternate layers of titanium dioxide (TiO₂) and silicon dioxide (SiO₂ ) instead of the traditional conducting silver (Ag) or aluminum (Al) coating. To demonstrate the idea, two cylindrical MIDRAs were designed and fabricated. The first one is excited in its TM_01δ mode by axially feeding it with a coaxial probe, giving an omnidirectional radiation pattern. For the second design, the unidirectional HEM_11δ mode is excited by using a slot-coupled source fed by a microstrip line. Experimental results show that these two MIDRAs radiate effectively as conventional DRAs. Since the proposed antenna appears to be a mirror, it can be an excellent hidden antenna that provides practical mirror functions.

An omnidirectional circularly polarized (CP) cylindrical dielectric resonator antenna is investigated in Chapter 4. Fed by a central coaxial cable, the DR operates in its TM_01δ mode and provides E_θ in the far field. In the meantime, the current on the slotted ground plane forms an in-phase loop current antenna generating E_ϕ in the far field. By combining these two orthogonal E-field components with a proper phase difference, omnidirectional CP fields can be obtained. This slotted ground technique can be applied to simple DRAs with conventional shapes, making its realization very easy. For demonstration, it was used to design a CP light-cover antenna. Its measured operating bandwidth is about 6.2%, covering the entire 2.4-GHz WLAN band.

A pattern diversity cylindrical DRA that makes use of two fundamental modes is investigated for the first time in Chapter 5. The idea is based on the fact that the HEM_11δ-mode E-field is vanishing weak at the center of the DRA where the TM_01δ-mode E-field is strongest. Therefore, increasing the dielectric constant (ϵ_r ) at the center of the DR can increase the electrical length and thus lower the resonant frequency of the TM_01δ mode. In our diversity design, the TM_01δ-mode is decreased until its resonant frequency coincides with the HEM_11δ-mode frequency. To realize this idea, a higher-ϵ_r cylindrical DR is inserted into the hollow region of a DR with a low dielectric constant, forming a two-layer DRA. The omnidirectional TM_01δ mode is excited by an axially coaxial probe. For the broadside HEM_11δ mode, it is excited by two open-circuited stubs coupled through a rectangular cross slot. The diversity antenna was designed at 2.4-GHz (WLAN band) using ANSYS HFSS. Two transparent materials with different ϵ_r were used in our design. One transparent prototype was fabricated and tested, of which the measured overlapping impedance bandwidth of the two ports is 6.9%. The prototype radiates like a conventional TM_01δ-mode HEM_11δ-mode DRA.

In this thesis, all designed antennas are totally transparent with good antenna performance, which are especially useful for indoor communications due to their aesthetics nature. Finally, some potential topics are suggested in Chapter 6.

Date of Award	8 Sept 2016
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwok Wa LEUNG (Supervisor)