With the rapid of growth of wireless connectivity more demand is placed on the need for innovative technologies capable of satisfying increasing user demand and network capacity. Adaptive antennas systems or most commonly known as Smart Antennas are expected to be implemented in the next generation of wireless systems. Their implementation avails in dynamic adaptation to spatial and temporal conditions affecting the quality of communication, while offering tremendous flexibility to wireless providers. However one of the major challenges facing Smart Antenna technology is the inherent complexity of the antenna structure, associated control algorithm and implemented RF components possibly contributing to the delay of commercial interest. This thesis will present various adaptive antenna configurations that utilize an embedded dielectric in order to achieve significant size reduction and mechanical rigidity while maintaining favorable electromagnetic performance. In order to constrict the lateral ground plane dimension, a cylindrical shaped hollow ground skirt was attached to the antenna structures effectively compromising between effective beam forming in the azimuth plane and physical size. The complexity of these antenna structures requires a more contemporary design approach which involved computer modeling using a commercial available Finite Element software package and optimization using a developed generic Genetic Algorithm based optimization program. A dielectric embedded 7-element monopole array antenna featuring switched parasitic elements is presented and optimized for maximum vertically polarized gain in the horizontal plane, producing an antenna structure with a radial length of less then 0.25λ and total height of 0.4&alamba which was shown to radiate a main lobe beamwidth of 80 degrees with an absolute gain of 4.8dBi at 2.45GHz. Further on a dielectric embedded 7-element monopole array antenna featuring parasitic elements terminated with finite set of terminating reactive loads is presented with a radial length of less then 0.25&alambda and total height of 0.4&alambda. The antenna structure and reactive load combination were optimized for maximum horizontal gain producing a principal main lobe with a measured gain of 5.1dBi and beamwidth of 110 degrees at 2.48GHz. Finally it was shown single and dual radiation lobes maybe produced when active monopoles elements are placed eccentric in a circular shaped dielectric material. A circular array of elements embedded in a dielectric material was realized with measured gains of single and dual beam radiation at 2.45GHz was shown to be 5.18dBi and 3.65Bi respectively with corresponding beamwidths of 78.5 degrees and 53 degrees.

The concept of a weak sound radiating cell is proposed to reduce the low frequency radiated noise from structures. The cell consists of two coupled surfaces such that, when placed on a vibrating structure, the responses of the two surfaces are nearly out-of-phase and of equal strength over a wide frequency range. This structure response leads the cell to behave as an acoustic dipole and thus as a poor sound radiating source. The control of low frequency structurally radiated noise is then achieved by covering the structure with an array of these weak radiating cells, i.e. surface treatment. Thus, the surface treatment essentially transforms the response of the structure to that of a distributed array of dipoles yielding a low sound radiating structure. Theoretical models are developed to predict the performance of the cell. Experimental verification is performed for a single cell applied to a piston-like structure to demonstrate the concept on a simple radiating structure. The results demonstrated an overall sound power level reduction of 5.2 dB between 400-1600 Hz with maximum reductions over 30 dB at discrete frequencies. Finally, an array of weak radiating cells is experimentally applied to a more complex structure, a rectangular plate. The results of the plate experiments reveal an overall sound power level reduction of 10.2 dB between 100-1600 Hz with maximum reductions of 25 dB at discrete frequencies. These results demonstrate the potential of the weak radiating cell concept to reduce low frequency structurally radiated noise. / Master of Science

The design of reflectarray surface currents that satisfy both radiation and user-defined antenna feasibility constraints is addressed through a novel paradigm which takes advantage of the non-uniqueness of inverse source (IS) problems. To this end, the synthesis is formulated in the IS framework and its non-measurable solutions are employed as a design DoF. Thanks to the adopted framework, a closed-form expression for the design of reflectarray surface currents is derived which does not require any iterative local/global optimization procedure and which inherently satisfies both the radiation and the feasibility design constraints. The features and potentialities of the proposed strategy are assessed through selected numerical experiments dealing with different reflectarray aperture types/sizes and forbidden region definitions.

