The aim of this study is to investigate the effects of changes in the physical parameters of tapered slot antennas (TSA) on their radiation characteristics and also to explore the possibility of reconfiguration in the radiation pattern of TSA by switching between two different types of tapering. There are mainly three physical parameters that affect the radiation pattern of TSAs. These are antenna length, aperture width and the ground extension. After designing a wideband microstrip line to slot line transition, the effect of antenna parameter variations on the beamwidth and sidelobe level of the antenna are investigated through the use of a commercially available electromagnetic simulation software HFSS by ANSOFT. The radiation characteristics of constant width slot antennas (CWSA) and linearly tapered slot antennas (LTSA) are compared. It is observed that CWSAs exhibit narrower beamwidth and higher sidelobe level whereas linearly tapered slot antennas (LTSA) have wider beamwidth with lower sidelobe level compared to each other. A novel switching architecture between CWSA and LTSA is proposed to obtain a reconfigurable antenna.

In this thesis, parametric study and design of Vivaldi antennas and arrays are studied. The parameters of single element antennas and arrays are investigated regarding their effects on the design. The return loss responses and radiation patterns are considered in the parametric study. The results of simulations realized using Ansoft HFSS, a high frequency electromagnetic field simulation program, are shown and discussed. Two different Vivaldi antennas operating in 8.5-10.5 GHz frequency band with return loss responses better than 15 dB are designed based on the results of parametric study. Stripline to slotline transition is used in the feeding section of both antennas. In the same manner, two different 8-element uniform linear arrays operating in 8.5-10.5 GHz with half power beam widths smaller than 12&amp / #730 / and side lobe levels smaller than 13 dB are designed. Binomial and Dolph-Chebyshev feeding techniques are also investigated in order to improve half power beamwidths and side lobe levels of the designed arrays. The designed single element Vivaldi antennas and a linear array of Vivaldi antennas are fabricated. The return loss response and radiation patterns of the fabricated antennas and the array are measured and compared with the simulation results.

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This dissertation contains two significant investigations. One is the development of the broadband microwave bandpass filters with high out-of-band performance. The other is the development of low-loss hybrids. These researches are parts of the National Aeronautic and Space Administrator (NASA)s mission to explore the universe. The former is focused on the techniques used in microstrip line bandpass filter design that help achieving both low in-band insertion loss and high out-of-band attenuation level. Moreover, these filters achieve very broadband out-of-band attenuation bandwidth. These techniques are related to the improvement of stepped impedance resonators, coupling between resonators and effective methods to allocate transmission zeros to suppress filters out-of-band spurious responses. The later is focused on the techniques used in planar magic-T designs such that the developed magic-T obtains high isolation between port E (difference port) and port H (sum port). Moreover, it obtains low-loss and broadband characteristics. These techniques are related to the development of the low-loss broadband microstrip-toslotline (MS-to-SL transition and the magic-T with a highly symmetric structure. The theoretical analysis and experimental measurements have been performed. The experimental results of both the filter and magic-T researches show significant improvement over their prior state-of-the-art designs by number of magnitude. The designs also reduce fabrication complexity.