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Title: ITERATIVE SOLUTION OF MULTIPLE RIGHT HAND SIDE MATRIX EQUATIONS FOR FREQUENCY DOMAIN MONOSTATIC RADAR CROSS SECTION CALCULATIONS
Abstract: Monostatic rcs characterisation using integral equation methods in the frequency domain requires the solution of very large matrix equations with multiple right hand sides. Although costly for a single right hand side, direct methods are attractive in that subsequent right hand sides are very cheap. Iterative methods are much cheaper for a single right hand side, but if the whole solution must be repeated for each, they become much more expensive. We investigate here the performance of a simple modification to the GCR algorithm, which allows solutions for an essentially unlimited number of right hand sides to be obtained for a modest multiple of the cost of the first. For the cases investigated, with up to 360 right hand sides on bodies up to 15 wavelengths long, with matrices up to 20,440 by 20,440 in size, this multiple was below ~10. Costs seem to rise with the number of right hand sides till the surface field is in some sense characterised, and thereafter subsequent illumination angles are essentially free. An investigation of cost scaling on a set of spheres, ranging from ~1 to 7 wavelengths in diameter, seems to indicate the cost of full monostatic characterisation to scale with about the fifth power of frequency
Author(s): M. D. Pocock, S. P. Walker
File Type: Journal Paper
Issue:Volume: 13      Number: 1      Year: 1998
Download Link:Click here to download PDF     File Size: 3462 KB

Title: A FDTD SURFACE IMPEDANCE BOUNDARY CONDITION USING Z-TRANSFORMS
Abstract: The surface-impedance boundary condition for the Finite-Difference, Time-Domain (FDTD) method is re-formulated using digital filtering theory and Z trans-forms. The approach expands upon recent work in de-veloping an efficient surface-impedance boundary condi-tion for FDTD. The present work involves formulating the surface-impedance boundary condition in the frequency domain for a lossy dielectric half-space and for a thin lossy dielectric layer backed by a perfect conductor. The impedance function of the lossy medium is approximated with a series of low-pass filters. This approximation is independent of material properties and these low-pass fil-ters are converted to corresponding digital filters using Z-transform theory. The FDTD surface-impedance bound-ary condition is reformulated in the Z domain, and the corresponding time-domain electric field sequence updat-ing equation involves a recursive formula. Results are presented for both one and two-dimensional test prob-lems.
Author(s): John H. Beggs
File Type: Journal Paper
Issue:Volume: 13      Number: 1      Year: 1998
Download Link:Click here to download PDF     File Size: 1351 KB

Title: SHIPBOARD HFDF SYSTEM SIMULATION
Abstract: NEC 4.1 has been used to compute the responses of the antennas in a shipboard high frequency direction finding system which employs the CIDF algorithm to derive bearing estimates. This paper discusses the computational results as well as the performance of the simulation in which the results were utilized.
Author(s): Jeffrey B. Knorr
File Type: Journal Paper
Issue:Volume: 13      Number: 1      Year: 1998
Download Link:Click here to download PDF     File Size: 3556 KB

Title: HYBRID MOM/SBR METHOD TO COMPUTE SCATTERING FROM A SLOT ARRAY ANTENNA IN A COMPLEX GEOMETRY
Abstract: A method of moments (MoM) code has been developed to compute the scattering from a pla-nar or cylindrically conformal slot array antenna. By hybridizing the MoM with the shooting and bouncing ray (SBR) method, the scattering from a large, complex target with a slot array antenna can be computed. The scattering problem can be decomposed using the field equivalence prin-ciple such that the MoM is employed to model the slot array while the SBR method is used to compute the scattering from the large, complex target. Sample results show the utility of the method and the need to include slot array scat-tering when computing the RCS of a complex target.
Author(s): Andrew D. Greenwood, Jian-Ming Jin
File Type: Journal Paper
Issue:Volume: 13      Number: 1      Year: 1998
Download Link:Click here to download PDF     File Size: 514 KB

Title: EFFECTS OF GAPS AMONG PANELS IN RADIO ASTRONOMY REFLECTOR ANTENNAS
Abstract: The main reflector of antennas used for radio astronomy consists of hundreds of panels among which, for various reasons, small gaps are left. In this paper, the effects of these gaps on the field scattered by the reflector are analyzed by means of a hybrid numerical technique which combines the Finite Ele-ment Method (FEM) and the Method of Moments (MoM). Numerical results pertaining to the case of an incident plane wave are presented, and the effects of the introduction of corrugations inside the gaps to minimize the power flowing through the gaps them-selves are discussed.
Author(s): Giuseppe Pelosi, Roberto Coccioli, Alessio Gaggelli
File Type: Journal Paper
Issue:Volume: 13      Number: 1      Year: 1998
Download Link:Click here to download PDF     File Size: 611 KB

Title: SOME EXPERIENCES IN USING NEC2 TO SIMULATE RADIATION FROM SLOTS ON CYLINDERS
Abstract: A numerical method based on use of the NEC2 code has been used to study radiation from a slot in the surface of a conducting cylinder and the results used in a comparison with the eigenfunction series solu-tion. The two cases of an axial and circumferential slot have been taken as representative between them of the more general case of an inclined slot. In the case of a circumferential slot, truncation of the cylin-der to finite length has been found in all cases to lead to numerical and analytic results which are in poor agreement and this has been accounted for with an argument based on the geometrical theory of dif-fraction. However this alone is not enough to explain what is observed with axial slots where, for small cyl-inders, agreement of the two approaches is good but fails for larger cylinders. It is postulated that this is due to excitation of waveguide modes on the inside of the larger cylinders and the result confirmed by clos-ing the ends of the cylinder in the NEC2 model with a system of radial wires, when agreement is again re-stored.
Author(s): S. H. H. Lim, H. E. Green, C. E. Brander
File Type: Journal Paper
Issue:Volume: 13      Number: 1      Year: 1998
Download Link:Click here to download PDF     File Size: 422 KB

Title: A KIRCHHOFF INTEGRAL APPROACH FOR DECIMETRIC RADIOWAVE PROPAGATION IN URBAN AREAS
Abstract: We consider a three-dimensional approach based on the Kirchhoff’s method in order to predict electromagnetic waves propagation in urban environments. In particular, we are interested here in the evaluation of the electro-magnetic field on very large three-dimensional domains (typically with linear dimensions of the order of hun-dreds of meters) generated by a high frequency source (typically of the order of 1GHz which corresponds to a wavelength of about 30 centimeters). Some numerical tests and comparisons with experimental measurements have been done to validate this approach.
Author(s): L. Pisani, F. Rapetti, C. Vittoli
File Type: Journal Paper
Issue:Volume: 13      Number: 1      Year: 1998
Download Link:Click here to download PDF     File Size: 4588 KB

Title: ON THE BOUNDED PART OF THE KERNEL IN THE CYLINDRICAL ANTENNA INTEGRAL EQUATION
Abstract: The kernel in the cylindrical antenna integral equation was partitioned by Schelkunoff into a com-plete elliptic integral and a bounded integral. This paper gives an exact expression for the bounded part.
Author(s): M. P. Ramachandran
File Type: Journal Paper
Issue:Volume: 13      Number: 1      Year: 1998
Download Link:Click here to download PDF     File Size: 343 KB