Cities are often warmer than their surroundings, and this can lead to more energy being used to cool buildings. This study looks at one city, London, and assesses the impact of the urban environment on the amount of energy used for air-conditioning. There are three main strands to the work. First, it was important to determine just how great the variation in air temperature is, and how this varies through time and through the urban space. Eighty measurement stations were established along radiating lines from the centre of London as far as rural areas. These measured the air temperature simultaneously at hourly intervals for over a year. Second, to support the main data acquisition, short-term tests within London looked at specific aspects of the urban environment that affect air temperature: the effects of vegetation in parks, and facade colour in streets. Third, the impact on energy use of the measured temperature variation was then determined by using simulation to model a standard building in different urban contexts. The mean heat island intensity was found to vary with distance from the centre of London, and with the local degree of urbanization at any given distance. The maximum intensity reached 8°C on occasion but was more usually 1-2°C in the daytime and 3-4°C at night. The proximity of areas of vegetation, such as parks, to a site was associated with cooler daytime temperatures. The annual cooling load for a standard building at the centre of London was found to be 25% more than at a rural site. However, at the most urban sites over-shadowing reduced the cooling load to 14% more than at a rural site. Heating load decreased towards the centre, but on balance total annual load (for heating and cooling) rose towards the centre to 8.5% more than rural use, and then reduced at the most overshadowed sites. The balance of the effect of urbanization on heating and cooling load depended on the level of internal gain in the building. This study makes a significant contribution to understanding the balance of the impact of urban environments on the energy used for cooling and heating buildings.

8-oxo-7,8-dihydroguanine (8oxoG) is one of the most important base lesions formed during oxidative damage of DNA. The aim of the present research was to investigate the effects of DNA concentration, G content, and the nature of oxidizing species on the kinetics of 8oxoG in model DNA solutions by using HPLC. The experimentally obtained yields of 8oxoG were typically in the range of 2-2.5% of total concentration of guanine. The ratios of the rate constant of hole diffusion in DNA to the rate constant of conversion of the hole into 8oxoG (kd/kr) were calculated from the experimental data using the diffusion model of charge transfer in DNA to be in the range of 200-300, in agreement with previously reported kd/kr ratios in the duplex DNA oligonucleotides (GGA)n or (GGTT)n. Our current diffusion model cannot satisfactorily explain the absence of the G content dependence of the 8oxoG yields, which indicates that a more advanced model is required.

Switched and adaptive parasitic monopole array radiating structures are investigated. Antenna design is orientated toward increasing practicability for implementation in terrestrial wireless communication systems. A number of antennas are designed with the aid of optimization and commercial simulation software. Simulation procedure was verified with the experimental manufacture and measurement of the arrays. The antennas presented in this thesis comprise an active monopole surrounded by a ring of parasitic monopoles. Parasitic radiators are constructed with static loading to enable simple experimental realization. Beam positions of an electrically steered equivalent antenna are thus simulated. Antenna symmetry ensures the beam can be reproduced throughout the azimuth. Complex antenna geometries require antenna design through optimization. A genetic algorithm is employed with HFSS and NEC for electromagnetic analysis. The robust optimization method couples with simulation software flexibility to provide an effective design tool for arbitrary structures. The genetic algorithm is employed strictly for design and not complete structural optimization. Dual band, five and six element switched parasitic antennas are presented. Lumped elemental loading along the radiators provide resonance and directed radiation at two GSM frequencies. Load value, radiator dimension and spacing are incorporated as design parameters. Experimentally built, 10dB return loss bandwidths of 17.2% and 9.6% and front to back ratios of 12.6dB and 8.4dB at 900MHz and 1900MHz respectively are measured. To reduce the ground requirements of monopole arrays, a skirted ground structure for switched parasitic antennas is analyzed. A six element switched parasitic monopole array with conductive ground skirt exhibits a front to back ratio of 10.7dB and main lobe gain of 6.4dBi at 1.575GHz. Radiation is not elevated despite lateral ground terminating at the parasitic elements. Skirt height is observed to linearly control radiation elevation, depressing the principal lobe through 40 degrees from 23 degrees above the horizontal. The Electronically Steerable Passive Array Radiator or ESPAR antenna is an adaptive parasitic monopole array. An ESPAR radiating structure incorporating a conductive ground skirt is designed for operation at 2.4GHz. Utility is confirmed with a frequency sensitivity analysis showing consistent electrical characteristics over an 8.1% bandwidth. The antenna design is improved with optimization to reduce average principal lobe elevation from 25 degrees to 9.7 degrees.

