Trough waveguides (TWG) have been utilized in a variety of radio frequency (RF) and other related applications including radar, the treatment of hypothermia and in the generation of plasmas. Perturbing the guided wave in these structures with blocks, rods, dielectrics, and other structures can create reconfigurable periodic line sources. These trough waveguide antennas (TWA) are then capable of providing both fixedfrequency and frequency-dependent beam steering. This was originally performed using electro-mechanical “cam-and-gear” mechanisms. Previous work related to the excitation of TWG and the performance of TWA topologies are limited when compared to more common antenna designs, yet they possess many desirable features that can be exploited in a modern system. This thesis will examines an S-band rectangular-to-trough waveguide transition and trough guide antenna that has been designed for broadband reconfigurable antenna applications considering as well the airflow characteristics for sensing applications. The design, fabrication, and electromagnetic performance (mode conversion, impedance matching, and antenna performance) are discussed, including the use of metallic cantilever perturbations placed along the troughguide sidewalls that are designed to provide improved impedance matching when steering the beam from the backward quadrant through broadside, towards the forward quadrant. Impedance matching techniques such as use of circular holes at the edge of each actuated cantilever are used to reduce power reflections and provide a low voltage standing wave ratio (VSWR) along the S-band. Finite element simulations will provide a demonstration of the airflow and turbulence characteristics throughout the entire structure, where the metallic cantilevers are used to manipulate the flow of air, to distribute it across the surfaces of the structure better and improve its potential for sensing operations.

Conventional rectangular waveguides are commonly used for high power and millimetre wave microwave applications. However their use at lower frequencies has been limited by their bulky nature at these frequencies. A new type of compact waveguide called a partial H-plane waveguide has previously been proposed that has only one quarter of the cross sectional area of a conventional waveguide. However, only limited information relating to the feeding of such waveguides is available. This study focuses on the development of a practical end-launched coaxial and a microstrip to partial H-plane waveguide adapters with similar or larger achievable bandwidths compared to conventional waveguide probe feed adapters. The two proposed waveguide transition geometries are investigated to determine which parameters can be used to optimise the insertion and return losses. The prototype waveguide transitions were optimised using numerous simulations and fabricated in the back-to-back configuration. Simulated single ended and back-to-back S-parameters yield good performance over the entire H-band (3.95 to 5.85 GHz, also known as G-Band). De-embedding techniques are used to measure the approximate single ended response of the two waveguide adapters based on the back-to-back measurements. The measurement and simulation results compare favourably and validate the designs. / Dissertation (MEng)--University of Pretoria, 2014. / tm2015 / Electrical, Electronic and Computer Engineering / MEng / Unrestricted

THz and mm-wave technology has become increasingly significant in a very diverse range of applications such as spectroscopy, imaging, and communication as a consequence of a plethora of significant advances in this field. However to achieve a mass production of THz systems, all the commercial aspects should be considered. The main concerns are attributed to the robustness, compactness, and a low cost device. In this regard, research efforts should be focused on the elimination of obstacles standing in the way of commercializing the THz technology. To this end, in this study, low cost fabrication technologies for various parts of mm-wave/THz systems are investigated and explored to realize compact, integrated, and rugged components. This task is divided into four phases. In the first phase, a robust fiber-based beam delivery configuration is deployed instead of the free beam optics which is essential to operate the low cost THz photomixers and photoconductive antennas. The compensation of different effects on propagation of the optical pulse along the optical fiber is achieved through all-fiber system to eliminate any bulky and unstable optical components from the system. THz measurements on fiber-coupled systems exhibit the same performance and even better compared to the free beam system. In the next phase, the generated THz wave is coupled to a rectangular dielectric waveguide through design of a novel transition with low insertion loss. The structure dimensions are reported for various range of frequencies up to 650GHz with insertion loss less than 1dB. The structure is fabricated through a standard recipe. In third phase, as consequence of the advent of high performance active device at mm-wave and THz frequency, a transition is proposed for coupling the electromagnetic wave to the active devices with CPW ports. Different approaches are devised for different frequencies as at higher frequencies any kind of metallic structure can introduce a considerable amount of loss to the system. The optimized structures show minimum insertion loss as low as 1dB and operate over 10% bandwidth. The various configurations are fabricated for lower frequencies to verify the transition performance. The last phase focuses on the design, optimization, fabrication and measurements of a new dielectric side-grating antenna for frequency scanning applications. The radiation mechanism is extensively studied using two different commercial full-wave solvers as well as the measured data from the fabricated samples. The optimized antenna achieves a radiation efficiency of 90% and a gain of 18dB. The measured return loss and radiation pattern show a good agreement with the simulation results.

