Reflector sidelobe degradation due to random surface errors

It is well known that the sidelobe structure of a reflector antenna is highly susceptible to random surface errors, and that in most applications it is not adequate to investigate only the average behavior of the antenna. In this study, an attempt is made to determine the probability distribution of the sidelobe level of a reflector antenna subject to some random surface errors. Specifically, the random pattern function is considered and its sidelobe level studied using the level-upcrossing theory. Both the degradation of the maximum sidelobe and the degradation of the sidelobe region with respect to an International Radio Consultative Committee (CCIR) sidelobe envelope are obtained. The theoretical results are found in excellent agreement with those obtained by Monte Carlo simulations. Finally, some useful tolerance charts are presented.     

Limitations on reflector antenna gain by random surface errors,pointing errors, and the angle-of-arrival jitter

The effects of random surface, pointing, and angle-of-arrival errors on the antenna gain are discussed. It is shown that for manufacturing tolerance ratios greater than 10⁵, the random surface errors decrease the antenna gain only when D/λ>3000. The pointing/angle-of-arrival error causes nonnegligible loss in gain when it exceeds approximately a few tenths of the antenna beamwidth. The gain (and the size) of very large reflector antennas is generally limited by the combined effects of the above errors     

Contribution of reflector profile errors to antenna sidelobe radiation

The validity of a simple formula for the level of sidelobe radiation due to profile errors in an antenna reflector has been verified by numerical evaluation of Ruze's integral. The statistical distribution of the sidelobes is discussed before the formula is applied to the practical case of large earth-station antennas. Profile errors may significantly impair the sidelobe performance of these antennas unless adequate tolerances are specified.     

Crosspolarisation due to radial field components in reflector antennas

The letter stresses the importance of including the radial component of the transmit field of the feed horn when analysing reflector antennas. Omission of this component leads to errors in the prediction of crosspolar patterns similar in magnitude to those produced by diffraction effects within a dual reflector antenna.     

表面误差对双弯曲反射面天线副瓣的影响

天线反射面的表面误差会引起反射面天线的副瓣发生变化。为确定赋形波束双弯曲反射面天线反射面的表面误差与反射面天线副瓣最大值变化之间的关系,采用数理统计的方法,对受到随机表面误差影响的面电流积分,得到天线辐射场。随机表面误差用相关半径和Z向随机误差两个参量表示。根据随机表面误差求出其天线最大副瓣样本分布函数,较好地展现了表面误差引起的副瓣最大值变化,使得反射面天线表面误差引起的副瓣变化可预测,为天线反射面加工的精度要求提供了理论依据。     

天线表面误差的精确计算方法及电性能分析

提出一种天线表面误差的计算方法,用来精确计算变形表面法向、径向及轴向误差,并推导了相应的计算公式.通过分析某工程7.3m圆抛物面天线在多种工况下的表面误差,及其与ANSYS软件计算结果的比较,证明了该方法的正确性,且计算结果仅为ANSYS软件分析结果的三分之一左右.实验结果说明此方法能够更准确地分析变形天线的表面误差及电性能.在工程中应用此方法分析大型空间天线与地面天线,可明显降低天线设计成本,缩短天线设计周期,并为电气设计人员提供准确的天线变形反射面信息.     

Subreflectarrays for Reflector Surface Distortion Compensation

With the increasing interest in the applications of large deployable reflector antennas operating at high frequencies, the requirement on the reflector surface accuracy becomes more demanding. Thermal effects inevitably cause certain reflector surface distortions, thus degrading the overall antenna performance. This paper introduces a novel reflector surface distortion compensation technique using a subreflectarray and presents detailed discussions. A microstrip reflectarray is used as a subreflector, illuminated by a primary feed. By properly adjusting the additional phase shift provided by the subreflectarray, the aperture phase errors caused by the main reflector surface distortions are compensated, resulting in a considerably improved antenna performance. As an example, a distorted 20-m offset parabolic reflector antenna operating at X-band is successfully compensated by a subreflectarray, and the simulation results are compared with those obtained by array feed and shaped subreflector compensation techniques. The microstrip subreflectarray is low-profile, lightweight, and cost-effective. Only one primary feed is required, and a reconfigurable design can be achieved if electronically reconfigurable reflectarray elements are adopted.      

