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Title: GTD, PO, PTD, AND GAUSSIAN BEAM DIFFRACTION ANALYSIS TECHNIQUES APPLIED TO REFLECTOR ANTENNAS
Abstract: Several available diffraction analyses techniques are compared in this paper. Techniques including GTD, PO, PTD, and Gaussian Beams, are used to analyze representative reflector antenna geometries. First the techniques are compared for a flat circular disc, representing an unfocused system. Next, the techniques are applied to offset ellipsoidal reflectors and the offset parabolic reflectors. Near-fields, focal- fields and far-fields are determined using these techniques. Both copolar and cross-polar fields are compared. The acceptability ranges of each technique is carefully investigated. Numerical data are presented for representative configurations and, in particular, field intensities are determined for high power microwave applications. [Vol. 6, No. 1, pp. 6-30 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Electromagnetlcs]
Author(s): Y. Rahmat-Samii, P. O. Iversen, D. W. Duan
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 1537 KB

Title: MODERN HIGH FREQUENCY TECHNIQUES FOR RCS COMPUTATION; A COMPARATIVE ANALYSIS
Abstract: In this article, a comparative analysis is given of various asymptotic high frequency methods for the computation of the Radar Cross Section (RCS) of complex targets. After a brief revue of their principle, the limitations of the most popular methods and of their recent developments: Physical Optics (PO), Physical Theory of Diffraction (PTD), Geometrical Theory of Diffraction (GTD), Uniform Theory of Diffraction (UTD) and PTD extended to creeping waves, are analysed in relation with their theoretical foundations and the critical aspects of their application to the computation of RCS are discussed and illustrated by some numerical examples. [Vol. 6, No. 1, pp. 31-58 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Electromagnetics]
Author(s): F. A. Molinet
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 1758 KB

Title: A HYBRID UTD-EIGENFUNCTION METHOD FOR SCATTERING BY A VERTEX
Abstract: Present solutions for the electromagnetic scattering by a vertex are either approximate or difficult to use for computations. For example, GTD (UTD) solutions for vertex scattering are not yet fully developed. The exact eigenfunction solution is both difficult to use and computationally inefficient due to the large number of eigenfunctions that must be retained. In this work, we obtain the scattering by a vertex (e.g., a quarter plane) by employing the exact eigenfunction solution only in a very small region close to the tip of the vertex. Thus, only a small number of eigenfunctions (e.g., two or three) are required to obtain the current in the tip region. Outside of this region, the UTD is employed to obtain the current. The changeover point is determined by finding the point where the eigenfunction current has decayed to that predicted by UTD wedge and vertex diffraction theory. Results will be shown for the scattered field from the plane angular sector. In addition, the field scattered by a rectangular plane using this method will be compared with that predicted by the UTD with vertex diffraction, and the results will be seen to be in very close agreement. [Vol. 6, No. 1, pp. 59-74 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Electromagnetics]
Author(s): John N. Sahalos, Gary A. Thiele, F. M. Tait
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 667 KB

Title: CALCULATION OF ANTENNA PERFORMANCE USING A HYBRID TECHNIQUE WHICH COMBINES THE MOMENT METHOD WITH AN ASYMPTOTIC CURRENT
Abstract: Previously, this hybrid technique, a hybrid theory of diffraction, which combines the moment method with an asymptotic current, was used by Kim and Thiele to solve several 2-dimensional and 3dimensional scattering problems when the incident field was a plane wave. In this paper the method is extended to problems in which the incident field is the near field of an antenna and also extended to calculate antenna impedance. As a sample problem, a monopole at the center of a circular disk is considered. Since the general procedure is an iterative one, the starting point in this problem is the assumption of the current distribution on the monopole. This current distribution generates the initial incident field which in turn gives the initial value of the asymptotic current on the circular disk (which is similar to the geometrical optics current). The current in the asymptotic region (i.e., away from the edge of the disk), which is dominated by the optics type current on the entire surface of the scatterer, is solved by an iterative method to give the approximate surface current using the magnetic field integral equation. The difference between the approximate and true surface currents is calculated from the moment method current, which exists near shadow boundaries and/or sharp discontinuities in geometry. That is, the moment method current induces a current in the asymptotic region which is the difference between the optics type current and the true current. The impedance may be calculated by using the infinite ground plane value plus the change caused by the finite extent of the ground plane through the use of equivalent currents. Results are shown for both the impedance and the radiation patterns. These results are compared with other known results and the agreement is seen to be very good for both large (e.g., ka > 6) and small (e.g., ka < 3) ground planes. [Vol. 6, No. 1, pp. 75-95 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Electromagnetics]
Author(s): Li Shi-zi, Gary A. Thiele
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 920 KB

