GTD analysis of a hyperboloidal subreflector with conical flange attachment
TL;DR: A geometrical theory of diffraction analysis of the principal plane far-field radiation patterns of a hyperboloidal subreflector with a conical flange attachment (HWF) fed by a primary feed located at its focus is presented in this article.
Abstract: A geometrical theory of diffraction (GTD) analysis of the principal plane far-field radiation patterns of a hyperboloidal subreflector with a conical flange attachment (HWF) fed by a primary feed located at its focus is presented. While using the uniform geometrical theory of diffraction (UGTD) for evaluating the nonaxial fields, the method of equivalent currents is used in the axial region. In this paper, both the diffraction by the wedge formed between the hyperboloid and the conical flange and the diffraction by the edge of the flange are considered. While considering the diffraction by the edge due to the diffracted ray from the wedge in the H -plane, the slope diffraction technique has been used. The computed diffracted farfields of a typical HWF illuminated by a high performance primary feed shows good agreement with the available measured data and with the results based on the method of physical optics (PO). The sharp cutoff and the low spillover characteristics of the HWF are highlighted by comparing its radiation pattern with that of a hyperboloid without a flange. Further, the effects of the different parameters of the HWF on its radiation pattern are also studied and plotted, so that these results can be utilized in the design of the HWF for a specific requirement.
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TL;DR: In this article, a geometrical theory of diffraction/physical optics (GTD/PO) analysis technique is used to investigate the improving effects of the extended subreflector, beyond its optical rim, on the reflector efficiency and farfield patterns.
Abstract: Both offset and symmetric Cassegrain reflector antennas are used in satellite and ground communication systems. It is known that the subreflector diffraction can degrade the performance of these reflectors. A geometrical theory of diffraction/physical optics (GTD/PO) analysis technique is used to investigate the improving effects of the extended subreflector, beyond its optical rim, on the reflector efficiency and farfield patterns. Representative numerical results are shown for an offset Cassegrain reflector antenna with different feed illumination tapers and subreflector extensions. It is observed that for subreflector extensions as small as 1 \lambda noticeable improvements in the overall efficiencies can be expected. Useful design data are generated for the efficiency curves and far-field patterns.
17 citations
TL;DR: In this paper, the uniform geometrical theory of diffraction (UGTD) is employed to analyze the far-field radiation patterns of a prime focus paraboloid with a cylindrical shroud.
Abstract: The uniform geometrical theory of diffraction (UGTD) is employed to analyze the far-field radiation patterns of a prime focus paraboloid with a cylindrical shroud. The blockage of the aperture illumination of the dish by the gooseneck and the primary feed is also taken into account in the analysis. Far-field radiation patterns (for 0\deg\leq\Theta \leq 180\deg ), calculated for a typical prime focus paraboloid with 2 m aperture diameter, designed and fabricated, are compared with the experimentally derived patterns at 8.8 GHz in the E - and H -planes. There is a satisfactory agreement between the two results.
4 citations
30 Jan 2011
TL;DR: In this paper, Rahmat-Samii et al. presented a mathematical model of the single-reflector paraboloid antenna and double-reflectors Paraboloid Cassegrain antenna based on physical optics method.
Abstract: The numerical modeling of reflector antennas is a necessary stage of their design Due to numerical modeling dimensions of all antenna elements are defined The more factors are accounted during antenna numerical modeling the more accurately the antenna elements dimensions are defined There are many methods used in the programs of antenna numerical modeling: geometric optics method; aperture method; geometric theory method of diffraction; physical optics method, integral equations method; finite elements method By now there are many papers in which the different aspects of reflector antenna numerical modeling are discussed For determination of the field antenna reflector in regions of main lobe and first side lobes in front semi-space the aperture method is used; for determination of the field in full semi-space the physical optics method is used (Chen & Xu, 1990; Charles, 1975; Rusch, 1974) The geometric theory of diffraction (Narasimhan & Govind, 1991; Rahmat-Samii, 1986; Narasimhan et al, 1981) and moment method (Khayatian & RahmatSamii, 1999) are used for determination of the field in back semi-space, for determination of field features in front semi-space related with diffraction of the field on the edge of paraboloid and hyperboloid surfaces and for modeling the feed-horn In a number of papers different approaches are used for simplification of analytical expressions for calculation of antenna fields to reduce a mathematical model of antenna and to simplify modeling program (Rahmat-Samii, 1987) A number of works deal with research into the field in nearfield zone (Narasimhan & Christopher, 1984; Fitzgerald, 1972; Houshmand et al, 1988; Watson, 1964) But the results are not reduced to numerical data in that volume which is necessary for antenna design The field distribution in near-field zone is described in detail for plane aperture at uniform its excitation (Laybros et al, 2005), but for reflector antennas such research was not provided The reflector antenna in receiving mode is not discussed in literature, however at designing antenna for radioimaging systems it is necessary to know of field distribution in the focal region at receiving of the wave from near-field zone points The issue of isolation of channels in multi-beam reflector antenna at receiving of the wave from near-field zone is not analyzed too Without analysis of the isolation between channels it is impossible to analyze the quality of imaging in radioimaging systems In literature a number of works deal with describing the feed-horns in monopulse reflector antennas (Hannan, 1961; Scolnic, 1970) There is a little information on numerical characteristics description the regularity in monopulse reflector antenna In the present chapter the mathematical model of the single-reflector paraboloid antenna and double-reflector paraboloid Cassegrain antenna is based on physical optics method
3 citations
Cites methods from "GTD analysis of a hyperboloidal sub..."