Predictive accuracy of the previously developed coupled code for the solution of the time-dependent Navier-Stokes equations in conjunction with the radiative transfer equation was first assessed by applying it to the prediction of thermally radiating, hydrodynamically developed laminar pipe flow for which the numerical solution had been reported in the literature. The effect of radiation on flow and temperature fields was demonstrated for different values of conduction to radiation ratio. It was found that the steady-state temperature predictions of the code agree well with the benchmark solution. In an attempt to test the predictive accuracy of the coupled code for turbulent radiating flows, it was applied to fully developed turbulent flow of a hot gas through a relatively cold pipe and the results were compared with the numerical solution available in the literature. The code was found to mimic the reported steady-state temperature profiles well. Having validated the predictive accuracy of the coupled code for steady, laminar/turbulent, radiating pipe flows, the performance of the code for transient radiating flows was tested by applying it to a test problem involving laminar/turbulent flow of carbon dioxide through a circular pipe for the simulation of simultaneous hydrodynamic and thermal development. The transient solutions for temperature, velocity and radiative energy source term fields were found to demonstrate the physically expected trends. In order to improve the performance of the code, a parallel algorithm of the code was developed and tested against sequential code for speed up and efficiency. It was found that the same results are obtained with a reasonably high speed-up and efficiency.

The electronic control, instrumentation and communication hardware is becoming more and more compact and faster in operation due to the increased use of large scale integration of semiconductor devices operating at higher speeds. The use of VLSI circuit based systems in various industrial and defence sectors is also increasing continuously. Since the operating threshold voltages and currents of these devices are very small they are very prone to electrical disturbance in their operation by the Electromagnetic Interference (EMI) signals. Their proper functioning is very important particularly in the case of systems used in mission mode, critical defence/industrial platforms. EMI can be generated within the electronic system/equipment itself or may result due to some external electromagnetic source. The high power Ultra Wide Band system is one such kind of external High Power Electromagnetic (HPEM) interference source which may cause malfunctioning/physical damage to the sensitive electronic systems. Hence it is necessary to test the susceptibility of electronics to such high power UWB based intentional EMI or IEMI sources. The sources for generating these transient EM fields may also be used in impulse radars and offensive applications to mal-operate/damage non-friendly electronics. The UWB system consists of a high voltage pulsed power source called pulser along with a high bandwidth (Ultra Wide Band) antenna to radiate the UWB signal. The pulse fed by the pulser to the antenna through a switch is of high voltage type (amplitude of few 10s of kV to about a MV) and has a sub-nanosecond rise time. Most of the UWB systems developed over the world have the switch employing gaseous dielectric switching media used at pressures above the atmospheric level to generate such a fast rise time voltage pulse. Use of gaseous switching media at sub-atmospheric pressures to achieve sub-ns rise time, short duration high voltage pulses required for the high power UWB applications is another possibility. This possibility has not been exploited till date. Hence it was decided to develop a pulser switch with gaseous switching media at sub-atmospheric pressures (up to 50 mbar) and achieve sub-ns rise time voltage pulses of up to 50 kV. The energy delivered out by the UWB system depends upon the pulser output energy per switching shot and the repetitive switching rate of the pulser. To achieve maximum energy output it is required to maximize either the energy per switching shot or the pulse repetition rate (PRR) of the pulser switch. The optimization of the pulser operation to achieve maximum pulser energy output in every switching shot has not been tried so far. In this work it was decided to analyze the circuit so as to achieve maximum pulser output energy per switching shot. Another objective of the study was to systematically characterize the pulser switch using various gases and gas mixtures as the