A description of the apparatus used in microwave phase and diffraction measurements is contained in the first chapter of this thesis. The field was investigated by a phase technique and the point source of radiation reasonably well established. Several diffraction patterns about a dielectric and a conducting rod were taken at several distances behind the rod with E vector of the field parallel to the axis of the rod. These were qualitatively analysed and compared with the patterns of one other worker at 1.25 cm. Agreements and differences were noted. / Thesis / Master of Science (MS)

The limit of current technology for mobile base station filters is the multimode filter, in which each cavity supports two (or possibly three) independent degenerate resonances. Shielded dielectric resonators with a rectangular cross-section are useful in this application. In the design of these filters, manufacturers are using software packages employing finite element or finite difference time domain techniques. However, for sufficient accuracy these procedures require large numbers of points or elements and can be very time consuming. Over the last decade research using the mode matching technique has been used to solve this kind of difficulty for various types of filter design and waveguide problems. In this thesis a mode matching method and computer program is developed to calculate the propagation coefficients and field patterns of the modes in a shielded rectangular dielectric rod waveguide. Propagating, complex, evanescent and backward wave modes are included and the work shows the presence of a dominant mode, and other fundamental modes, not previously identified. The effect of the shield proximity on the propagation characteristics and mode spectrum is investigated, together with the limitations on the accuracy of the mode matching method. In addition, the fields within the shielded rectangular dielectric rod waveguide, are used to calculate the attenuation coefficient of the dominant and fundamental modes. The influence on the attenuation coefficient of the proximity of the shield to the rod is also evaluated for these modes and limitations on accuracy are discussed. The calculated numerical results for the propagation and attenuation coefficient values are verified by measurement. The propagation coefficients results are typically within 2% of those measured. Verification of the attenuation coefficient results is achieved by comparing calculated and measured Q at the resonant frequencies of a number of shielded rectangular dielectric rod resonators. The difference between calculated and measured Q values is on average less than 4%. In the absence of a full solution of the shielded rectangular dielectric rod resonator, these results provide useful design information for this structure. In addition, the work reported in this thesis provides a basis for a full electromagnetic solution of this type of resonator. This would encompass the cubic dielectric resonator in a cubical cavity.

Ever-increasing interest in millimeter-wave and terahertz spectrum has prompted research and development of novel passive components working at these frequencies. Compared with the conventional planar components, non-planar dielectric devices become more attractive as frequencies increase due to their higher quality factors and dimensional tolerances. In this thesis, we present fast methods to analyze the millimeter-wave dielectric resonator and rod antenna. First, an analytical method has been developed to evaluate resonant frequencies, quality factors of the Whispering Gallery Mode (WGM) disk resonators and also the resonator-waveguide coupling. A numerical solver based on full-wave finite element method is implemented to verify the analytical result. This analytical model provides a solution for fast design and optimization of WGM resonators in filter and sensor applications. Secondly, a fast analytical approach based on local mode theory is introduced to calculate the radiation from tapered dielectric rod antenna. This efficient approximate model consumes much less computing resources and time, and demonstrates good agreements with full-wave numerical results. It supplies a quantitative way to understand the radiation mechanism and interaction between different parts of the antenna. Based on this, design criteria for the taper profile of rod antennas are given.

Ever-increasing interest in millimeter-wave and terahertz spectrum has prompted research and development of novel passive components working at these frequencies. Compared with the conventional planar components, non-planar dielectric devices become more attractive as frequencies increase due to their higher quality factors and dimensional tolerances. In this thesis, we present fast methods to analyze the millimeter-wave dielectric resonator and rod antenna. First, an analytical method has been developed to evaluate resonant frequencies, quality factors of the Whispering Gallery Mode (WGM) disk resonators and also the resonator-waveguide coupling. A numerical solver based on full-wave finite element method is implemented to verify the analytical result. This analytical model provides a solution for fast design and optimization of WGM resonators in filter and sensor applications. Secondly, a fast analytical approach based on local mode theory is introduced to calculate the radiation from tapered dielectric rod antenna. This efficient approximate model consumes much less computing resources and time, and demonstrates good agreements with full-wave numerical results. It supplies a quantitative way to understand the radiation mechanism and interaction between different parts of the antenna. Based on this, design criteria for the taper profile of rod antennas are given.

Recently, there is emerging interests in deploying wireless sensor networks in forests for applications such as forest fire detection, environmental monitoring and remote surveillance. One challenge in the design of such networks is to ensure reliable communication between sensors located near the ground and over short distances. However, the propagation mechanisms in this type of scenario are complex and not well understood. Furthermore, the design of antennas that can exploit the resulting propagation mechanisms for optimal power transfer remains an open question. The objective of this dissertation is to understand wave propagation and antenna radiation in forested environments in the HF/VHF frequency range. To achieve this objective, several forest scaled models are introduced. The first scaled forest model is a periodic metal cut-wire array. The transmission data inside the cut-wire array are simulated and measured. The propagation mechanisms inside the array are extracted. Several interesting propagation phenomena associated with surface waves and leaky waves are observed and explained. Next, a dielectric rod array consisting of water-filled straws is investigated as a more realistic forest model. Water is chosen since its dielectric constant in the microwave range is close to that of tree trunks in the HF/VHF frequencies. The propagation mechanisms in the water rod array are investigated through scaled model measurements in the laboratory, numerical simulations and an effective medium theory. Randomization effects due to rod spacing and rod height on the propagation mechanisms are also studied. Finally, the transmission data in a real forest are collected in the HF/VHF frequency range to corroborate the findings from the models. The measurement site is located at Bastrop, Texas. For comparison, the transmission data are also measured in an open field. The transmission data are processed and the resulting propagation mechanisms are extracted and compared with the model predictions. As an extension of the propagation study, the potential to achieve directive antenna radiations in a forest is explored. A simple metal cut-wire array environment is considered for ease in modeling. For the case when both the transmit antenna and the receive antenna are embedded inside the array, two design ideas are presented. The first design tries to couple the antenna radiation into the dominant propagation mechanism through phase matching and the second design uses a closely spaced Yagi array to decouple the antenna from its surrounding medium. For the case when the transmit antenna is embedded inside the array and the receive antenna is located outside the array, the leaky wave mechanism is explored to achieve directive radiation. These designs are verified through theoretical predictions, numerical simulations and prototype measurements. / text

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This thesis describes the development of antennas suitable for mobile coinmunication systems based on a dielectric rod antenna fed from circular waveguide. Pattern control of the antenna is implemented using a combination of Frequency Selective Surface (FSS) elements and metallic endcaps placed on the antenna Both linear and circular polarised feeds have been made for these antennas to ensure they are suitable for a wide range of applications. The suitability of the dominant and next, higher order, waveguide mode were investigated and conclusions drawn as to their suitability for this type of antenna. The antennas were extensively modelled using a commercial TLM based solver and the results of these simulations were compared to the comprehensive set of antenna pattern measurements and S-parameter measurements obtained for the prototype antennas.