THz and mm-wave technology has become increasingly significant in a very diverse range of applications such as spectroscopy, imaging, and communication as a consequence of a plethora of significant advances in this field. However to achieve a mass production of THz systems, all the commercial aspects should be considered. The main concerns are attributed to the robustness, compactness, and a low cost device. In this regard, research efforts should be focused on the elimination of obstacles standing in the way of commercializing the THz technology. To this end, in this study, low cost fabrication technologies for various parts of mm-wave/THz systems are investigated and explored to realize compact, integrated, and rugged components. This task is divided into four phases. In the first phase, a robust fiber-based beam delivery configuration is deployed instead of the free beam optics which is essential to operate the low cost THz photomixers and photoconductive antennas. The compensation of different effects on propagation of the optical pulse along the optical fiber is achieved through all-fiber system to eliminate any bulky and unstable optical components from the system. THz measurements on fiber-coupled systems exhibit the same performance and even better compared to the free beam system. In the next phase, the generated THz wave is coupled to a rectangular dielectric waveguide through design of a novel transition with low insertion loss. The structure dimensions are reported for various range of frequencies up to 650GHz with insertion loss less than 1dB. The structure is fabricated through a standard recipe. In third phase, as consequence of the advent of high performance active device at mm-wave and THz frequency, a transition is proposed for coupling the electromagnetic wave to the active devices with CPW ports. Different approaches are devised for different frequencies as at higher frequencies any kind of metallic structure can introduce a considerable amount of loss to the system. The optimized structures show minimum insertion loss as low as 1dB and operate over 10% bandwidth. The various configurations are fabricated for lower frequencies to verify the transition performance. The last phase focuses on the design, optimization, fabrication and measurements of a new dielectric side-grating antenna for frequency scanning applications. The radiation mechanism is extensively studied using two different commercial full-wave solvers as well as the measured data from the fabricated samples. The optimized antenna achieves a radiation efficiency of 90% and a gain of 18dB. The measured return loss and radiation pattern show a good agreement with the simulation results.

This thesis introduces a topology optimization approach to design, from scratch, efficient microwave devices, such as antennas and waveguide transitions. The design of these devices is formulated as a general optimization problem that aims to build the whole layout of the device in order to extremize a chosen objective function. The objective function quantifies some required performance and is evaluated using numerical solutions to the 3D~Maxwell's equations by the finite-difference time-domain (FDTD) method. The design variables are the local conductivity at each Yee~edge in a given design domain, and a gradient-based optimization method is used to solve the optimization problem. In all design problems, objective function gradients are computed based on solutions to adjoint-field problems, which are also FDTD discretization of Maxwell's equations but solved with different source excitations. For any number of design variables, the computation of the objective function gradient requires one solution to the original field problem and one solution to the associated adjoint-field problem. The optimization problem is solved iteratively using the globally convergent Method of Moving Asymptotes (GCMMA). By the proposed approach, various design problems, including tens of thousands of design variables, are formulated and solved in a few hundred iterations. Examples of solved design problems are the design of wideband antennas, dual-band microstrip antennas, wideband directive antennas, and wideband coaxial-to-waveguide transitions. The fact that the proposed approach allows a fine-grained control over the whole layout of such devices results in novel devices with favourable performance. The optimization results are successfully verified with a commercial software package. Moreover, some devices are fabricated and their performance is successfully validated by experiments.