In recent decades phased array antennas with scanning capabilities have shown wide range of applications. One of the major problems in designing of phased array antennas is the presence of blind angles in their scanning range which limits the maximum scan angle considerably. At a blind angle, the reflection coefficient approaches unity, and all input power is reflected back to the Transmitter, in which case there is a possibility of serious damage to the Transmitter circuits. Similarly in the receiver antenna, no power is received at this angle. In this angle based on blindness condition the input impedance becomes short circuit, open circuit or an impedance with a large reactive part in which voltage and current become in phase quadrature. This phenomenon was studied for phased array antennas with various elements. Reason for occurrence of this phenomenon is strong coupling between the radiation modes of the array elements and the guided modes of slow wave structures in the array (such as substrates in planar arrays) at a specific angle. Based on grating lobe circles of the array in blind scan angle the surface wave circles contact with visible circle and input power of array couples with surface waves. In this thesis the reasons for occurrence of this phenomenon in Microstrip Antennas and methods for determining the blind angles are studied in detail. It is shown that by using grating lobe circle diagram we can predict scan angles. Floquet modal analysis is used for calculation of Floquet impedance of infinite array of current sources. Floquet Impedance is defined as Impedance seen from array where all elements are fed with uniform amplitude and linear progressive phase. Spectral method of moment is used for calculation of reflection coefficient versus scan angle from infinite array of probe fed rectangular microstrip patch. Thereafter, electromagnetic band gap structures (EBG) and their properties are introduced. In phase reflection coefficient and band gap characteristics of EBG structures have been studied and a method for calculation of dispersion diagram and reflection coefficient of EBG structures is introduced. Some well-known EBG structures and their reflection and band gap properties are studied using Ansoft HFSS. Finally their applications in removing the blind angles will is demostrated. It was shown that by using EBG structures surface waves are suppressed and no blind angle occurs in the array. Also Specific EBG structure introduced and designed to have band gap in desired frequency range. After that designed Keywords: Phased array antenna, scan blindness, Electromagnetic Band Gap structures, Floquet analysis, spectral domain moment method.
