Roberto G. Rojas

Electromagnetic diffraction of an obliquely incident plane wave field by a wedge with impedance faces

A uniform asymptotic solution is presented for the electromagnetic diffraction by a wedge with impedance faces and with included angles equal to 0 (half-plane), pi /2 (right-angled wedge), pi (two-part plane) and 3 pi /2 (right-angled wedge). The incident field is a plane wave of arbitrary polarization, obliquely incident to the axis of the wedge. The formal solution, which is expressed in terms of an integral, was obtained by the generalized reflection method. A careful study of the singularities of the integrand is made before the asymptotic evaluation of the integral is carried out. The asymptotic evaluation of the integral is performed taking into account the presence of the surface wave poles in addition to the geometrical optics poles near the saddle points. This results in a uniform solution which is continuous acros the shadow boundaries of the geometrical optics fields as well as the surface wave fields. >

Efficient analysis of input impedance and mutual coupling of microstrip antennas mounted on large coated cylinders

An efficient and accurate hybrid method, based on the combination of the method of moments (MoM) with a special Greens function in the space domain is presented to analyze antennas and array elements conformal to electrically large material coated circular cylinders. The efficiency and accuracy of the method depend strongly on the computation of the Greens function, which is the kernel of the integral equation that is solved via MoM for the unknown equivalent currents representing only the antenna elements. Three types of space-domain Greens function representations are used, each accurate and computationally efficient in a given region of space. Consequently, a computationally optimized analysis tool for conformal microstrip antennas is obtained. Input impedance of various microstrip antennas and mutual coupling between two identical antennas are calculated and compared with published results to assess the accuracy of this hybrid method.

Efficient computation of surface fields excited on a dielectric-coated circular cylinder

An efficient method to evaluate the surface fields excited on an electrically large dielectric-coated circular cylinder is presented. The efficiency of the method results from the circumferentially propagating representation of the Greens function as well as its efficient numerical evaluation along a steepest descent path. The circumferentially propagating series representation of the appropriate Greens function is obtained from its radially propagating counterpart via Watsons transformation and then the path of integration is deformed to the steepest descent path on which the integrand decays most rapidly. Numerical results are presented that indicate that the representations obtained here are very efficient and valid even for arbitrary small separations of the source and field points. This work is especially useful in the moment-method analysis of conformal microstrip antennas where the mutual coupling effects are important.

Wiener-Hopf analysis of the EM diffraction by an impedance discontinuity in a planar surface and by an impedance half-plane

The Wiener-Hopf technique is used to solve two canonical problems. The first problem considered is the electromagnetic (EM) diffraction, by a planar surface with an impedance discontinuity (two-part surface), of an arbitrarily polarized plane wave obliquely incident to the axis of the two-dimensional objects. The second problem considers the EM diffraction by a half-plane with equal impedances on both sides. The solutions obtained are cast in a matrix notation which is useful for diffraction problems. The exact formal solutions are expressed in terms of integrals which can be asymptotically evaluated. Uniform asymptotic expressions are obtained where the presence of the geometrical optics (GO) poles as well as the surface-wave poles near the saddle point are fully taken into account. Several numerical examples are presented and it is shown that the solutions are continuous across the shadow boundaries of the GO and surface-wave fields. >

Scattering by an inhomogeneous dielectric/ferrite cylinder of arbitrary cross-section shape-oblique incidence case

A moment method (MM) solution is developed for the fields scattered by an inhomogeneous dielectric/ferrite cylinder of arbitrary cross-section. The incident field is assumed to be a plane wave of arbitrary polarization with oblique incidence with respect to the axis of the cylinder. The total electric and magnetic fields are the unknown quantities in two coupled equations from which a system of linear equations is obtained. Once the total electric and magnetic fields within the cylinder are computed, the scattered fields at any other point in space can be calculated. It is noted that for the case of oblique incidence, the scattered field has TE/sub z/ and TM/sub z/ polarized fields regardless of the polarization of the incident field. The echo widths of cylinders and shells of circular, semicircular, and rectangular cross section are calculated for TE/sub z/ and TM/sub z/ polarized incident fields. It is shown that the results obtained for dielectric/ferrite cylinders and shells of circular cross section with the solutions developed here agree very well with the corresponding exact eigenfunction solutions. >

