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.     

Compensation of the beam squint in axially symmetric, large dualreflector antennas with large-ranging laterally displaced feeds

The beam squint in axially symmetric reflector antennas with laterally displaced feeds is manifested by a small shift of the radiation patterns for left- and right-hand circular polarization. It is caused by the cross-polarized component in the reflected field. The authors' interest in this effect, and especially in how to minimize or to compensate for it, was stimulated by some measurements with the 100-m radio telescope of the Max-Planck-Institut fuer Radioastronomie. They have carried out a series of computer simulations for different geometrical antenna configurations for the Cassegrain optics. The presented results show that depending upon various geometrical antenna parameter values, while keeping the laterally displaced feed in a gain optimized orientation, i.e., directed to the primary focus, the angular shift between the squinted beams can even reverse. This offers the chance to compensate the beam squint for large range lateral displacement of the feed by choosing an appropriate antenna/feed configuration     

Beam squint in a linearly polarised offset reflector antenna

A beam squint has been found to occur in the field radiated by an offset paraboloid antenna illuminated by a feed with linear polarisation in the plane of offset. The squint, whose existence has been confirmed by computed and measured radiation patterns, occurs in the offset plane, and is typically 0.03 of a beamwidth for a reflector of 6-wavelengths diameter. An assessment is given of the likely effect of the squint on the performance of an offset reflector radiating a circularly polarised field     

Beam squint in axially symmetric reflector antennas with laterallydisplaced feeds

Numerical calculations have been carried out and the beam squint for circular polarized excitation has been measured using a 100 m telescope. The telescope was operated in the Gregorian mode, where the equivalent focal length equals 387.5 m. The feed horn was laterally displaced by 1.364 m from the optical axis at the system focus. Good agreement was obtained between the numerical calculations and the experimental results. The authors found a shift of the two radiation patterns of ≃2 arcsec. The orientation of the beam squint in the configuration with a laterally displaced feed is different from the orientation in offset reflector antennas     

Cross polarization in beam waveguide-fed Cassegrain reflectorantennas

The sensitivity of the cross-polarization level to a deviation from the geometrical condition derived by M. Mizusawa and T. Kitsuregawa (1973) for different geometrical configurations is studied. This condition restricts the number of possible beam waveguide configurations for beam waveguide-fed Cassegrain reflector (BFCR) antennas. For a symmetrical feed, this condition results in a symmetrical aperture distribution with no cross-polarized component. By examining a number of beam waveguide configurations satisfying the condition, it was observed that for a linearly polarized feed, the cross-polarization level is very sensitive to a deviation from this condition. For a circularly polarized feed, deviation from this condition does not increase the cross-polarization level; however, it results in the squinting of the beam for BFCRs     

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.     

Circularly polarized contoured beam antenna with offset reflectorarray feeds

A mathematical model of the circularly polarized feed is developed in this paper. The approach for the Jacobi-Bessel series expansion of the physical optics radiation integral studied by R. Mittra et al. is extended to the case of circular polarizition. A computation method for the circularly polarized contoured beam of the offset reflector antennas is analysed. Representative numerical result is given for effective isotropic radiated power contour-plot of the western zone coverage of Chinese territory by the 7-element array illumination.     

Lateral-feed displacement in a paraboloid

The beam shift and degradation of a paraboloidal reflector with an offset feed is analyzed by the scalar plane wave theory. Higher order coma terms are included with the feed at its optimum axial position. The beam characteristics for a tapered circularly symmetric illumination are presented. The range of validity of the approximate analysis is indicated.     

Optimum beam scanning in offset single and dual reflector antennas

Optimum beam scanning in offset single reflector (paraboloid) and dual reflector (Cassegrain and Gregorian) antennas is considered. Analytical, computationally efficient solutions and results are presented for the optimum feed position, the constant beam direction feed loci, and the optimum feed position locus. Examples are presented to verify that the analysis technique yields the optimum feed position, which exhibits better gain, pattern symmetry, and sidelobe levels when compared with other feed positions producing beams scanned to the same direction. The solutions described were obtained under the ray optics approximation and a "receive mode" analysis. Although the developed method was applied to the antennas listed above, it can be easily extended to othern-reflector systems, shaped reflector antennas, lenses, and other similar systems.     

Synthesis of offset dual shaped subreflector antennas for control of Cassegrain aperture distributions

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.     

偏置抛物面天线圆极化版图成形波束工程理论的研究

本文根据矩形波导中隔片式圆极化器传输波型的渐变过程,导出了圆极化馈源的辐射场公式,并利用R.Mittra等作者提出的物理光学辐射积分Jacobi-Bessel级数展开方法对偏置抛物面天线圆极化版图成形波束的工程理论进行了研究。它包括:圆极化馈源辐射场公式;偏置抛物面电流等效投影孔径布分;二次辐射场计算公式;版图成形波束等效各向同性辐射功率(EIRP)工程计算公式;天线坐标、版图经纬度坐标与卫星坐标之间的变换关系;给出了七元馈源阵成形的我国西部版图圆极化波束的数值结果,其EIRP等电平曲线是计算机自动打印的,本文研究的理论方法可作为研制版图成形波束天线各阶段的基本分析工具。     

Forward radiation from axially symmetric reflector antennas

The fields radiated from focus-fed symmetric reflector antennas in the vicinity of forward axial direction are determined by a series representation of the physical optics integral. The considered feeds have cosine-tapered patterns with different tapers inE- andH-planes. The influence of feed pattern asymmetry, subreflector blockage, feed taper, focal distance to diameter (f/D) ratio and the reflector size on the performance parameters of paraboloidal, Cassegrain, near-field Cassegrain, and corresponding Gregorian antennas is investigated. Design curves are presented to predict the performance parameters of the considered antennas.     