switching media to evaluate the switch performance as a function of gas pressure and switch breakdown voltage. The effect of pulser and antenna performance parameters on the UWB system performance was also decided to be evaluated. Hence the present thesis work deals with the design, development, evaluation and performance optimization of a 50 kV, 25 MW UWB system based on Half Impulse Radiating Antenna (HIRA) fed by a coaxial capacitive pulser. The spark gap type self triggered pulser switch is designed to have a fixed gap spacing and variable gas pressure in order to vary the switch breakdown voltage. The switch is designed for operation with dry air, nitrogen, sulphur hexafluoride (SF6) and a mixture of different gases as the dielectric switching media with pressures of up to 5 bar above the atmospheric level and up to 50 mbar below the atmospheric level. Physical placement of the switch just above the coaxial pulser capacitor terminal offered a low inductance geometry. The rise time estimation of the switch has been carried out as a function of gas pressure and the switch arc inductance. These rise time values have been compared with the measured ones and a good agreement was found between the two. The rise time values indicate that an inverse relationship exists between the gas pressure and the rise time. The rise time was found to decrease at increased pressures. SF6 gas offered the minimum rise time out of all the gases/mixtures studied. The pulse repetition rate (PRR) of the UWB system depends upon the dielectric recovery of the gaseous switch and the charging time of the pulser capacitor. To estimate the PRR a circuit model has been proposed based on these parameters. The model shows an inverse relationship between the switch breakdown voltage (BDV) and the gas pressure with the PRR. The estimated PRR values were found to vary between 800 Hz and 5 kHz in the experimented range of the switch breakdown voltage. The PRR values have also been experimentally measured. There is a good match between the measured and the estimated values up to the switch BDV of 12.5 kV after which the difference is increased to about 20 %. The feed for the reflector of the HIRA antenna consists of a pair of coplanar conical transverse electromagnetic (TEM) feed plates as they have a better antenna aperture blockage performance. The angles of the TEM feed plates have been chosen using stereographic projections of the feed plates into the HIRA reflector. Each TEM feed plate of 200  characteristic impedance has been terminated by matched resistor. An analytical expression has been derived to optimize the pulser output voltage at which the energy output per switching shot of the UWB system is maximum. It was found that when the pulser output voltage i.e. the switch breakdown voltage is 75 % of the dc source voltage the output energy delivered is maximum. It was possible to achieve a maximum output energy of 10 J per switching shot for the designed 25 MW high power UWB system. The HIRA antenna has been analysed for the impedance profile for frequencies up to 3.5 GHz and was found to maintain a reflection performance better than -10 dB over the frequency range. The radiated field analysis of the antenna was carried out using an analytical model and numerically by using a commercially available software. It was found that as per the analytical model, the Figure of Merit (FoM) of the designed UWB system is 1.41 V for a normalized excitation feed pulse of 1 V and the 3 dB spectral content of the radiated field is between 180 MHz-1.8 GHz. The corresponding results using computer simulations of the UWB system indicate a slightly lesser FoM of 1.1. Higher FoM obtained using the analytical model is due to ignoring the antenna aperture blockage and the field diffraction effects over the TEM feed arms as well as from the rim of the reflector of the antenna. The radiated field amplitude and gain of the HIRA antenna were found to be a direct function of the frequency of the radiated signal. Higher gains and narrower beam width for the radiated field were observed with an increase in the frequency. The radiated field spectral waveform in the near field region was observed to have a notch at a particular frequency and its harmonics. The notch frequency was found to be a function of the propagation time difference called clear time. The effect of pulser rise time, antenna feed arm impedance and position on the radiated far field amplitude and wave shape was analysed. It was observed that with decrease in the pulser rise time from 700 ps to 100 ps, the radiated field amplitude increases by about 600 %. A matched