Design of Frequency Reconfigurable Antennas Using the Theory of Network Characteristic Modes

This paper demonstrates a design procedure for frequency tunable reconfigurable antennas based on the application of reactive loads. Unlike other design procedures, antennas of arbitrary geometry can be tuned utilizing the proposed design framework. The design technique utilizes the theory of network characteristic modes to systematically compute reactive load values required to resonate any antenna at many frequency points in a wide frequency range. For simplicity, a 1.2 m dipole antenna is used to demonstrate the design procedure by tuning it at four loading ports along the antenna body. Both simulations and measurements demonstrate wide frequency tunability characteristics of the dipole input impedance (tunability range wider than 1:4) while preserving the radiation pattern and polarization at seven different frequency states. Lastly, a loaded PIFA is briefly examined as a more complex application of the procedure.

Diffraction of EM waves by a dielectric/ferrite half-plane and related configurations

Uniform asymptotic high-frequency ray solutions are developed for the problems of diffraction of electromagnetic plane, cylindrical, and surface waves normally incident on the edge of an isotropic, homogeneous dielectric/ferrite half-plane. The boundary conditions developed, which are referred to as the generalized impedance boundary conditions (GIBC), yield solutions that recover the exact geometrical constants of the surface wave fields in these problems. In the limiting case of small thickness, the solutions recover the solutions based on the boundary conditions given by L.A. Weinstein (1969). Furthermore, the relatively simple ray solutions developed are shown to be very accurate by comparing them with those obtained by an independent approach based on the moment method. >

Integral Equations for the Scattering by a Three Dimensional Inhomogeneous Chiral Body

Integral equations are obtained for the electromagnetic (EM) scattering by an inhomogeneous, isotropic, three dimensional chiral body. The chiral body is assumed to be in free space, and it can be attached to a perfect electric conducting (PEC) body. The integral equations are obtained with the help of vector-dyadic identities and the free space dyadic Greens function. These equations are expressed in terms of a volume integral with the electric field as the unknown and surface integrals where the tangential components of the electric field and its curl are the unknowns. The integral equations are then transformed into a linear system of simultaneous equations by means of the moment method technique. Expressions for the scattered field in the far-zone are also obtained by replacing the dyadic Greens function and its curl with their large argument approximations. Furthermore, closed form expressions are obtained for the fields and dipole moments induced inside an electrically small, homogeneous chiral sp...

Electromagnetic plane wave diffraction by a planar junction of two thin dielectric/ferrite half planes

A uniform geometrical theory of diffraction solution is developed for the two-dimensional problem of high-frequency plane wave diffraction by a planar junction of two thin dielectric/ferrite half planes. Each material half plane in this two-part configuration is assumed to be electrically thin so that it can be replaced in the analysis by a generalized resistive boundary condition of 0(t), where t is the corresponding material slab thickness. The solution obtained is based on the Wiener-Hopf technique, and it is shown that the present boundary value problem can be completely solved by imposing a junction condition at the junction of the two dielectric half planes in addition to the boundary and radiation conditions as well as the usual edge condition. This junction condition can be obtained if the field near the junction is modeled by a quasi-static solution. The solution developed here can be further specialized to limiting cases for which either of the material half planes can become free space, perfectly conducting, or purely resistive, respectively. Several numerical examples are presented, and it is shown that the present solution reduces to known results and agrees very well with a moment method solution.

Comparison between two asymptotic methods

Two complete asymptotic expansions of an integral with many simple pole singularities and a first-order, isolated, saddle point evaluated by two different methods are compared. It is shown that both expansions are exactly the same (term by term) inside and outside the transition regions.