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Title: A THIN DIPOLE ANTENNA DEMONSTRATION OF THE ANTENNA MODELING CAPABILITIES OF THE FINITE DIFFERENCE TIME DOMAIN TECHNIQUE
Abstract: The Finite Difference Time Domain (FDTD) technique has been successfully applied for modeling the electromagnetic scattering from and coupling into a variety of objects. In this communication we use FDTD to compute the input impedance of a thin dipole antenna. The short circuit current and open circuit voltage at the antenna terminals are computed over a wide bandwidth using pulsed plane wave excitation, then Fourier transformed to the frequency domain and divided to obtain the complex input impedance over a wide bandwidth using one FDTD computation. These results are compared with thin wire antenna results using the Method of Moments and good agreement is obtained except at very low frequencies, where the FDTD results obtained using this approach lose accuracy due to the imperfect outer absorbing boundary. [Vol. 5, No. 1, pp. 2-18 (l990)]
Author(s): K. S. Kunz, R. J. Luebbers, F. Hunsberger
File Type: Journal Paper
Issue:Volume: 5      Number: 1      Year: 1990
Download Link:Click here to download PDF     File Size: 533 KB

Title: MODELING ELECTRICALLY SMALL, THIN SURFACES WITH WIRE GRIDS
Abstract: Wire grids are widely used to model conductive surfaces. Wire grid models reported in the literature have primarily addressed electrically large or resonant configurations. Electrically small surfaces, however, can also have a significant effect on radiation characteristics and these surfaces are often difficult to model using other methods. A structure containing an electrically small, thin plate with wires attached to both sides is analyzed using different wire-grid models. The thin plate is modeled as either an open or closed surface using single and double-layer grids. Comparing the model results with measurements suggests that a closed surface (double-layer) grid may be more accurate and efficient than an open surface (single-layer) grid in many cases. Unlike single-layer grids, doublelayer grids do not permit current to flow through the surface and hence enforce an important boundary condition. [Vol. 5, No. 1, pp. 19-24 (l990)]
Author(s): Todd H. Hubing, J. Frank Kauffman
File Type: Journal Paper
Issue:Volume: 5      Number: 1      Year: 1990
Download Link:Click here to download PDF     File Size: 449 KB

Title: A LIMITED COMPARISON OF PREDICTED AND MEASURED RESULTS FOR AN HF GROUND- ARRAYED LOG PERIODIC DIPOLE ARRAY
Abstract: Measurements of the in-situ radiation pattern of a full scale horizontally polarised HF log periodic antenna are compared with predicted results from two different computer programs. The limited results suggest that more attention should be paid to ground effects at low take-off angles. While the predicted results are in good agreement with each other for take-off angles below the peak of the pattern, they tend to diverge above the peak. [Vol. 5, No. 1, pp. 25-38 (l990)]
Author(s): Duncan C. Baker
File Type: Journal Paper
Issue:Volume: 5      Number: 1      Year: 1990
Download Link:Click here to download PDF     File Size: 890 KB

Title: RADAR ANTENNA PATTERN ANALYSIS FOR THE SPACE SHUTTLE USING NEC-BSC
Abstract: In order to improve tracking capability, radar transponder antennas will be mounted on the space shuttle solid rocket boosters (SRB). These four antennas, each being identical cavity-backed helices operating at 5.765 GHz, will be mounted near the top of the SRB's, adjacent to the intertank portion of the external tank. The purpose of this study is to calculate the roll-plane pattern (the plane perpendicular to the SRB axes and containing the antennas) in the presence of this complex electromagnetic environment. The large electrical size of this problem mandates an optical approach, thus a general purpose code, the Numerical Electromagnetics Code - Basic Scattering Code, was chosen as the computational tool. This code is based on the modern Geometrical Theory of Diffraction and allows computation of scattering of bodies composed of canonical shapes such as plates and elliptic cilinders. Apertures mounted on a curved surface (the SRB) cannot be accommodated by the code, so an antenna model consisting of wires was devised that approximated the actual performance of the antennas. Although the method of moments (MM) was not used in developing the antenna model, the code's MM input option proved instrumental in implementing the scheme. The improvised antenna model matched well with measurements taken at the NASA/Marshall Space Flight Center (MSFC) range. The SRB's, the external tank, and the shuttle nose were modeled as circular cylinders, and the code was able to produce what is thought to be a reasonable roll-plane pattern. [Vol. 5, No. l, pp. 39-46 (1990)]
Author(s): Roger Radcllff, Warren Harper
File Type: Journal Paper
Issue:Volume: 5      Number: 1      Year: 1990
Download Link:Click here to download PDF     File Size: 338 KB

Title: AN ALTERNATIVE DESCRIPTION OF THE MAGNETIC CURRENT ANNULAR RING (FRILL) SOURCE
Abstract: An alternative method is presented of considering, and of deriving expressions for the fields generated by, an annular ring of magnetic current (magnetic frill source). The magnetic frill appears to offer a means of numerical model excitation that is more realistic than the pulse source and, moreover, provides analytical expressions for fields in some cases. The fields for the magnetic frill source were first derived by Tsai from the electric vector potential produced by the magnetic current. The method described shows the source to resemble a toroidal transformer and the field expressions are derived from the magnetic vector potential produced by electric currents. The expressions derived are, in essence, the same as those of Tsai, but it is considered that the method yields greater physical insight into the source and so facilitates modification to suit particular applications. Tsai's expression which is of most interest for numerical calculations is derived by inspection using the method described. For calculations using point matching however, the benefits of using the frill source seem more apparent than real. [Vol. 5, No. l, pp. 47-62 (1990)]
Author(s): J. W. R. Cox
File Type: Journal Paper
Issue:Volume: 5      Number: 1      Year: 1990
Download Link:Click here to download PDF     File Size: 714 KB

Title: BICONICAL TRANSMITTING ANTENNAS, A NUMERICAL ANALYSIS
Abstract: We have obtained a general, numerical solution of an ideal biconical transrnitting antenna, with arbitrary arm length and conic angle. We evaluate all necessary spherical functions, including Legendre functions of integer and noninteger degrees, and spherical Hankel functions of integer degrees. Using Schelkunoff s solution, field coefficients appear as an infinite set of unknowns that satisfy a linear equation. We truncate the infinite set at 16; for a 5O antenna the Legendre functions have maximum degree 33.3 in the interior region and 31 in the exterior region. To minimize the error, we discard the last two terms in all field, power, and impedance calculations. Solutions are checked in several ways for consistency, including evaluating and comparing calculated fields across the antenna aperture. Results obtained are input impedance, radiation pattern, all fields including near and far ones, and antenna surface current and charge density. Representative plots of all results are included. [Vol. 5, No. l, pp. 62-93 (l990)]
Author(s): Vahid Batii, Ken Tomiyama, Dale M. Grimes
File Type: Journal Paper
Issue:Volume: 5      Number: 1      Year: 1990
Download Link:Click here to download PDF     File Size: 1098 KB