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1.
In a recent paper [Oct. 2003], Van Bladel discussed in detail the behavior of fields and surface charges near singularities of metallic structures, such as a hollow circular cylinder, a circular cone, a thin prolate spheroid, and a uniformly charged segment. At times, the reasoning in [Van Bladel, Oct. 2003] leads to conclusions not as definitive as one might wish because of the various approximations introduced, such as the electrostatic limit for the field or idealized singularities for the conductors (e.g., the tip of a cone). In this work, an example is provided that may assist in shedding some light on the questions raised in [Van Bladel, Oct. 2003], because it consists of an exact, closed-form, simple solution to a boundary-value problem that is valid at all frequencies. The problem consists of a plane wave propagating in free space and axially incident on the convex side of a paraboloid of revolution. This problem was solved by Schensted [1955] for a perfectly conducting (PEC) paraboloid, and by Roy and Uslenghi [Oct. 1997] for an isorefractive paraboloid, which comprises the PEC paraboloid as a particular case. A remarkable fact is that these exact solutions are also the geometrical optics solutions. 相似文献
2.
Narasimhan M. Ramanujam P. Raghavan K. 《Antennas and Propagation, IEEE Transactions on》1981,29(4):654-660
A geometrical theory of diffraction (GTD) analysis of the principal plane far-field and near-field patterns of a near-field Cassegrainian subreflector is presented. The uniform geometrical theory of diffraction (UGTD) [1] that drastically reduces the computation time has been utilized to analyze the subreflector in the form of a paraboloid illuminated by a plane wave. The numerical computations of the far-field amplitude and phase patterns of a typical paraboloidal subreflector based on the above technique correlate well with the results obtained by physical optics current integration, justifying the validity of the analysis presented. The GTD near-field analysis presented here is an improvement over that published earlier [2] and removes some of its limitations. 相似文献
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Grids of thin wires are mounted between a paraboloid and a microwave feed at its focus, so as to obtain linear polarization everywhere in the far-field of the paraboloid. Three types of grids are proposed: 1) a family of hyperbolae on an arbitrary planeS , 2) projections of these hyperbolae on an arbitrary surface, and 3) a set of straight lines that converge on a certain pointF' on a planeS . If in cases 1) and 2) a point source is placed at the focus, the resulting plane wave reflected by the paraboloid will be polarized in one direction. In ease 3), there will also he a small cross-polarized component reflected by the paraboloid; the optimum choice ofF' that minimizes this undesirable component is determined. Only for certain orientations of S will the optimum choice ofF' be at infinity (in which case the grid is an ordinary parallel wire grid). A simple technique for fabricating grids on curved surfaces is also described. Using the grids discussed here, perfect (or nearly perfect, in case 3)) polarization diplexing at microwave frequencies can be achieved. 相似文献
5.
Many existing large ground reflector antennas have been designed as Cassegrain systems-i.e., paraboloid/hyperboloid combinations. Other large ground antennas are simply paraboloid designs. Upgrading the gain of these systems to a gain comparable to that obtainable with a dual shaped reflector antenna system has been an important and costly objective of many such ground stations. A potentially economic method for such an antenna upgrade is presented herein. It involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a parabaloid. A pair of offset subreflectors are synthesized which will give a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method that is used is based on an approximate formulation for an offset dual shaped high gain antenna that was first presented by Galindo-Israel and Mittra in 1977. In that approximate formulation, the geometrical optics (GO) energy was scattered from a subreflector and then from a second large reflector which reflected a uniform phase distribution. In the present offset dual shaped subreflector (DSS) antenna, the second reflection is from a smaller (sub) reflector and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. Excellent results are shown for the approximate synthesis of the DSS. 相似文献
6.
For a Cassegrain antenna system, a simple and general procedure is presented for computing the effective surface root mean square (rms) and beam deviations considering primary surface distortions and relative translations and rotations of the antenna components. It is shown that certain primary reflector distortions can be compensated for by an appropriate adjustment of the subreflector position. Different methods in use for calculating surface rms are reviewed. It is shown that the method in which the deformed configuration of the primary reflector is best fitted with another paraboloid yields a surface rms almost equal to that obtained by the optimal positioning of the antenna components. The actual deformation patterns of several large Cassegrain antenna systems with different structural concepts are reviewed. It is shown that for a class of antenna structures, the gain can be significantly improved by an optimal positioning of the subreflector. 相似文献
7.
Rusch W.V.T. Prata A. Jr. Rahmat-Samii Y. Shore R.A. 《Antennas and Propagation, IEEE Transactions on》1990,38(8):1141-1149
The equivalent paraboloid is derived for classical offset Cassegrain and Gregorian antennas. The important practical case of systems with circular exit apertures is discussed in detail, and a condition for a symmetric equivalent paraboloid is derived. For such systems, diffraction effects are investigated using tapered and scanned feeds to illuminate the equivalent paraboloid and the two-reflector system 相似文献
8.
《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1985,73(8):1338-1339
The purpose of this letter is to discuss the application of a modified gradient filtering technique proposed by Friedlander [1] to the modified recursive least mean square (RLMS) algorithm of Hsia [2]. 相似文献
9.