termination impedance with position of 30of the TEM feed arms with respect to the vertical symmetry axis of the antenna provides a higher radiated field amplitude and lower post pulse oscillations in the radiated field waveform. The pulser switch was evaluated systematically for various performance parameters such as BDV, rise time, PRR, voltage recovery and jitter characteristics as a function of switch gas pressure, type of gaseous switching media and breakdown voltage at pressures above and below the atmospheric level. The switch BDV was found to be a linear function of pressure of the gas used i.e. dry air, nitrogen, sulphur hexafluoride (SF6) and a mixture of air and SF6. The measured rise times of all the gases were found to be in inverse proportion to the switch gas pressure. SF6 gas offered the best rise time and hence was found to be a good contender for achieving higher radiated field amplitudes and bandwidth. The voltage recovery characteristics of SF6 gas and air were experimentally studied as a function of the recovery time. It is found that both the gases have similar recovery characteristics having a distinct saturation plateau region. It was found that for a given recovery time SF6 recovers to a higher voltage than air and the recovery further improves for SF6 at increased pressures (between 0.5-2 bar). The effect of the number of switching shots on the jitter in the switch rise time was measured by operating the switch continuously at a PRR of 1 kHz and for total number shots up to 10.8 M. It was observed that the jitter increases by an order of magnitude after 10.8 M shots. This indicates that for the present switch design, the switch electrodes require maintenance (buffing, polishing, etc.) after every 3.5 M shots to maintain a reasonably low jitter. SF6 gas was characterized for a fixed source voltage to determine the effect of pressure on rise time in the sub atmospheric regime (up to 50 mbar). It was found that the rise time vs. pressure characteristics follows the Paschen’s curve with a value of pressure at which rise time is the lowest for a given source voltage. With increase in the source voltage the rise time was found to decrease. The HIRA based UWB radiating system was evaluated for radiated fields in the near and far field region for the temporal and spectral characteristics. It was found that for the source voltage of 25 kV, the FoM in the near and far field region are 29.4 kV and 28.9 kV respectively. The fields in the distant far field region have more oscillatory post pulses due to the effect of ground reflections and the low frequency dipole moment mismatch of the antenna. Since SF6 gas offered the best rise time of 193 ps at a voltage of 46 kV than the other gases tried, the radiated field is the highest (5.3 kV/m) with SF6 at a distance of 10 m offering a gain factor of 1.15. Dry air offered a radiated field gain factor of 0.83 which got improved by 33 % by just 30 % addition of SF6 gas into the air. The field amplitudes measured were in good agreement with those computed using the analytical model and the computer simulations and they follow the 1/R rule as a function of the far field distance, R in the bore sight direction. The measured radiation pattern of the UWB system showed a focussed and narrow radiated field beam at higher frequencies with a half field beam width (HFBW) of 8 at 2 GHz. The UWB system was measured to have dominant highest cut off frequency of 1.79 GHz with a band ratio and percentage band width of 9.56 and 162.11 % respectively. This confirmed that the developed system is of sub-hyper band radiator type. The UWB system developed through this work is having a better performance than some of the other systems developed elsewhere in the world, in terms of FoM (53 kV) and the PRR (> 1 kHz). The system can be further improved in terms of consistency (jitter) and intensity by use of a triggered switch and hydrogen gas at 100 bar pressure as the switching medium respectively. The profile of the TEM feed plates of the HIRA antenna may be further improved to have a better antenna aperture fill factor. Such multiple systems in an arrayed manner may be used either for higher power output/better agility of the radiated field beam. This system will be fully exploited for the applications of susceptibility evaluation of electronic circuits, non-friendly applications as well as impulse radars

