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A dual-frequency system is needed to better understand natural processes that constitute the environment and seasonal cycles of the Earth. A system working at two different wavelengths acquiring data simultaneously will give a valuable dataset since the conditions on the ground will be exactly the same. Hence, elements such as wind, soil moisture or any other changes on the ground will not interfere in the mea- surements. This thesis explains how an S-band radar was built and tested. Moreover, the experiments done with a Ka-band radar used as a scatterometer are explained as well as the data processing and analysis. Finally, the two systems are used to get dual-frequency measurements from an airborne platform. The dual-frequency data is explored, showing the differences in normalized radar cross-section between frequencies and discussing the interferometric measurements.

This thesis presents experimental studies of low-temperature atmospheric pressure plasma sources with generic ability to effectively treat large-scale three-dimensional (3D) objects. The reported large-scale plasma sources are developed through parallelisation of single plasma jets. This strategy outshines the other reported strategies for treatment of uneven surfaces by being able to produce spatially extended plasma directly onto the surface of heavily three-dimensional objects. Comparable studies of the design of elemental plasma jets bring out a hybrid electrode configuration, the capillary-ring jet, as the best elemental jet to be used in the parallelisation. It is found that the introduction of a ballast resistor to the individual jet circuit or built-in capacitance is important to assure the jet-to-jet synchronism, stability and uniformity. Electrical and optical analyses of one-dimensional (1D) array of atmospheric pressure plasma jets demonstrate robust temporal and spatial jet-to-jet uniformity both for flat and sloped surfaces. Hexagonally-arranged two-dimensional (2D) arrays of atmospheric pressure plasma jets show good level of insusceptibility to variations of the downstream samples in their physical dimensions as well as structural and material properties. The reaction chemistry impact area of a 2D 37-jet array is estimated to be 18.6 cm2. These confirm the plasma jet arrays as a viable option as large-scale atmospheric plasma sources, well suited for many processing applications including plasma medicine. The spatially separated dual-frequency excitation further benefits the plasma jet in that separate control of different important plasma parameters is possible. Enhanced plasma properties achieved by the dual-frequency offer greater potential to the jet arrays.

The designs of various feeding networks for microstrip antennas have been investigated in this dissertation. In the active microstrip antenna designs, an amplifier-type microstrip antenna integrates a two- port amplifier circuitry to the various passive antennas through a 50£[ inset microstrip feed line which can function in broadband operation¡ACP operation and dual-frequency operation. As for the broadband circularly polarized microstrip antenna with a dual-perpendicular feed design, antenna with a dual-perpendicular feed which was formed by a Wilkinson power divider with a quarter-wavelength section of microstrip line for providing equal splitting of the input power and 90¢X phase difference to the two feed points has been presented. The broadband CP bandwidth can be achieved due to the good isolation between the two output lines of the feed network. In the microstrip antennas with CPW feed designs, we first proposed a microstrip antenna with three-dimensional CPW feed. A separate feed substrate is used which can provide a large-area space for the layout of complicated microwave circuitry. In addition, the feed substrate can also be oriented to the patch substrate with various angles, resulting in more flexibility in the proposed three-dimensional feed design. Finally, we proposed a design with two slot lines located at both sides of the coupling slot which can lengthen the excited patch surface current path, thus the resonant frequency is reduced and an antenna size reduction up to 40% has been obtained.

Neste trabalho é apresentada uma análise da antena planar F-invertido (PIFA) com o objetivo de identificar os parâmetros geométricos que influenciam nas características de desempenho: frequência de ressonância, perda de retorno e largura de banda. Para realizar este estudo é desenvolvido um algoritmo em linguagem C baseado no método das Diferenças Finitas no Domínio do Tempo (FDTD). Duas PIFAs de geometria distintas foram investigadas. O primeiro modelo trata da geometria tradicional de elemento irradiador e plano de terra, ambos retangulares. Já o segundo modelo apresenta a fenda em formato de L no elemento irradiador, para operação em frequência dupla, e o plano de terra em formato de T, visando a aumentar a largura de banda. Os resultados da simulação do algoritmo identificam os parâmetros que influenciam no aumento nas duas larguras de banda, e são aplicados na prototipação de uma PIFA GSM-900 e GSM-1800. As medidas de laboratório do protótipo evidenciam os resultados previstos pelo algoritmo. / A Planar Inverted-F Antenna (PIFA) analysis is presented in this work, aiming to identify the geometric parameters that influence the antenna performance, such as resonant frequency, return loss and bandwidth. To accomplish this goal, an algorithm in C language based on the Finite-Difference Time-Domain (FDTD) method is developed. Two PIFAs with distinct geometries are investigated. The first one corresponds to a typicall rectangular patch and ground plane, The second model presents an L-shaped slot for dual frequency band operation, combined with a T-shaped ground plane, in order to increase the bandwidth. The simulated results obtained by algorithm identify the parameters that respond for both bandwidths enhancement, and are applied in a PIFA GSM-900/GSM-1800 practical project. The prototype’s measurements confirm the simulated results.