...The geometric theory of diffraction (Narasimhan & Govind, 1991; Rahmat-Samii, 1986; Narasimhan et al, 1981) and moment method (Khayatian & RahmatSamii, 1999) are used for determination of the field in back semi-space, for determination of field features in front semi-space related with diffraction…...
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TL;DR: In this paper, the results for 3 to 8 λ -diameter hyperboloidal subreflectors, using the geometrical theory of diffraction (GTD) are presented and compared with the physical optics approach and other data.
Abstract: Cassegrain antennas have been utilized in applications where small size is critical. It has been observed that for antennas having subreflectors of the order of only several λ in diameter, near-in sidelobe levels rise beyond normally predicted values. E-plane sidelobes are often several dB higher than those in the H-plane. Disturbances in the illumination of the primary reflector due to diffraction limitations of the electrically small subreflector are a possible cause of the sidelobe phenomenon. In this work, the results for 3 to 8 λ -diameter hyperboloidal subreflectors, using the geometrical theory of diffraction (GTD) are presented and compared with the physical optics approach and other data. Degradation of the scatter patterns are observed as the subreflector size is decreased, and the differences in the E-and H-plane phase and amplitude patterns are observed in the GTD solution.
01 Oct 1986
TL;DR: In this article, a geometrical theory of diffraction/physical optics (GTD/PO) analysis technique is used to investigate the improving effects of the extended subreflector, beyond its optical rim, on the reflector efficiency and farfield patterns.
Abstract: Both offset and symmetric Cassegrain reflector antennas are used in satellite and ground communication systems. It is known that the subreflector diffraction can degrade the performance of these reflectors. A geometrical theory of diffraction/physical optics (GTD/PO) analysis technique is used to investigate the improving effects of the extended subreflector, beyond its optical rim, on the reflector efficiency and farfield patterns. Representative numerical results are shown for an offset Cassegrain reflector antenna with different feed illumination tapers and subreflector extensions. It is observed that for subreflector extensions as small as 1 \lambda noticeable improvements in the overall efficiencies can be expected. Useful design data are generated for the efficiency curves and far-field patterns.
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01 Nov 1974
TL;DR: In this article, a compact dyadic diffraction coefficient for electromagnetic waves obliquely incident on a curved edse formed by perfectly conducting curved plane surfaces is obtained, which is based on Keller's method of the canonical problem, which in this case is the perfectly conducting wedge illuminated by cylindrical, conical, and spherical waves.
Abstract: A compact dyadic diffraction coefficient for electromagnetic waves obliquely incident on a curved edse formed by perfectly conducting curved ot plane surfaces is obtained. This diffraction coefficient remains valid in the transition regions adjacent to shadow and reflection boundaries, where the diffraction coefficients of Keller's original theory fail. Our method is based on Keller's method of the canonical problem, which in this case is the perfectly conducting wedge illuminated by plane, cylindrical, conical, and spherical waves. When the proper ray-fixed coordinate system is introduced, the dyadic diffraction coefficient for the wedge is found to be the sum of only two dyads, and it is shown that this is also true for the dyadic diffraction coefficients of higher order edges. One dyad contains the acoustic soft diffraction coefficient; the other dyad contains the acoustic hard diffraction coefficient. The expressions for the acoustic wedge diffraction coefficients contain Fresenel integrals, which ensure that the total field is continuous at shadow and reflection boundaries. The diffraction coefficients have the same form for the different types of edge illumination; only the arguments of the Fresnel integrals are different. Since diffraction is a local phenomenon, and locally the curved edge structure is wedge shaped, this result is readily extended to the curved wedge. It is interesting that even though the polarizations and the wavefront curvatures of the incident, reflected, and diffracted waves are markedly different, the total field calculated from this high-frequency solution for the curved wedge is continuous at shadow and reflection boundaries.