Narasimhan M. Raghavan K. Ramanujam P. 《Antennas and Propagation, IEEE Transactions on》1983,31(5):792-794
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 (for0degleqTheta leq 180deg ), 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 theE - andH -planes. There is a satisfactory agreement between the two results. 相似文献
10.
A potentially economic method for upgrading the gain of the large earth reflector antenna Cassegrain system to a gain comparable to that obtainable with a dualshaped reflector antenna system is presented herein. It involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a paraboloid. A pair of offset subreflectors are synthesized which will give a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method that is used is based on an approximate formulation for an offset dual shaped high gain antenna where the geometrical optics energy was scattered from a subreflector and then from a second large reflector which reflected a uniform phase distribution. In the present offset dual shaped subreflector (DSS) antenna, the second reflection is from a smaller subreflector and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. Excellent results are shown for the approximate synthesis of the DSS. 相似文献
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Mittra R. Rahmat-Samii Y. Galindo-Israel V. Norman R. 《Antennas and Propagation, IEEE Transactions on》1979,27(3):294-304
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. 相似文献
14.
《IEEE transactions on information theory / Professional Technical Group on Information Theory》1985,31(5):589-593
A unified approach is given for constructing cross entropy and dissimilarity measures between probability distributions, based on a given entropy function or a diversity measure. Special properties of quadratic entropy introduced by Rao [7] are described. In particular it is shown that the square root of the Jensen difference (dissimilarity measure) arising out of a quadratic entropy provides a metric on a probability space. Several characterizations of quadratic entropy are obtained. 相似文献
15.
Coaxial feeds produce an approximate sector-shaped pattern, an almost optimum pattern of a feed for high aperture efficiency and low spillover of paraboloid antennas. Such a coaxial feed consists of a central circular waveguide which is surrounded by one or more conductors with circular cross sections. Theoretical and experimental investigations on coaxial feeds excited by H11 modes have shown that the first ring yields the highest increase in the aperture efficiency of paraboloid antennas illuminated by them. Measurements performed on paraboloid antennas illuminated by a coaxial feed with only one ring yielded aperture efficiencies of 68 to 75 percent for angular apertures of the paraboloidal reflector of100deg to160deg . Circularly symmetric patterns in conjunction with almost linearly polarized aperture fields can be achieved by multimode coaxial feeds. The values for the aperture efficiency, which are calculated for paraboloid antennas illuminated by multimode coaxial feeds, nearly reach the theoretical optimum. The measured values are 68 to 80 percent. In addition, the multimode feeds produce very little cross polarization. 相似文献
16.
A multipole expansion technqiue is developed for calculating the co-polar and cross-polar radiation patterns of a prime-focus paraboloid fed by a dominant-mode rectangular waveguide feed. Initially, the feed radiation pattern is expressed in terms of electric and magnetic multipoles of different orders which are subsequently used to obtain the secondary radiation patterns (co-polar and cross-polar) in explicit closed form in the form of a rapid by a converging algebraic series. The validity of the analysis is effectively demonstrated by calculating the co-polar and cross-polar patterns of a typical paraboloid and comparing and results with calculations based on the direct numerical integration of the physical optics currents induced on the reflector surface. 相似文献
17.
利用电磁场分析软件FEKO,研究了抛物面天线在破片穿孔时天线方向图的变化规律。通过对物理光学法(PO)和物理绕射理论(PTD)的混合使用,对战损后抛物面天线的电气性能进行了建模与分析,以比较反射面损坏前后对天线方向图的影响。着重研究了天线反射面毁伤后,毁伤孔数、孔径及其位置变化对方向图的影响。 相似文献
18.
一种L波段背射模螺旋馈源 总被引:1,自引:1,他引:0
螺旋天线具有较好的宽频带圆极化特性,在L或S波段体积和增益都很适中,加工制造公差要求较低,因此在该频段螺旋天线得到了广泛的应用。由背射模螺旋构成的L波段宽带馈源具有结构简单,馈电、调试方便,口面遮挡小,频带宽(大于25%),可以承受大功率等优点,可以满足L或S波段测控系统对天线的要求。用该背射模螺旋馈源研制的前馈天线的口径效率超过了53%,第一副瓣小于-20 dB,在工程中得到了很好的应用。 相似文献
19.
In Cassegrain antennas, the whole focusing system is usually replaced by an equivalent paraboloid of focal length mf, where f is the focal length of the paraboloid and m is the magnification of the Cassegrain system. Here, it is shown that this equivalent paraboloid is useful only for feeds near the Cassegrainian focus. The discussion of series expansions of the phase in the system aperture yields severe differences between the Cassegrain system and the prime focus-fed equivalent paraboloid for larger radial or axial defocusing. 相似文献
20.
The uniform geometrical theory of diffraction (UGTD) has been applied successfully to analyze the near-field patterns of a prime-focus paraboloid. In order to establish the validity of the analysis, near-field amplitude and phase patterns have been computed over the principal planes at several observation distances for a typical prime-focus paraboloid. These calculations compare very favorably with the corresponding results obtained numerically with the aid of Silver's near-field aperture integration formula. 相似文献