反射面天线馈源的混合相位中心

本文基于场相关定理严格导出了轴对称反射面天线中馈源的混合相位中心表达式,证明了对于线极化和园极化馈源,馈源混合相位中心表达式是完全一致的.为反射面天线尤其是修正卡塞格伦天线的设计和调整提供了理论依据.     

Dual offset reflector multibeam antenna for international communications satellite applications

Dual offset reflector antenna systems offer exciting possibilities for achieving both low scan losses and low cross polarization in geosynchronous communications satellite antennas providing narrow (100 leq D/lambda leq 400) and multiple beam frequency reuse coverages over an18degconical field of view. Novel geometrical configurations for the reflectors are characterized by simultaneously achieving: 1) blockage free apertures for all element beams within the18degconical field of view, 2) compatibility with large planar feed arrays, 3) additional degrees of design freedom by orientation and shaping of reflector surfaces for depolarization and scan loss optimization, and 4) large effectivef/Dratio achieved in compact and foldable geometries. A comparison of new front-fed offset Cassegrain (FFOC) and side-fed offset Cassegrain (SFOC) systems is made.     

Subreflector extension for improved efficiencies in Cassegrain antennas--GTD/PO analysis

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 as1 lambdanoticeable improvements in the overall efficiencies can be expected. Useful design data are generated for the efficiency curves and far-field patterns.     

一种星载通信混合反射面天线的设计方法

为了使星载通信天线产生1个赋形波束覆盖服务区,同时产生1个固定点波束和1个有限扫描点波束,该文提出一种由2个赋形反射面和3个馈源组成的混合反射面天线。该天线是以赋形主反射面共用为基础,等效为2副单馈源单偏置反射面天线和1副双偏置格里高利型赋形反射面天线,分别产生赋形波束、固定点波束和有限扫描点波束。通过对一副口径为1.2 m的天线各个波束进行仿真实验,赋形波束在Ku收、发频段时波束覆盖区边缘(EoC)方向性系数为27.5 dBi,固定点波束在C收、发频段时天线口径效率高于70%,通过将赋形副反射面及对应馈源横向偏焦实现Ka收、发频段的点波束在服务区内外的扫描。仿真结果表明,该混合反射面天线可实现C/Ku/Ka频段的同时通信任务。     

双馈源偏置卡塞格伦天线研究

文章根据偏置卡塞格伦天线设计原理,通过双馈源、副反射面的正反面将两种不同结构的偏置卡塞格伦天线结合在一起,研究出一种双馈源-单副反射面-双主反射面的新型偏置卡塞格伦天线。该天线充分利用了偏置双反射面天线的扫描特性好、可避免遮挡效应等优点,在两个不同方向上产生双波束,实现双方向扫描,拓宽了卡塞格伦天线的应用领域。     

Gaussian beam techniques for illuminating reflector antennas

Simple design procedures are presented for use when a Gaussian beam is used to illuminate a classical reflector antenna. Displacement of the location of the beamwaist toward the focusing element in the case of electrically small antennas where the aperture is in the near field of the feed was calculated together with modification of the required beamwaist radius. Dual reflector antennas were discussed and design procedures appropriate for systems with large and small focal length to diameter ratio developed. Cases where a reflector or subreflector is electrically small, or in the near field of a feed, are readily treated. For elliptical beam antennas, a simple illumination system using only a scalar horn and a single cylindrical lens can generally be found; this has no ray optics analogue. A configuration of this type is discussed, with a practical case study of a 28-by-80-λ elliptical Cassegrain antenna operating at a wavelength of 3 mm. The design process for designing the feed system is discussed in detail. Despite the small size and relatively large aperture blockage, an aperture efficiency of 0.48 was measured, which compared quite well with the expected efficiency of 0.53, thus verifying the validity of the Gaussian beam design approach     

La synthèse d’antennes à deux réflecteurs conformés à alimentation décalée

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.     

The current state of the reflector antenna art

The current state of the reflector antenna art is presented in the form of a review article, intended primarily for engineers who are new in the field. The properties of paraboloidal antennas are related to aperture efficiency, surface tolerance, feed defocusing, and aperture blocking. Other reflector shapes considered are offset-fed paraboloids, classical and offset Cassegrain systems, shaped-dual reflectors, and scanning reflectors. Properties of contoured-beam systems are reviewed. Structures and materials are reviewed for ground stations and satellite-borne reflectors. Emphasis is placed on specific hardware examples for each principle or concept. Near-field probing, dichroic surfaces, and techniques of surface metrology are presented as examples of promising areas for new developments.