2,582 citations
TL;DR: In this paper, the authors used the geometrical theory of diffraction to obtain the backscattered field for plane-wave incidence on a target with particular emphasis on those regions that are usually avoided, namely, the caustic region and its immediate vicinity.
Abstract: The fields diffracted by a body made up of finite axially symmetric cone frustums are obtained using the concepts of the geometrical theory of diffraction. The backscattered field for plane-wave incidence on such a target is obtained with particular emphasis on those regions that are usually avoided, namely, the caustic region and its immediate vicinity. The method makes use of equivalent electric and magnetic current sources which are incorporated in the geometrical theory of diffraction. This solution is such that it is readily incorporated in a general computer program, rather than requiring that a new program be written for each shape. Several results, such as the cone, the cylinder and the conically capped cylinder, are given. In addition, the method is readily applied to antenna problems. An example which is reported consists of the radiation by a stub over a circular ground plane. This present theory yields quite good agreement with experimental results reported by Lopez, whereas the original theory given by Lopez is in error by as much as 10 dB.
191 citations
TL;DR: In this article, the problem of a half plane illuminated by a nonplanar wave is investigated using the concept of the plane wave spectral representation, and a new higher-order asymptotic solution for the total field up to and including terms of order k−5/2 relative to the incident field is derived.
Abstract: The knowledge of high-frequency diffraction of an arbitrary wave incident on an edge is important in many applications, such as antennas mounted on aircraft and reflector antennas illuminated by complex feeds. In this paper the problem of a half plane illuminated by a nonplanar wave is investigated using the concept of the plane wave spectral representation. For large wave number k, a new higher-order asymptotic solution for the total field up to and including terms of order k−5/2 relative to the incident field is derived. The behavior of the solution for the observation points which coincide with shadow boundary directions of a multipole line source is discussed in detail. Furthermore, numerical solution of the field integral representation is constructed for the observation angles in the transition regions. The results are compared with those of the Geometrical Theory of Diffraction (GTD), the Uniform Asymptotic Theory (UAT), the Uniform Theory of Diffraction (UTD) and the Modified Slope Diffraction (MSD).
51 citations
TL;DR: In this article, the spherical wave expansion technique is used to analyze the general properties of paraboloidal antenna feed systems, and fundamental aperture efficiency and noise temperature limitations are established as quantitative functions of antenna wavelength size.
Abstract: In this paper the spherical wave expansion technique is used to analyze the general properties of paraboloidal antenna feed systems. As a result, fundamental aperture efficiency and noise temperature limitations are established as quantitative functions of antenna wavelength size. A boundary-value solution is found for synthesis of ideal subreflector shapes in a Cassegrain-type feed system. The resulting surface is found to reduce the classical hyperboloid in the limit of zero wavelength. Applications of this synthesis technique to high performance feed systems and subreflector matching are discussed. Finally, an interesting quantitative cross-check between vector spherical wave and vector diffraction theories is obtained.
48 citations
TL;DR: In this article, the authors calculated the scattered field from a hyperboloidal reflector by integrating the induced current density over the front of the hyperboloid, and the resulting integral expressions for the fields possess a stationary term which, when evaluated, yields the geometrical ray-optics approximation to the scattering problem.
Abstract: The scattered field from a hyperboloidal reflector is calculated by integrating the induced current density over the front of the hyperboloid. The resulting integral expressions for the fields possess a stationary term which, when evaluated, yields the geometrical ray-optics approximation to the scattering problem. The complete field, including diffraction effects, may be obtained by numerical evaluation of the integrals. The formulas are applied to a hyperboloid illuminated by an idealized, sharply cut off uniform feed pattern. Characteristic diffraction phenomena are reduced with increasing D/\lambda until the geometrical ray-optics result is obtained in the limit of vanishing wavelength. Theoretical field patterns are also obtained for a horn-fed hyperboloidal subreflector in a Cassegrainian feed system; they indicate that for moderately large hyperboloidal reflectors spillover may be reduced to an acceptable level, but there is a tendency toward increased forward spillover. The results of 9600-Mc model tests compare favorably with the theoretical patterns.
44 citations