Conference Paper

Offset-fed radial-rib umbrella reflector antenna analysis using Zernike polynomials

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Abstract

Zernike polynomials were used to perform the physical optics scattering integral of umbrella reflectors. A computationally efficient numerical procedure was obtained for the umbrella reflector. To demonstrate the method, it was applied to a typical offset umbrella reflector. The radiation patterns for a focused and a scanned feed are shown. Three patterns are shown superposed: one was computed using a conventional two-dimensional integration, and the other two using the Zernike expansion procedure. The agreement between the Zernike approach and the conventional method is good, provided that enough gore-related expansion terms are included in the computations.

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Article
Modern reflector theory and practice, with emphasis on the recent past are reviewed. Included are a brief historical review and performance definitions for the nonspecialist. Special sections are devoted to shaped multiple reflectors, reflector surface metrology, and reflectors for the next decade and beyond. In spite of decreasing budgets for scientific, commercial, and military space programs, the 1990s promise to be a true golden age for the reflector antenna art
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Offset antennas with aperture diameters up to 12 m and a medium surface accuracy, will be required for some future European communication satellites. A special offset reflector design with deployable ribs running radially outwards from the apex and covered by a mesh will be suitable for such applications. This paper presents a parametric investigation of the rf characteristics of such an unfurlable antenna concept with single mesh as described above. For a 4.5-m-diam reflector (4/6 GHz application) a lightweight mechanical design has been elaborated based on the design principle already shown. Main features of this reflector and the deployment principle are given. © American Institute of Aeronautics and Astronautics, Inc., 1983, All rights reserved.
Conference Paper
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Article
The properties of the offset-fed radial rib umbrella reflector are analyzed. A simple formula for positioning the feed to maximize the gain is given. The best aim point for the feed is numerically determined to be very close to the rib center. The exponent of a cos^{n} alpha feed pattern, evaluated for maximum gain, and the loss due to the gores is presented. It is shown that the offset configuration exhibits the same behavior and position of the gore-related sidelobe as the front-fed umbrella reflector.
Article
A series approach for the rapid computation of the vector secondary pattern of offset paraboloid reflectors wherein the feed is displaced is presented. We show that the Jacobi polynomial series method, which has been demonstrated to provide an efficient means for evaluating the radiation integral of symmetric paraboloid reflectors, can be extended to the case of an offset paraboloid without compromising the ease or speed of computation. The analysis leading to the series formula is also useful for deriving an analytic expression for the optimum scan plane for the displacement of the feed. Representative numerical results illustrating the application of the method and the properties of the offset paraboloid are presented.
Article
Given the true or any approximate current on a reflector, the radiated far-field is determined from a rapidly convergent series representation of the radiation integral. The leading term is a well-shaped J_{1}(x)/x beam pointing in a desired direction. Higher order terms provide perturbations to the leading term. The coefficients of the series are independent of the observation angles. Hence, once they are computed, the field may be determined very rapidly at large numbers of points. Initially, a suitable small angle approximation is made that places the radiation integral in the form of a Fourier transform on a circular disk. The theory is then extended such that the results are valid in both the near and the wide angle regions. Application to a rotationally symmetric paraboloid is presented herein. Other applications include the offset and dual reflectors and near- to far-field integrations. A modified form of the series can also be used for Fresnel zone computations.
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