Title: ANALYSIS AND SIMULATION OF COLLISION AVOIDANCE TCAS ANTENNAS MOUNTED ON AIRCRAFT
Abstract: Traffic-alert and Collision Avoidance systems (TCAS) are being developed by order of the Federal Aviation Administration (FAA) to assist aircraft pilots in mid-air collision avoidance. This paper discusses computer models developed to simulate the radiation patterns of TCAS arrays mounted on the fuselage of any aircraft. Computer models are also used to calculate and evaluate the errors in estimating the bearing of aircraft in the vicinity of a TCAS-equipped airplane. The performance of two TCAS systems mounted on a Boeing 737 aircraft are studied and compared. [Vol. 6, No. 1, pp. 112-139 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Electromagnetics]
Author(s): R. G. Rojas, K. S. Sampath, W. D. Burnside
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 1594 KB

Title: NUMERICAL TECHNIQUES TO DETERMINE THE UTD BISTATIC CREEPING WAVE PATHS AND PARAMETERS FOR AN ELLIPSOID
Abstract: In applying the Uniform Geometrical Theory of Diffraction (UTD) to evaluate the scattering patterns of a doubly curved surface, the determination of correct ray paths is one of the most important and difficult tasks. In this paper, an efficient numerical technique to obtain the complete ray path of the creeping wave for the bistatic scattering of an ellipsoid is discussed. Also, the numerical method to evaluate the energy spreading factor of the creeping wave and the caustic distance at the diffraction (launching) point are described. An ellipsoid is chosen because of its modeling capability to represent the fuselage of an aircraft and similar objects. The same numerical techniques for an ellipsoid can be extended to a general doubly curved surface as well. [Vol. 6, No. 1, pp. 140-158 (1991), Special Issue on Applicatlons of High Frequency Methods and Computer Techniques in Electromagnetics]
Author(s): Jaehoon Choi, Ronald J. Marhefka
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 932 KB

Title: DESCRIPTION OF A RADAR CROSS SECTION PREDICTION CODE WITH APPLICATIONS TO INDUSTRIAL PROBLEMS
Abstract: Following the recent advances in the fields of electromagnetic prediction theory and computer power, and a growing interest to low observables or stealth platforms [1], new software tools for the calculation of radar backscattering have been developed in several companies/institutions. While the basics of the algorithms used are the same, each code has been focused on different ranges of frequencies, kinds of platforms, and so on. This paper describes the prediction code developed at IDS, which is currently used in consultancy activities in the fields of design of new platforms with reduced/controlled RCS, as well as evaluation of the radar signature of existing systems. [Vol. 6, No. 1, pp. 159-174 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Electromagnetics]
Author(s): Francesco Zacca, Giuliano Bettini, Riccardo Cioni
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 3549 KB

Title: NUMERICAL MODEL OF MICROWAVE BACKSCATTERING AND EMISSION FROM TERRAIN COVERED WITH VEGETATION
Abstract: A computational model for estimating microwave scattering and emission from the earth's surface covered with vegetation (MESCAM) has been developed. The model is founded on relevant electromagnetic properties of the vegetation and of the underlying terrain and takes into account multiple scattering both within the vegetation and between vegetation and underlying soil. By selecting the appropriate scattering functions, the backscattering coefficient and the emissivity can be estimated in a wide range of frequencies, for different sensor configurations, and for a variety of terrain and vegetation characteristics. [Vol. 6, No. 1, pp. 175-191 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Electromagnetics]
Author(s): P. Ferrazzoli, L. Guerriero, D. Solimini
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 1124 KB

Title: ANALYTIC RAY TRACING FOR THE STUDY OF HF MAGNETO-IONIC RADIO PROPAGATION IN THE IONOSPHERE
Abstract: Ray tracing provides a quantitative tool for the study of HF radio wave propagation in the ionosphere, but its use is limited in many applications by the time required to numerically trace many separate rays. One approach to overcoming this problem is the use of ionospheric models that admit analytic solutions of the ray tracing equations. Such a model may be based on a vertical plasma frequency profile consisting of what are known as quasi-parabolic segments. Analytic tracing is much faster than numerical tracing (from 5 to 10 times), but more limited in the range of situations that can be considered. The main thrust of the paper is to present and test a technique, based on the use of an equivalent operating frequency, that allows the analytic results to be extended to take approximate account of the magneto-ionic effects associated with the earth's magnetic field. Errors in group path, phase path and ground range are generally less than 5 km, errors in absorption generally less than 5%. [Vol. 6, No. 1, pp. 192-210 (1991), Special Issue on Applications of High Frequency Methods and Computer Techniques in Elcctromagnetics]
Author(s): J. A. Bennett, J. Chen, P. L. Dyson
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 1468 KB

Title: Antennas on dielectric coated convex surfaces : theory and experimentations
Abstract: The purpose of this paper is to analyze the diffraction phenomenon introduced by perfectly conducting convex surfaces with dielectric coating, resulting from an interaction of these structures with an electromagnetic radiation created essentially by sources which are set on these objects at high frequencies.
Author(s): M. -A. Blondeel-Fournier, W. Tabbara, L. Beaulieu
File Type: Journal Paper
Issue:Volume: 6      Number: 1      Year: 1991
Download Link:Click here to download PDF     File Size: 1033 KB