SABOR: description of the methods applied for a fast analysis ofhorn and reflector antennas

The program presented in this paper is intended to provide a valuable aid for teaching antennas to electrical engineers, and to provide fast and accurate pre-designs for professionals. This paper covers the theory and numerical techniques used in SABOR. In summary, this program computes the radiated field of an aperture antenna (horn or reflector), using a common engine based on the Gauss-Legendre quadrature method for evaluating the radiation integrals. For horn, the aperture fields are the usual dominant modes of the feed waveguide, with a quadratic phase correction. For reflectors, the aperture fields are computed using geometrical optics ray tracing from the feed horn. Also, equivalent-reflector concepts are applied for dual-reflector antennas. The paper includes some examples to demonstrate the most important features of the program     

Performance and design procedure of dual parabolic cylindricalantennas

The radiation characteristics of dual parabolic cylindrical antennas are studied, and the dependence of the principal plane beamwidths and the peak cross-polarization on their geometrical parameters is determined. The antenna aperture is rectangular in shape and generates an elliptical beam pattern, with a beamwidth ratio that can be controlled by the main and subreflector focal lengths. The far-field patterns are determined by an extended aperture integration method that includes the contributions of the reflected and the main diffracted rays. It is found that the cross-polarization depends of the offset angle between the axis and the direction of the normal to the subreflector surface and can be minimized by optimizing the relative angle between the reflectors. Other pattern characteristics are controlled by the antenna geometrical parameters and the feed illumination. A procedure for the design of these antennas and the expressions for determining the reflector geometries are provided     

Étude en optique physique d’une antenne a double réflecteur

After having recalled to mind some fundamentals of the electromagnetic theory, the authors compare experimental and theoretical results obtained in the approximation of physical optics for a few known antennas. For the Cassegrain antenna, the surfaces of the main reflector and of the auxiliary reflector and the incident field generated by the primary source, constitute data. First, the distribution of the field on the auxiliary reflector is determined. After that, the radiation in the area near the auxiliary reflector with regard to the main reflector, is calculated, and then the electromagnetic field radiated at a great distance by the antenna. The calculation programme is a general one, and applies to reflectors of any shape. Results obtained on two Cassegrain antennas belonging to the CNET are presented, one of them being of the revolution type and the other of the asymmetrical dual reflector type.     

Effect of disc reflectors on radiation impedance of short-backfire antenna

Using near-field analysis, the influence of large and small disc reflectors used in short-backfire antennas on the radiation impedance of a dipole feed element has been investigated. The effect of each reflector is evaluated separately, and the overall change in the radiation impedance of the dipole is predicted.     

矩形反射柱体近旁天线辐射特性的计算应用几何绕射理论

本文用几何绕射理论分析计算了矩形反射柱体近旁偶极子和双环天线的水平辐射特性。研究了到达场点的直达波、反射波和柱体稜边各次绕射波之间的量值关系。分析了柱体边长、天线与柱体之间的相对位置和天线的馈电方式对合成场强方向图的影响。计算结果表明,需要对天线系统的有关参数进行优化设计,以得到最佳的全向辐射特性,本文的分析方法和结果为这类天线系统的优化设计提供了手段和依据。     

Radiating Mechanisms in a Reflector Antenna System

The radiating mechanisms of a reflector antenna system are discussed. Means of computing the various component fields are given, including aperture radiation, direct feed radiation, and diffracted radiation. An offset parabolic reflector antenna system fed with a corrugated horn is treated as an example. Means of reduction of the undesired components of radiation are suggested.     

