Gary A. Thiele

A hybrid technique for combining moment methods with the geometrical theory of diffraction

A technique for combining moment methods with the geometrical theory of diffraction (GTD) is presented, which permits the application of the method of moments to a larger class of problems. The fundamental idea used to develop the hybrid technique is to modify the usual impedance matrix that characterizes, for example, a wire antenna such that a metallic body or discontinuity on that body is properly accounted for. It is shown in general that one can modify the impedance matrix for any basis and/or weighting functions if one can compute the correct modification to the impedance matrix element. The modification is readily accomplished using the geometrical theory of diffraction and/or geometrical optics. Several example problems are considered to illustrate the usefulness of the technique. First, the canonical problem of a monopole near a conducting wedge is investigated. Second, a monopole at the center of a four-sided and an eight-sided flat plate is considered. Impedance results for the latter case are in good agreement with measurements. Third, a monopole at the center of a circular disc is examined and compared with experimental measurements in the literature, and fourth, the problem of a monopole near a conducting step is solved and the dependence of the input impedance upon the step height shown.

A hybrid technique for combining the moment method treatment of wire antennas with the GTD for curved surfaces

This hybrid technique is a method for solving electromagnetic problems in which an antenna is located near a conducting body. The technique accomplishes this by casting the antenna structure in a moment method (MM) format, then modifying that format to account for the effects of the conducting body via the geometrical theory of diffraction (GTD). The technique extends the moment method to handle problems that cannot be solved by GTD or the moment method alone. Wire antennas are analyzed to find their input impedance when they are located near perfectly conducting circular cylinders, although the methods used are not restricted to circular cylinders. Three orthogonal orientations are identified, and antennas to match them are analyzed. For each case, the hybrid solution is checked with one of three independent solutions: an MM-eigenfunction solution, image theory, or experimental measurement. In almost all cases, excellent agreement is obtained due in large part to the fact that the moment method near fields are, for the first time, cast into a ray optical form.

Analysis of yagi-uda-type antennas

A method of analyzing Yagi-Uda-type antennas is presented. Since the Yagi-Uda array is a fairly well-known antenna, it is used as an example to demonstrate the application and accuracy of the method. However, the method of solution is not limited to a planar array, such as the Yagi, but can be applied to arrays of nonplanar linear elements. The approach taken in analyzing Yagi-Uda antennas is based on rigorous equations for the electric field radiated by the elements in the array. All interactions are taken into account. Calculated results are presented for the Yagi-Uda array that show excellent agreement with experimental results reported in the literature. In addition, the dependence of the far-field patterns on the phase velocity is shown. It is also demonstrated that the phase velocity is generally nonuniform along the array.

Comments on "A field iterative method for computing the scattered electric fields at the apertures of large perfectly conducting cavities"

An iterative method is developed for computing the scattered electric fields at the apertures of large perfectly conducting cavities. The field iterative method (FIM) uses Kirchhoffs approximation to initiate a two stage iterative process (i.e., the method of successive approximations), involving both the magnetic field integral equation and the electric field integral equation, to calculate the electric currents on the internal cavity walls and the electric fields across the aperture of the cavity. The technique combines the flexibility of the boundary-integral method with the speed necessary to efficiently analyze large scale cavity problems. The paper presents the general theory, and applies the technique to the problem of TE scattering from two-dimensional perfectly conducting cavities. >

On the application of the GTD-MM technique and its limitations

In 1975 two techniques were published that combined the method of moments (MM) and the geometrical theory of diffraction (GTD). One technique extended the moment method through the use of the GTD while the second used the moment method to solve for unknown diffraction coefficients, thereby extending the use of the GTD. It is the latter method that is considered in this paper and is referred to as the GTD-MM technique. One problem area that existed with the original GTD-MM work was associated with a field incident along or nearly along one wall of a wedge structure. An improved series representation for the diffracted current that is sufficient at all incidence angles is shown. The improved formulation is then applied to the problem of bistatic scattering by a three-sided pyramid. Radar cross section (RCS) results that compare very well with experimental measurements are obtained. This is believed to be the first use of the GTD-MM technique in treating a three-dimensional geometry.

Application of the hybrid technique to time domain problems

A hybrid technique which formally combines the method of moments with the geometrical theory of diffraction is used to efficiently generate sufficient frequency domain data so that accurate transformation to the time domain can be accomplished via the fast fourier transform. The advantage of the hybrid technique is that it permits one to solve problems that cannot readily be solved by either method alone. For example, the problem of a monopole at the center of a circular disk is considered. The monopole is characterized by the method of moments, and the finite size of the disk is accounted for by geometrical theory of diffraction techniques. The technique applies equally well, however, to antennas on other bodies such as satellites and aircraft and could be used to investigate the electromagnetic pulse (EMP) response of antennas on such bodies.

Design of a small conformal array

The principal objective of this investigation was to determine how to efficiently excite a small conical body over a 2:1 frequency bandwidth such that a prescribed minimun value of gain is exceeded in a 60\deg conical sector about the forward axis of the cone. To achieve the necessary bandwidth, two approaches have been considered. First, an electrically small moderately efficient tunable antenna can be employed to excite currents on the cone. Second, one can employ a wide-band antenna having at least a 2:1 bandwidth. This latter approach was investigated theoretically and found to be feasible but experimental confirmation was not attempted in deference to the first approach that has the potential of greater gain (efficiency). To demonstrate the validity of the first approach a four element conical array was constructed and is described in this paper. Experimental results agreed well with theoretical expectations.

Application of the Hybrid Technique to Time Domain Problems

A hybrid technique which formally combines the method of moments with the geometrical theory of diffraction is used to efficiently generate sufficient frequency domain data so that accurate transformation to the time domain can be accomplished via the fast fourier transform. The advantage of the hybrid technique is that it permits one to solve problems that cannot readily be solved by either method alone. For example, the problem of a monopole at the center of a circular disk is considered. The monopole is characterized by the method of moments, and the finite size of the disk is accounted for by geometrical theory of diffraction techniques. The technique applies equally well, however, to antennas on other bodies such as satellites and aircraft and could be used to investigate the electromagnetic pulse (EMP) response of antennas on such bodies.

Radar cross section of open circular loops

Two methods of calculating the monostatic and bistatic scattering by an open circular loop are briefly discussed. Theoretical and experimental backscatter results are presented that show how the echo area varies when a straight wire is curved into an arc of decreasing radius of curvature. For small radii of curvature, an interesting resonance phenomenon is observed.

Geodesic lens antennas for low-angle radiation

Previous unit-index geodesic lens antenna designs have not been able to produce a lens with good radiation characteristics in or near the plane of the lens rim. This paper extends previous work to permit the design of a geodesic lens for angles within approximately 20\deg of the plane of the lens rim. This is accomplished by requiring that less than the full semicircular aperture be exactly focused, and also by dividing the outer annulus into two or more constant-slope sections. The capability of these lenses for beam elevation positioning is also discussed.