A thesis submitted to the Faculty of Engineering,University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree or Doctor of Philosophy. / The aim of the thesis is to develop techniques for the analysis of antennas composed of dipoles and folded dipoles,that are efficient relative to the method-of-moments. (Abbreviation abstract) / AC2017

No description available.

No description available.

In this dissertation a new type of infrared focal plan array (IR FPA) was investigated, consisting of antenna-coupled microbolometer fabricated using electron-beam lithography. Four different antenna designs were experimentally demonstrated at 10-micron wavelength: dipole, bowtie, square-spiral, and log-periodic. The main differences between these antenna types were their bandwidth, collection area, angular reception pattern, and polarization. To provide pixel collection areas commensurate with typical IR FPA requirements, two configurations were investigated: a two-dimensional serpentine interconnection of individual IR antennas, and a Fresnel-zone-plate (FZP) coupled to a single-element antenna. Optimum spacing conditions for the two-dimensional interconnect were developed. Increased sensitivity was demonstrated using a FZP-coupled design. In general, it was found that the configuration of the antenna substrate material was critical for optimization of sensitivity. The best results were obtained using this membranes of silicon nitride to enhance the thermal isolation of the antenna-coupled bolometers. In addition, choice of the bolometer material was also important, with the best results obtained using vanadium oxide. Using optimum choices for all parameters, normalized sensitivity (D*) values in the range of mid 108[√Hz/W] were demonstrated for antenna-coupled IR sensors, and directions for further improvements were identified. Successful integration of antenna-coupled pixels with commercial readout integrated circuits was also demonstrated.

Millimeter waves, corresponding to the frequency range 30 to 300 GHz, have characteristics which make them ideal for many applications. Antennas at these frequencies have the advantage of reduced size and weight and can be fabricated as an integral part of the system they are used in. Millimeter wave microstrip antennas have been extensively researched over the past decade. The purposed of this report was to build and test 35 GHz microstrip antennas as well as put into operation a high voltage klystron power supply, Micro-Now Model 756. The antennas were fabricated and tested in the lab and the results obtained are reported. The operation of the Model 756 power supply is also outlined in detail.

The spherical helical antenna is investigated as a new variation of the conventional helical antenna. The spherical helix is a wire antenna in a helix shape that is wound over a spherical surface instead of the standard cylindrical one. Analysis of this structure requires numerical methods and experimental measurements because its complex geometry makes it very difficult to develop analytic expressions for its radiation characteristics. The wire antenna code ESP, based on the method of moments, is used to calculate both the gain and phase of the electric-field components, E(} and E¢J' in the far field for spherical helices having between 3 and 10 turns. Several of these antennas were also constructed and tested on the Virginia Tech antenna test range. The numerical analysis and experimental measurements were performed over a wide range of frequencies corresponding to spherical circumferences of between 0.7 and 2.8 wavelengths. A literature survey of the conventional helical antenna is presented so that comparisons can be made with the spherical helix. The numerical and experimental results of the spherical helical antenna indicate that its behavior is markedly different than the cylindrical helix. It is a broad beam elliptically polarized radiator, in general, and operates over a relatively wide bandwidth. Under certain conditions, it is circularly polarized over a major portion of the main beam for a narrow bandwidth, a result which has potential applications. The behavior of the spherical helical antenna is not significantly affected by the number of turns and is discussed in this work for a few representative cases. / Master of Science

The aim of this study is to analyze radiation from surface currents lying on an interface between a grounded, planar, lossless, isotropic, chiral slab and a simple lossless dielectric half-space. An eigenvalue-eigenfunction approach, specifically the Ohm-Raleigh method, is employed to obtain a modal expression for the full-wave Green's function. By using asymptotic techniques, the far-field electric field is found for an elemental point dipole source. The theory is directly applicable to radiation from a printed circuit antenna having a chiral substrate. Current research efforts show that under certain circumstances, the introduction of chiral materials as substrates of microstrip antennas offers novel radiation properties over achiral microstrip antennas. A number of representative antennas are analyzed with respect to radiation pattern, main beam position, and directivity. / Master of Science