A study of three techniques used in the diffraction analysis ofshaped dual-reflector antennas

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     

偏馈FZP天线的远场辐射特性

利用物理光学法分析了偏馈FZP反射面天线的辐射特性，考察了喇叭馈源的最强辐射中心带来的影响，计算机模拟结果表明了该天线具有较好的辐射特性。     

Cross-polarization in satellite and earth-station antennas

Cross-plarization in axially symmetric reflector antennas can be reduced, theoretically, to zero by use of special feeds like the Huygens' source. Alternatively, paraboloidal reflectors with large f/D ratio do not deteriorate further the cross polarization level relative to the value due to the feed itself. The Cassegrainian optics is equivalent to a large f/D paraboloid. The reflector of linearly polarized off set fed antennas contribute more cross-polarization than symmetrical reflectors fed by the same feed. With symmetrical reflectors the cross-polarized component generated by the reflector vanishes in the principal planes and is confined to four main lobes that have peak values in planes at 45° to the principal planes. In the case of offset fed reflectors cross-polarization vanishes in the plane of symmetry and has its peak in the plane of asymmetry. The reflector generated cross-polarization with offset fed antennas may be reduced by use of small offset angles and large f/D ratios. Feed offsetting has but little effect on the peak level of cross-polarization. This is usually accompanied with an asymmetry in the cross-polarization radiation pattern. Feed offsetting also results in spatial tilt in the copolarized and cross-polarized lobes with the cross-polar minimum always coinciding with the main beam peak. The effect of surface errors on the antenna cross-polarization is to partly fill the cross-polar along boresight. The peak cross-polarization, however, changes but slightly.     

Back-to-Back Reflector Antennas With Reduced Moment of Inertia for Spacecraft Spinning Platforms

A back-to-back reflector antenna system with reduced moment of inertia is proposed in order to address the demanding problem of supporting large reflector antennas on spinning platforms. The configuration provides additional potential advantages, such as reducing the spinning speed by half for a given sampling rate when both back-to-back reflectors are utilized. Geometrical parameters of the reflector are determined such that the moment of inertia of the rotating system is reduced. It is shown that these back-to-back reflectors suffer from a high cross-pol level in the asymmetrical plane due to the large feed offset angle. Two different methods are explored to alleviate the high cross-pol level problem. In the first method, a sub reflector is utilized to minimize the cross-pol level by satisfying the Mizugutchi condition. In the second method, a tri-mode matched feed horn is suggested to achieve a similar result. The suppressed cross-pol level puts forward the gravitationally balanced back-to-back reflector antenna systems as a potential candidate for future spacecraft antennas on spinning platforms.     

Complete dual-offset reflector antenna analysis includingnear-field. paint-layer and CFRP-structure effects

The computer analysis of dual-offset reflector antennas fed by a circular corrugated horn, using an accurate feed model, is described. Near-field, thermal-paint, and carbon-fiber-reinforced plastic (CFRP)-structure effects are included in order to simulate a realistic design scenario and predict their impact on the radiation characteristics. The theoretical results have been verified by experiment for a near-field illuminated offset Gregory antenna     

Single shaped reflector antennas for broadcasting satellites

For future direct broadcasting satellites in Japan, precisely contoured beam antennas will be required for onboard antennas. Single shaped-reflector antennas are suitable for that purpose, since they do not need complex feed networks. However, in a previous study, discontinuities in the reflector surface were observed. The problem of the discontinuity was circumvented in this work and the shaped reflectors were successfully designed to produce contoured beams for covering the Japanese islands for the downlink and feederlink antennas. The downlink antenna was fabricated, and the radiation pattern was verified by measurement. The radiation pattern meets the radio regulations imposed on the onboard antenna, such as sidelobe and cross-polarization characteristics     

Depolarization properties of offset reflector antennas

The cross polarized radiation for linearly polarized excitation and the beam displacement for circularly polarized excitation have been investigated for offset reflector antennas. Numerical calculations are given to illustrate the dependence upon the angletheta_{0}between the feed axis and the reflector axis as well as upon the half-angletheta_{c}subtended at the focus by the reflector. In the casetheta_{0} = theta_{c} = 45deg, measured results have been obtained for both linearly and circularly polarized excitations with a dual mode feed illuminating an offset paraboloid. The cross polarized radiation of horn reflector and open Cassegrainian antennas rises sharply to rather high values off the beam axis; however, in general, the maximum cross polarized radiation of offset reflector antennas can be made small by using a small angle between the feed and reflector axes. The cross polarization caused by offset is compared with that caused by an unbalanced feed pattern. The effect of the longitudinal current distribution and of departure of the surface from a paraboloid on cross polarization are also examined. The clarification of these cross polarization properties is found to be valuable in the design of reflector antennas.     

