Analysis of near-field Cassegrain reflector: plane wave versuselement-by-element approach

A near-field Cassegrain reflector (NFCR) is an effective way to magnify a small phased array into a much larger aperture antenna for limited scan applications. Traditionally the pattern wave approach, i.e. the field from the feed array incident on the subreflector is approximated by a truncated collimated beam with planar phase and tapered amplitude distribution. This approach simplifies the computation tremendously, but fails to provide design information about the most critical component of the whole antenna system, namely, the feed array. With the help of today's computers, it is now feasible to calculate the pattern of a NFCR by a more exact element-by-element approach. Each element in the feed array is considered individually and the diffraction pattern from the subreflector is calculated by the geometrical theory of diffraction (GTD). The field contributions from all elements are superimposed at the curved main reflector surface, and a physical optics integration is performed to obtain the secondary pattern     

Self-similar prefractal frequency selective surfaces for multiband and dual-polarized applications

Frequency-selective surfaces (FSS), that have been designed using fractal iterative techniques, have been fabricated and measured. Fractals contain many scaled copies of the starting geometry, each of which acts as a scaled version of the original. A multiband FSS can be designed that uses several iterations of the geometry to form a prefractal that resonates corresponding to each of the scales present in the geometry. Minkowski and Sierpinski carpet fractals have been utilized in the design of three surfaces which exhibit two or three stopbands depending on how many iterations are used to generate the geometry of the cell. These surfaces are dual polarized due to the symmetry of the geometry. Simulation capabilities have been developed to analyze these periodic structures, including periodic method of moments (MOM) and finite-difference time-domain (FDTD) techniques which show good correlation to the measured results.     

Beam efficiency of reflector antennas: the simple formula

Beam efficiency has proven to be an important measure for characterizing the performance of reflector antennas. In this paper, a simple formula is presented to allow for a quick estimate of the beam efficiency of reflector antennas. This estimate includes the effects of edge tapers, feed spillover, random surface errors, and central blockage. The application of this formula is detailed in a case study, and its accuracy is demonstrated in comparison with a numerical simulation using diffraction analysis     

A parallel electromagnetic genetic-algorithm optimization (EGO) application for patch antenna design

In this paper, we describe an electromagnetic genetic algorithm (GA) optimization (EGO) application developed for the cluster supercomputing platform. A representative patch antenna design example for commercial wireless applications is detailed, which illustrates the versatility and applicability of the method. We show that EGO allows us to combine the accuracy of full-wave EM analysis with the robustness of GA optimization and the speed of a parallel computing algorithm. A representative patch antenna design case study is presented. We illustrate the use of EGO to design a dual-band antenna element for wireless communication (1.9 and 2.4 GHz) applications. The resulting antenna exhibits acceptable dual-band operation (i.e., better than -10 dB return loss with 5.3 and 7% operating bandwidths at 1.9 and 2.4 GHz) while maintaining a cross-pol maximum field level at least 11 dB below the co-pol maximum.     

Communicating from space [antenna metrology]

Worldwide, microwave holographic metrology is being considered as a very powerful diagnostic tool for identifying reflector surface distortions and defective radiating elements of an array. Important measurement improvements, algorithm developments, error evaluations, and display methodologies have been reported in recent years. The author discusses the application of microwave holographic metrology to space communication antennas     

Scattering from a circular disk: a comparative study of PTD and GTDtechniques

Two groups of techniques, PTD (physical theory of diffraction) and GTD (geometric theory of diffraction), are studied by analyzing the scattering from a conducting circular disk. The authors include comparisons of calculations based on several different asymptotic high-frequency theories with the method of moments (MoM) as a baseline. The latter is treated as numerically exact. Features of particular interest include the performance at the reflection boundaries, boresight caustics, and far-angle sidelobes. The bistatic radar cross sections (RCSs) calculated by these techniques are examined. Although only far-field radiation is considered, these methods can also be used for near-field calculations     

Array feeds for reflector surface distortion compensation: conceptsand implementation

The development of a compensation algorithm based on the use of array feeds and a focal-plane conjugate field approach is discussed. Results of numerical simulations performed to verify the accuracy of the computer programs are examined, and an experimental study is described. Both the numerical and experimental results show that the array will compensate for the slowly varying surface distortion. In particular, for some typical, slowly varying thermal or gravitational surface distortions, a 19-element array feed can improve the reflector performance considerably. Additionally, it is demonstrated that it will be possible to compensate for the surface distortion without actual knowledge of the distortion itself     

Scattering by superquadric dielectric-coated cylinders using higherorder impedance boundary conditions

A general method for deriving higher order impedance boundary conditions is described. It is based on solving an appropriate canonical problem exactly in the spectral domain. After approximating the spectral impedance terms as a ratio of polynomials in the transform variable, elementary properties of the Fourier transform are used to obtain the corresponding boundary condition in the spatial domain. The method is applicable to multilayer coatings with arbitrary constitutive relations. Higher-order boundary conditions which neglect the effects of curvature are derived for a dielectric coating using the method. The boundary condition equation and the magnetic field integral equation are solved simultaneously using the method of moments, yielding the bistatic and monostatic radar cross section for dielectric-coated superquadric cylinders. The method is also applicable to a combined field integral equation (CFIE) solution, which can be used to eliminate the internal resonance problem associated with either the electric field integral equation (EFIE) or magnetic field integral equation (MFIE)     

Comments on "New plane wave spectrum formulations for the near-fields of circular and strip apertures"

The above paper (see ibid., vol.24, p.438-449, July (1976)) applies a plane wave spectrum (PWS) formulation to diffraction problems involving circular and strip apertures and gives new results in terms of Fresnel integrals for the electric field near the aperture. In this note, a discussion of those new results is presented; conclusions are: As a technique for solving electromagnetic aperture diffraction problems, the particular PWS described gives inadequate results, especially for near-fields, and by using the standard Keller's formula, a geometrical theory of diffraction (GTD) solution for the diffracted field from a circular aperture is obtained, but the solution does not in general agree with the one given