The Direct Simulation Monte Carlo method is a computational tool for modelling rarefied flows. The Macroscopic Chemistry Method was developed to simplify the modelling of dissociation and recombination reactions in DSMC. The ability to understand and predict the behaviour of chemically reacting, rarefied flows is a critical aspect in the development of high altitude, high speed bodies such as re-entry craft, high altitude aircraft, space transport vehicles and missiles. Computational methods are an invaluable source of information when experimental techniques are difficult, costly or time-consuming. However, traditional methods of modelling chemical kinetics using DSMC suffer from a number of drawbacks. The Macroscopic Chemistry Method overcomes a number of these problems, but has previously only been applied to simulations of a single diatomic gas. The Macroscopic Chemistry Method (MCM) is extended to consider multiple species and multiple reaction sets, thermal non-equilibrium effects, trace species modelling, unsteady flows, vibrational state specific chemistry, electronic excitation, relaxation and ionization and coupled nonequilibrium radiation emission. The Macroscopic Method is described as a general DSMC modelling philosophy rather than as a single formulated method. That is, the flexibility and utility of the method are shown through examples of applying a macroscopic approach to a number of problems, and by highlighting instances where a macroscopic approach is useful or even necessary. The problems investigated include reservoir relaxation calculations, 1-D shock, expansion and shock-expansion calculations, two-dimensional flows over a vertical step and through a cavity, and axis-symmetric flow about a sphere. The studies demonstrate that although MCM may often present a simplified approach as compared to traditional 'non-macroscopic' methods, it does not necessarily lead to more approximate solutions. On the contrary, the ability of macroscopic methods to combine different models of physical processes with the most recent (verified) data means that they are particularly suited to simulate high altitude, rarefied flows. It is also shown that, like any model approach, the validity of the approximations employed must be justified for a particular problem. In general, macroscopic methods of varying complexity and accuracy may be implemented to model a specific physical process. Adoption of the Macroscopic Chemistry Method in DSMC has the potential to enhance the modelling of chemical kinetics, charged-particle effects and radiation in rarefied hypersonic flows. This capability may be attributed to the simplicity and flexibility which the macroscopic approach affords over methods which seek to avoid the use of collective information. Macroscopic methods have already been employed to model weakly ionized flows. Their further application to model chemical kinetics and other processes would be useful for modelling and understanding the behaviour of objects in rarefied hypersonic flow-fields.

A non-iterative pressure based algorithm which consists of splitting the solution of momentum energy and species equations into a sequence of predictor-corrector stages was developed for the simulation of transient reacting radiating flows. A semi-discrete approach called the Method of Lines (MOL) which enables implicit time-integration at all splitting stages was used for the solution of conservation equations. The solution of elliptic pressure equation for the determination of pressure field was performed by a multi-grid solver (MUDPACK package). Radiation calculations were carried out by coupling previously developed gray and non-gray radiation models with the algorithm. A first order (global) reaction mechanism was employed to account for the chemistry. The predictions of the algorithm for the following test cases: i) non-isothermal turbulent pipe flow and ii) laminar methane-air diffusion flame / were benchmarked against experimental data and numerical solutions available in the literature and the capability of the code to predict transient solutions was demonstrated on these test cases. Favorable agreements were obtained for both test cases. The effect of radiation and non-gray treatment of the radiative properties were investigated on the second test case. It was found that incorporation of radiation has significant effect on Temeprature and velocity fields but its effect is limited in species predictions. Executions with both radiation models revealed that the non-gray radiation model considered in the present study produces similar results with the gray model at a considerably higher computational cost. The algorithm developed was found to be an efficient and versatile tool for the timedependent simulation of different flow scenarios constitutes the initial steps towards the computation of transient turbulent combustion.