Front-to-back ratio of paraboloidal reflectors

An analysis is presented which uses the uniform geometrical theory of diffraction for determining the near fields diffracted by a paraboloid either with or without a conical flange attached to its circular rim when an axially propagated plane wave is incident on the concave or convex portion of the paraboloidal reflector. The field correlation theorem is used to determine the power coupled by a prime focus feed associated with the paraboloid which is being illuminated by a uniform plane wave. Based on this analysis, the front-to-back ratio of unflanged and flanged paraboloids is computed. Computed results show satisfactory agreement with the available measured as well as computed results based on alternative procedures. The variation in the on-axis gain on a prime-focus reflector when the feed is displaced from the focus is studied. Typical computed results are presented and compared with the available measured data. Computed results on the front-to-back ratio of paraboloids (flanged or unflanged) illuminated by a PFF whose radiated field exhibits phase variation over a constant radius are also presented     

Symmetric reflectors with zero depolarisation

The polarisation performance of a symmetric reflector antenna illuminated by an ideal feed is limited by depolarisation introduced by the reflector, even if other sources of cross-polarisation are negligible. It is shown that depolarisation can theoretically be eliminated by the use of anisotropic reflectors.     

Coaxial feeds for high aperture efficiency and low spillover of paraboloidal reflector antennas

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 H11modes 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 of100degto160deg. 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.     

A generalized diffraction synthesis technique for high performancereflector antennas

Stringent requirements on reflector antenna performances in modern applications such as direct broadcast satellite (DBS) communications, radar systems, and radio astronomy have demanded the development of sophisticated synthesis techniques. Presented in the paper is a generalized diffraction synthesis technique for single- and dual-reflector antennas fed by either a single feed or an array feed. High versatility and accuracy are achieved by combining optimization procedures and diffraction analysis such as physical optics (PO) and physical theory of diffraction (PTD). With this technique, one may simultaneously shape the reflector surfaces and adjust the positions, orientations, and excitations of an arbitrarily configured array feed to produce the specified radiation characteristics such as high directivity, contoured patterns, and low sidelobe levels, etc. The shaped reflectors are represented by a set of orthogonal global expansion functions (the Jacobi-Fourier expansion), and are characterized by smooth surfaces, well-defined (superquadric) circumferences, and continuous surface derivatives. The sample applications of contoured beam antenna designs and reflector surface distortion compensation are given to illustrate the effectiveness of this diffraction synthesis technique     

Backfire antennas constructed with coaxial circular loops

The radiation characteristics of backfire antennas constructed entirely of circular loops are studied. A comparison between this type of backfire antenna and the conventional one, made of a solid reflector, reveals that the optimum dimensions of the reflector and the peripheral rim are approximately the same for both kinds of antennas. In addition, it is found that as few as six loops are sufficient to construct the optimum reflector, and almost the same number of loops are required to form the optimum peripheral rim. The new structure reduces the weight, windage, and obstruction of view which accompany solid reflectors. In addition, the new antenna lends itself to exact analytical investigation.     

Effect of surface distortions on the crosspolarisation performance of paraboloidal reflectors

The effects on the crosspolarisation and the radiation patterns of circular paraboloidal reflectors produced by (a) reflector distortions which are periodic in the circumferential direction and (b) a lateral offset of the feed have been evaluated for a wide range of reflector and feed parameters. It is shown that the use of a balanced hybrid-mode feed minimises crosspolarisation introduced by reflector distortions.     

Offset-reflector antennas with offset feeds

The development of a simple mathematical model for the vector radiation fields from offset-parabolic-reflector antennas is outlined. The model permits the primary feed to be offset from the reflector geometric focus, and thus allows the study of multiple-beam antennas. Predicted results are shown and correlated with measurements made on a 30 GHz antenna system.