IN an age when access to information, communication and infotainment, any time, any place, anywhere has become a pre-requisite for modern life, it is not surprising that the wireless technology has been the focus of attention of technocrats and scientists.
The field of wireless communication has been undergoing a revolutionary growth for the last decade. This is attributed to the invention of portable mobile phones some fifteen years ago. The access of the second generation (2G) cellular communication services motivates the development of wideband third generation (3G) cellular phones and other wireless products and services, including wireless local area networks, home RF, bluetooth, wireless local loops, local multipoint distributed networks etc. The crucial component of a wireless network or device is the antenna. We can see our cities are flooded with antennas of different kinds and shapes. On the other hand for safety and portability reasons, low power, multi functional and multiband wireless devices are highly preferred. All these stringent requirements demand the development of highly efficient, low profile and small size antennas that can be embedded into wireless products.
In the last two decades, two classes of novel antennas have been investigated. They are the Microstrip Patch Antenna (MPA) and the Dielectric Resonator Antenna (DRA). Both are highly suitable for the development of modern wireless communications. The use of dielectric resonator antenna was first proposed by Prof. S. A. Long in the early nineteen eighties. DRA has negligible metallic loss, and hence it is highly efficient than its counterpart when operated in microwave and millimeter wave frequencies. Also low loss dielectric materials are now easily available commercially at very low cost, which attracts more system engineers to choose dielectric resonator antenna to design their wireless products.
Dual or multifrequency operation is highly attractive in current wireless communication systems. If a single DRA can support multiple frequencies, then there is no need for multiple single frequency antennas. Applications requiring different frequency bands can be addressed simultaneously with one radiating element. This reduces the circuit size and leads to compact systems. In addition, when multiple frequencies are located close to each other, the antenna may have a broad operating bandwidth. Many investigators have reported on DRA with dual frequency operation using various approaches. But in all these cases dual frequencies are achieved by using either dual feed lines or multiple radiating elements or hybrid radiating structure, which causes design complexity and large size.
In this thesis, the author proposes a new geometry of hexagonal shape DR to the DR antenna community - Hexagonal Dielectric Resonator Antenna (HDRA) for multi frequency operation with a single feed of excitation, which is the highlight of this work. These multiple frequency bands are suitable for Digital Cordless Telephones (DCT), Personal Communication Systems (PCS) and Wireless Local area Networks (WLAN) bands.
This thesis is organized into seven chapters which describe the problem addressed, methodology adopted, results obtained, comparison between measured and theoretical results, and conclusions arrived at.
First chapter explains the development of electromagnetism, microwaves and its application from the origin. Also describes the dielectric resonator antennas (DRAs), its features over conventional microstrip antennas, different coupling methods used for exciting the DRAs and different geometries of DRAs already developed. The last section of this chapter gives a comparative study of different numerical methods used for modeling the antenna.
Chapter II provides the review of dielectric resonator antennas from its beginning to the current development. It includes the different coupling mechanism used to excite the DRAs, various geometries of DRAs developed, different techniques for improving the bandwidth, gain and the sequential theoretical analysis of DRAs. The diverse methods for producing circular polarization, air gap effect on resonant frequency and bandwidth, variety of techniques for producing multi frequency operation are reviewed in chronological order.
Chapter III describes the research methodology opted for this work. The experimental setup used for measuring the return loss, radiation pattern, gain and polarization is explained. Moreover it describes the simulation software used for characterizing the hexagonal dielectric resonator antenna.
The step-by-step development of hexagonal dielectric resonator antenna from the basic material is explained in chapter IV. It includes weighing, mixing, sintering, pressing, shaping etc. The setup used for measuring the dielectric permittivity of the material is also explained.
In Chapter V, the first part introduces the new hexagonal shaped dielectric resonator antenna with coaxial feed as excitation. The optimizations of coaxial probe length, probe feed location and aspect ratio of HDRA are performed experimentally. Besides, it explains the radiation pattern, gain and polarization of the antenna for different bands using the coaxial feed excitation. The second part explains the HDRA characteristics using microstrip feed excitation. A comparison of experimental results with the simulated results, using Ansoft HFSS, is also included in this chapter.
The theoretical study using finite difference time domain (FDTD) method for modeling the microstrip fed and coaxial fed HDRA is explained in chapter VI. It describes the theoretical concepts of FDTD in electromagnetics and the perfect matched layer (PML) concepts for absorbing boundary condition (ABC). All the necessary equations for three-dimensional electric and magnetic field variables are derived from the fundamental Maxwell’s curl equations and are given in this chapter. It also uses Lubber’s feed techniques for reducing the number of time steps required for modeling the whole structure. The FDTD results are compared with the measured values.
Chapter VII provides the conclusions and highlights drawn from this work. Advantages of new geometry hexagonal DRA and its possible applications in wireless communication are specified. Furthermore, it gives the future scope of the work in this area.
There are two Appendices included.
In Appendix – A, a metal-coated cylindrical dielectric resonator antenna for producing multiple resonances is given. Coaxial probe is used for exciting the cylindrical DRA. All the characteristics of the antenna are explained in this section.
In Appendix – B, the development of a novel coupling media and phantom material constituent for microwave medical imaging applications using sodium meta silicate gel is explained. Dielectric parameters, heating and absorption coefficient of this material are studied and discussed. Comparative studies of the suitability of gel with various biological tissues are also given in the last part.
In view of the fact that, microwave medical tomography is promising a novel non- hazardous method of imaging for the detection of tumors in soft tissues. The tomographic set up consists of antennas, coupling media and the object to be imaged. The antenna must be operated at ISM frequency. The purpose of coupling media is to enhance the coupling of electromagnetic energy between the antenna and the object to be imaged. The object is placed at the center of the imaging set up from where the scattered microwave data is collected and analyzed at the various locations of the receiver and the orientation of the object. As the HDRA developed in the core work of the thesis operates at 2.4 GHz-ISM frequency can as well be used in tomographic set up.
