An asymptotic transition region theory for calculation of diffraction losses in multiple reflector antennas
08 Jun 1986-Vol. 24, pp 247-250
TL;DR: In this paper, an asymptotic transition region theory of diffraction (TRT) is presented to predict and minimize losses due to diffraction from the feed system in a reflector antenna, if the feed systems is a line feed, or a point source plus one or more subreflectors.
Abstract: JntrOdUCtiOQ: The paper presents an asymptotic transition region theory of diffraction (TRT), which can be used to predict and minimize losses due to diffraction from the feed system in a reflector antenna, if the feed system is a line feed, or a point source plus one or more subreflectors. The theory is a simplification and specialization of the uniform geometrical theory of diffraction (UGTD) and of well-known asymptotic radiation integral solutions as well as an extension of these teories to include asymptotic secondary results like efficiency losses and diffraction spillover. Compared with numerical integration, these results are much faster. provide considera* more insight into the diffraction process, and are better adapted to the design stage. The complete theory will be presented in C 1 3 . and is an extension and generalization of that presented in C23 and applied in C33.
Citations
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TL;DR: In this paper, a dual-reflector feed for the spherical reflector antenna in Arecibo is presented, which is analyzed over a large frequency range: at the lower frequencies by physical optics (PO) integration, and at the higher ones by a geometrical optic (GO) ray tracing technique described in another work.
Abstract: A proposed dual-reflector feed for the spherical reflector antenna in Arecibo is presented. This is analyzed over a large frequency range: at the lower frequencies by physical optics (PO) integration, and at the higher ones by a geometrical optic (GO) ray tracing technique described in another work. The latter calculations are extended with the transition region theory (TRT) to include edge diffraction. The results clearly demonstrate the usefulness of the time efficient TRT method. However, they also show that PO integration is important, as this has detected an underillumination of the central region of the aperture. This effect is related to a similar problem with the line feeds, but can in the present case be reduced by moving the subreflectors away from the paraxial focus.
19 citations
06 Jun 1988
TL;DR: In this paper, the synthesis of offset dual-reflector antennas is reduced to solving nondifferential equations, which is made possible by approximating a known solution for the geometrical optics (GO) field reflected from a surface, and using local parabolic expansions of the reflector surfaces.
Abstract: An approach is presented by which synthesis of offset dual-reflector antennas is reduced to solving nondifferential equations. This is made possible by approximating a known solution for the geometrical optics (GO) field reflected from a surface, and using local parabolic expansions of the reflector surfaces. The advantages of this approach are that the calculation is easy to carry out, the condition of low cross-polarization is included, and the question of existence clearly can be answered. The synthesis method can also readily be extended to the synthesis of multireflector antennas. >
8 citations
TL;DR: In this article, the authors examined three techniques used for the efficient computation of fields diffracted by a subreflector that has been shaped by geometrical optics synthesis and found that these techniques produce errors in the computed fields that are specific to shaped reflectors.
Abstract: An examination is presented of three techniques used for the efficient computation of fields diffracted by a subreflector that has been shaped by geometrical optics synthesis. It is found that these techniques, which are based on the geometrical theory of diffraction (GTD), produce errors in the computed fields that are specific to shaped reflectors. These errors are examined for a reflector system shaped to produce maximum gain from a tapered feed illumination. The discrepancies are directly related to the caustic being located near an observation point of the GTD calculations. The errors found are localized, and they increase in magnitude as the caustic approaches the main reflector. In a general offset geometry, the location of the caustic may be located arbitrarily close to the main reflector given a prescribed output aperture distribution. For the specific case considered here-the common situation of shaping to produce maximum gain-the caustic is located near the edge of the main reflector and on the reflection shadow boundary. A local correction is derived which creates a uniform solution through the caustic and across the reflection shadow boundary. Away from this point the calculation recedes to the standard GTD solution. >
3 citations
References
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TL;DR: In this article, an asymptotic theory is presented with which the reduction in aperture efficiency caused by diffraction from a subreflector edge can be calculated for any dual-reflector system.
Abstract: An asymptotic theory is presented with which the reduction in aperture efficiency caused by diffraction from a subreflector edge can be calculated for any dual-reflector system. The theory is applied to conventional Cassegrain antennas, for which approximate analytical effieiency formulas are derived. These formulas show that subreflector diffraction may represent a significant efficiency loss even for subreflector diameters as large as 20 wavelengths. The formulas are used to obtain an optimum subreflector size which represents the best trade-off between losses due to subreflector diffraction and geometrical shadowing.
40 citations
TL;DR: In this paper, an alternative formulation of endpoint diffraction is given which is similar to the formulation of edge diffraction in the uniform geometrical theory of diffraction, which is used to obtain simple analytic diffraction corrections to geometric optics solutions, for example, calculations of the aperture efficiencies of dual-reflector antennas and cylindrical reflector antennas.
Abstract: The asymptotic approximation of a radiation integral, in which the integrand has a stationary phase point near one of the integration boundaries, is well known (endpoint diffraction). An alternative formulation of endpoint diffraction is given which is similar to the formulation of edge diffraction in the uniform geometrical theory of diffraction. In many applications the endpoint diffraction solution is the integrand of a new secondary radiation integral over another surface. An asymptotic approximation to one such class of secondary integrals, which represent double-endpoint diffraction in the direction of a stationary phase caustic, is evaluated explicitly. This explicit expression can be used to obtain simple analytic diffraction corrections to geometric optics solutions, for example, calculations of the aperture efficiencies of dual-reflector antennas and cylindrical reflector antennas.
17 citations