Neste trabalho é apresentada uma análise da antena planar F-invertido (PIFA) com o objetivo de identificar os parâmetros geométricos que influenciam nas características de desempenho: frequência de ressonância, perda de retorno e largura de banda. Para realizar este estudo é desenvolvido um algoritmo em linguagem C baseado no método das Diferenças Finitas no Domínio do Tempo (FDTD). Duas PIFAs de geometria distintas foram investigadas. O primeiro modelo trata da geometria tradicional de elemento irradiador e plano de terra, ambos retangulares. Já o segundo modelo apresenta a fenda em formato de L no elemento irradiador, para operação em frequência dupla, e o plano de terra em formato de T, visando a aumentar a largura de banda. Os resultados da simulação do algoritmo identificam os parâmetros que influenciam no aumento nas duas larguras de banda, e são aplicados na prototipação de uma PIFA GSM-900 e GSM-1800. As medidas de laboratório do protótipo evidenciam os resultados previstos pelo algoritmo. / A Planar Inverted-F Antenna (PIFA) analysis is presented in this work, aiming to identify the geometric parameters that influence the antenna performance, such as resonant frequency, return loss and bandwidth. To accomplish this goal, an algorithm in C language based on the Finite-Difference Time-Domain (FDTD) method is developed. Two PIFAs with distinct geometries are investigated. The first one corresponds to a typicall rectangular patch and ground plane, The second model presents an L-shaped slot for dual frequency band operation, combined with a T-shaped ground plane, in order to increase the bandwidth. The simulated results obtained by algorithm identify the parameters that respond for both bandwidths enhancement, and are applied in a PIFA GSM-900/GSM-1800 practical project. The prototype’s measurements confirm the simulated results.

Neste trabalho é apresentada uma análise da antena planar F-invertido (PIFA) com o objetivo de identificar os parâmetros geométricos que influenciam nas características de desempenho: frequência de ressonância, perda de retorno e largura de banda. Para realizar este estudo é desenvolvido um algoritmo em linguagem C baseado no método das Diferenças Finitas no Domínio do Tempo (FDTD). Duas PIFAs de geometria distintas foram investigadas. O primeiro modelo trata da geometria tradicional de elemento irradiador e plano de terra, ambos retangulares. Já o segundo modelo apresenta a fenda em formato de L no elemento irradiador, para operação em frequência dupla, e o plano de terra em formato de T, visando a aumentar a largura de banda. Os resultados da simulação do algoritmo identificam os parâmetros que influenciam no aumento nas duas larguras de banda, e são aplicados na prototipação de uma PIFA GSM-900 e GSM-1800. As medidas de laboratório do protótipo evidenciam os resultados previstos pelo algoritmo. / A Planar Inverted-F Antenna (PIFA) analysis is presented in this work, aiming to identify the geometric parameters that influence the antenna performance, such as resonant frequency, return loss and bandwidth. To accomplish this goal, an algorithm in C language based on the Finite-Difference Time-Domain (FDTD) method is developed. Two PIFAs with distinct geometries are investigated. The first one corresponds to a typicall rectangular patch and ground plane, The second model presents an L-shaped slot for dual frequency band operation, combined with a T-shaped ground plane, in order to increase the bandwidth. The simulated results obtained by algorithm identify the parameters that respond for both bandwidths enhancement, and are applied in a PIFA GSM-900/GSM-1800 practical project. The prototype’s measurements confirm the simulated results.