no abstract provided by author / Master of Science

Narrow beamwidth antenna systems are important to remote sensing applications and point-to-point communication systems. In many applications the main beam of the antenna radiation pattern must be scannable over a region of space. Scanning by mechanically skewing the entire antenna assembly is difficult and in many situations is unacceptable. Performance during scan is, of course, also very important. Traditional reflector systems employing the well-focused paraboloidal-shaped main reflector accomplish scan by motion of a few feeds, or by phase steering a focal plane feed array. Such scanning systems can experience significant gain loss. Traditional reflecting systems with a spherical main reflector have low aperture efficiency and poor side lobe and cross polarization performance. This dissertation introduces a new approach to the design of scanning spherical reflector systems, in which the performance weaknesses of high cross polarization and high side lobe levels are avoided. Moreover, the low aperture utilization common in spherical reflectors is overcome. As an improvement to this new spherical main reflector configuration, a flat mirror reflector is introduced to minimize the mechanical difficulties to scan the main beam. In addition to the reflector system design, reflector antenna performance evaluation is also important. The temperature resolution issue important for earth observation radiometer antennas is studied, and a new method to evaluate and optimize such temperature resolution is introduced. / Ph. D.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Recent progress in laser and semiconductor technology has allowed for far easier generation and measuring of coherent terahertz radiation, a previously difficult region in the radiation spectrum to coherently generate. Time based terahertz spectroscopy is a rather unique form of spectroscopy. Not only is it time based, but the electric field is measured instead of the intensity. This allows for the measurement of the complex refractive index. From this one can obtain certain details of the structure and environment of the sample being studied. A terahertz time-domain spectroscopy setup was constructed during this project. This setup used low temperature grown GaAs photoconductive antennae, with multiple antenna size options available for both the receiving and transmitting antennae. After the construction and alignment of this setup, the antennae were characterized. Lastly measurements were performed on the background, sugar and silicon to demonstrate the capabilities of the system. It was found that the measured terahertz electric field amplitude increased with the intensity of the pump pulse and that the amplitude of the measured terahertz electric field was dependent on the polarization of the pump pulse. As the size of the antenna was increased so too did the amplitude of the measured electric field and conversely the bandwidth of the measured terahertz electric field decreased with the increase of antenna size. This held true for both the transmitting and receiving antennae. / AFRIKAANSE OPSOMMING: Danksê onlangse tegnologiese onwikkelings in lasers en halfgeleier het dit veel makliker geraak om terahertz straling te genereer wat fase samehangendheid toon. Voor hierdie ontwikkelings was straling in hierdie spektrale gebied moeilik om te genereer op ’n wyse wat fase samehangendheid toon. Tyd verwante terahertz spektroskopie is taamlik uniek, aangesien die metings in tyd geneem word en die elektriese veld amplitude word pleks van die intensiteit gemeet. Een van die voordele van hierdie metode is dat dit toelaat vir die meeting van die komplekse brekingsindeks van monsters. Dit is moontlik om van die komplekse brekingsindeks strukturele en omgewings eienskappe van die monster af te lei. Gedurende die projek was ’n tyd verwante terahertz spektroskopie sisteem gebou wat gebaseer was op lae temperatuur gegroeide GaAs foto-geleidende antennas. Die sisteem bevat vier antennas van verskillende groottes aan beide die sender en ontvanger kant. Die antennas was gekarakteriseer na die bou en belyning van die terahertz sisteem en meetings was gedoen op die agtergrond, suiker en silikon om die sisteem se vermoë te demonstreer. Dit was gevind dat die amplitude van die gemete terahertz elektriese veld groter geraak het soos die intensiteit van die pomp puls verhoog was en dat die die amplitude van die gemete terahertz electriese veld afhanklik was van die polarisasie van die pomp puls. Die amplitude van van die gemete terahertz elektriese veld het gegroei met die grootte van die antenna, maar hoe groter die antenna geraak het, hoe kleiner was die bandwydte van die gemete terahertz elektriese veld. Hierdie was die geval vir beide die sender en ontvanger antennas.