تعيين تابع گرين دياديكي با حضور استوانه هادي الكتريكي كامل بزرگ پوشيده شده با فريت

چكيده انگليسي :

Determination of Dyadic Green s Function of Ferrite Coated Electrically Large PEC Circular Cylinder Mohammad Hossein Sadrearhami mh sadrearhami@ec iut ac ir Date of Submission 2011 4 18 Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSuprevisor Dr Abolghasem Zeidaabadi Nezhad zeidabad@cc iut ac irAbstractToday conformal antennas are widely used in airborne radar applications One of the important issues related tothese antennas are calculating radiation pattern and mutual coupling between antennas mounted on aircraft platform Because these antennas are usually mounted on surfaces whose radius of curvature is larger than wavelength numerical methods are not practical in these cases Furthermore to reduce radar cross section RCS of aircrafts their bodies are usually coated with absorbing material One of the most common used radar absorbing materials RAM is ferrite coatings Due to tensor permeability of these materials calculations in the presence of coatings aremuch more complicated To solve problems including geometries larger than wavelength asymptotic methods suchas uniform theory of diffraction UTD are used This method is highly efficient as compared to the conventionalnumerical methods such as method of moment MOM More importantly it provides useful physical insights intothe antenna radiation and coupling mechanisms To use asymptotic methods we need Green s function in anaccurate and simple form But for ferrite coated cylinder calculation of Green s function is difficult and thereforeapproximate methods must be used A common and simple approximate method in electromagnetic and alsoacoustic problems is impedance boundary condition IBC With this method electromagnetic fields is solved onlyfor outer region of ferrite layer and as a result complexity of calculation reduced considerably Impedance boundaryconditions are widely used to analyze problems with dielectric structures but this method has not been used foranalyzing ferrite layers In this thesis we first derive impedance boundary condition for ferrite layers To do thisfields in ferrite medium are expanded using Taylor series and then appropriate impedance boundary condition isderived The resulting condition is general and can be used in every orthogonal curvilinear coordinate The validityand accuracy of computed IBC is also examined We observed that by increasing radius of cylinder accuracy of ourapproximate impedance boundary condition increased In addition accuracy of our method is decreased byincreasing ferrite layer thickness Using this impedance boundary condition Green s function of ferrite coatedcylinder is calculated Because Green s function is in infinite series form and converges slowly specially for surfacefields we expand it with asymptotic methods First by using Watson transform we transform infinite series solutionto integral form Then we use Fock theory and method of steepest descent to transform the integral form into aclosed and computationally efficient one To verify our method surface fields and far fields pertaining to the surfacemagnetic current is computed by exact series solution and our asymptotic solution and results are compared Theresults show that the accuracy of asymptotic methods in the far field is better than those of surface fields Ourproposed closed form solution is accurate within the nonparaxial region and become less accurate in the paraxial close to axial region of the cylinder Also by increasing radius of the cylinder accuracy of proposed method isimproved Key WordsFerrite coating Dyadic Green s function Impedance boundary condition Asymptotic method Uniform Theory ofDiffraction UTD Surface fields Far field

صدرارحامي، محمد حسين

تعيين تابع گرين دياديكي با حضور استوانه هادي الكتريكي كامل بزرگ پوشيده شده با فريت

شرط مري امپدانس , روش هاي مجانبي , تئوري تغرق يكنواخت , ميدان هاي سطحي ميدان هاي راه دور