The ionosphere is a layer of ionised gas in the upper layers of the atmosphere around the Earth that plays a critical role in satellite communication, military communication and space science. The influence that the ionosphere has on communication systems can be quantified if the distribution of the electron density within the ionosphere is known. Several methods and instruments to determine the distribution of electron density are currently being used: satellites, ionosondes, incoherent scatter radars and computerised ionospheric tomography based on dual-frequency GPS signals. The present study investigates a novel way of using GPS receivers on mobile platforms to achieve near real-time ionospheric characterisation over locations beyond the reach of land-based ionospheric characterisation methods. GPS observations were collected, pre-processed and inverted by means of tomography to generate three-dimensional electron density maps. These electron density maps were analysed and verified. The viability of using observations from a mobile GPS receiver for ionospheric tomography was investigated. The algorithms were verified by means of a model ionosphere and a simulated GPS receiver. Furthermore, electron density maps generated from GPS observables from a mobile receiver were verified against ionosonde-derived electron density profiles, static land-based GPS receivers and known high-frequency propagation paths using propagation path prediction. The results were evaluated and the conclusion was that, although some aspects still have to be addressed, a dual-frequency GPS receiver on a ship can provide useful ionospheric characterisation in areas which are otherwise poorly or not covered by land-based receivers. / Dissertation (MEng)--University of Pretoria, 2011. / Electrical, Electronic and Computer Engineering / unrestricted

In this dissertation, liquid crystal (LC) materials and devices are investigated in order to meet the challenges for photonics and displays applications. We have studied three kinds of liquid crystal materials: positive dielectric anisotropic LCs, negative dielectric anisotropic LCs, and dual- frequency LCs. For the positive dielectric anisotropic LCs, we have developed some high birefringence isothiocyanato tolane LC compounds with birefringence ~0.4, and super high birefringence isothiocyanato biphenyl-bistolane LC compounds with birefringence as high as ~0.7. Moreover, we have studied the photostability of several high birefringence LC compounds, mixtures, and LC alignment layers in order to determine the failure mechanism concerning the lifetime of LC devices. Although cyano and isothiocyanato LC compounds have similar absorption peaks, the isothiocyanato compounds are more stable than their cyano counterparts under the same illumination conditions. This ultraviolet-durable performance of isothiocyanato compounds originates from its molecular structure and the delocalized electron distribution. We have investigated the alignment performance of negative dielectric anisotropic LCs in homeotropic (vertical aligned, VA) LC cell. Some (2,3) laterally difluorinated biphenyls, terphenyls and tolanes are selected for this study. Due to the strong repulsive force between LCs and alignment layer, (2,3) laterally difluorinated terphenyls and tolanes do not align well in a VA cell resulting in a poor contrast ratio for the LC panel. We have developed a novel method to suppress the light leakage at dark state. By doping positive [Delta][epsilon] or non-polar LC compounds/mixtures into the host negative LC mixtures, the repulsive force is reduced and the cell exhibits an excellent dark state. In addition, these dopants increase the birefringence and reduce the viscosity of the host LCs which leads to a faster response time. Dual-frequency liquid crystal exhibits a unique feature that its dielectric anisotropy changes from positive to negative when we increase the operating frequency. Submillisecond response time can be achieved by switching the frequency of a biased voltage, rather than switching the voltage at a given frequency. In this dissertation, we investigate the dielectric heating effect of dual-frequency LCs. Because the absorption peak of imaginary dielectric constant occurs at high frequency region (~ MHz), there is a heat generated when the LC cell is operated at a high frequency voltage. To measure the transient temperature change of the LC inside the cell, we have developed a non-contact method by utilizing the temperature-dependent birefringence property of the LC. Most importantly, we have formulated a new dual-frequency LC mixture which greatly reduces the dielectric heating effect while maintaining good physical properties. Another achievement in this thesis is that we have developed a polarization independent phase modulator by using a negative dielectric anisotropic LC gel. With ~20 % of polymer mixed in the LC host, the LC forms polymer network which, in turn, exerts a strong anchoring force to the neighboring LC molecules. As a result, the operating voltage increases but the response time is significantly decreased. On the phase shift point of view, our homeotropic LC gel has ~0.08 [pi] phase shift, which is 2X larger than the previous nano-sized polymer-dispersed liquid crystal droplets. Moreover, it is free from light scattering and requires a lower operating voltage. In conclusion, this dissertation provides solutions to improve the performance of LC devices both in photonics and displays applications. These will have great impacts in defense and display systems such as optical phased array, LCD TVs, projectors, and LCD monitors.