الفهرس 
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Abstract
Microstrip patch antennas are widely used because of their many advantages, such as low-profile, light weight, and conformity. However, patch antennas have some disadvantage: narrow bandwidth and surface waves. Researchers have made many efforts to overcome these problems. The purpose of this thesis is to investigate a novel method to reduce the mutual coupling in microstrip patch antennas using substrates containing photonic crystals. Photonic crystals are a class of periodic dielectric, metallic, or composite structures that when introduced to an electromagnetic signal can exhibit a forbidden band of frequencies (or band-gap) in which the incident signal destructively interferes and thus is unable to propagate. It is proposed that such photonic crystals will reduce surface waves and prohibit the formation of substrate modes, which are commonly known inhibitors of patch antenna designs. In this thesis the Plane Wave Expansion Method (PWM) is used to calculate band structures of photonic crystals having a frequency independent dielectric function. The dispersion curves band-gaps are found to correspond to the frequency bands which do not allow any propagating modes. Utilization of electromagnetic bandgap (EBG) struc¬tures is becoming attractive in the electromagnetic and antenna community. In this thesis, the effects of a two-dimensional photonic EBG structures on the performance of microstrip patch antenna arrays are investigated using the Ansoft High Frequency Selective Simulator (HFSS™). HFSS is a software package for electromagnetic modeling and analysis of passive, three dimensional structures. It helps the user to observe and analyze various electromagnetic properties of the structure such as radiation patterns and scattering parameters. A mushroom-like EBG structure is compared with a 2-D photonic bandgap (PBG) structures. HFSS™ is employed to determine the effects of different structure parameters on a two element microstrip patch antennas array. Microstrip patch antenna arrays on a uniform substrate suffer from strong mutual coupling due to the pronounced surface waves. Therefore, the use of EBG substrate would improve the performance of the array. Different forms of 2-D photonic bandgap (PBG) substrates have been considered, like: small number of holes, large number of holes, defect mode and different number of mushroom- patches columns structur. The two element microstrip patch antenna array is placed on a defect in the photonic bandgap (PBG) substrate which localizes the energy under the antennas. The excitation frequency of the two element microstrip patch antennas array near the resonance frequency of the defect mode can be used to control the coupling between antennas that are placed in an array. The mutual coupling is found to be reduced by using large number of mushroom- patches columns structure.