A new aperture plane method of measuring antenna surface errors

A scheme for measuring surface errors of reflector antennas is proposed in which the phase of the signals from small areas of the wavefront are compared one by one to the phase of the signal from the rest of the wavefront. The comparison is accomplished by periodically changing the phase of the signal from each small area by ±90°. For convenience and economy, the measurement can be made in an aperture plane of an antenna feed system, and can be done in real time with the antenna pointing steadily at the source being observed. Estimates are made of the signal power required for the measurement, and three means of modulating the phase are suggested, one of which allows the surface error of the antenna to be corrected as well as measured     

Effect of support strut diffraction on reflector surface profile assessment

The phase-imaging properties of microwave holographic processing are used to provide an inward-looking reconstruction of support strut diffraction and its effect on reflector surface profile assessment. Measured near-field data are employed to illustrate the contribution made by the support strut geometry to the apparent profile error for a 2.5 m reflector antenna operating at 10 GHz. The relative significance of real profile errors and those deriving from strut effects are studied by using a diffraction model. It is concluded that certain configurations of support struts can give rise to significant modification of the effective profile error distribution.     

Random surface error effects on offset cylindrical reflector antennas

The focus of this paper is to characterize the average power pattern of an offset cylindrical parabolic reflector antenna subject to random surface errors. A novel computational method is developed for studying the random surface error effects on the boresight gain loss and the sidelobe levels of the average power pattern. The particular antenna dimensions used for this study are similar to the specifications for the second generation precipitation radar (PR-2) mission. In keeping with the requirements in the PR-2 mission, both the Ku and Ka frequency bands are considered. Random surface error effects are also studied for different edge taper levels. In addition to analysis of a reflector with uniform random surface errors, a nonuniform surface error case is presented in this study. This is an important consideration for evaluating the suitability of large deployable membrane reflector antennas for the PR-2 mission.     

反射面天线机电场耦合关系式及其应用

针对反射面天线随机误差和系统误差的特点,通过对Ruze公式、最佳吻合和提出的机电场耦合关系式的分析比较,从理论上研究了三者的差别.即Ruze公式未考虑反射面的系统误差,从而导致对天线结构设计和制造精度要求往往过高,而最佳吻合虽考虑了系统误差,但未考虑随机误差.机电场耦合关系式则将两种误差同时考虑进去了,并将电性能表示为...     

An approximation of the radiation integral for distorted reflectorantennas using surface-error decomposition

The physical optics/aperture integration (PO/AI) formulation is often used to analyze the radiation patterns of reflector antennas. In this study, the PO/AI radiation integrals for distorted reflector antennas are addressed. The surface error of the antennas is approximated by a series of surface expansion functions. The radiation integral is decomposed into a series of radiation-type integrals, each of which corresponds to one of the surface expansion functions. Each of these radiation-type integrals is then weighted by amplitude coefficients. The advantage of performing the decomposition is that each of the radiation-type integrals can be computed and the pattern data stored. The computation of the pattern for a distorted reflector antenna with a changing error profile is performed by recalling the pattern data for each perturbation term and weighting it with the amplitude coefficient. This facilitates rapid evaluation of the radiation integral in cases where the error profile is changing (for example, time-varying errors). The superposition of integrals presented in this paper was shown to be valid for surface-error profiles up to 0.1 λ rms amplitude     

Aperture efficiency of Itapetinga 45-ft antenna at λ = 3.3 mm

The aperture efficiency atlambda = 3.3mm of the 45-ft Itapetinga reflector was found to be of about 25 percent. This radiometric result suggests an equivalent surface accuracy of less than 0.25 mm, root sum square (rss) assuming a Gaussian distribution of errors, which is much smaller than the actual rss estimated for the dish (i.e., 0.37 mm, rss) and, represents a 5 dB improvement in gain. This result confirms that non-Gaussian errors across a smooth surface have negligible effects on the aperture efficiency. It is demonstrated that there is little effect on gain produced by error correlation regions that are small compared to the antenna diameter and that antenna efficiency may be predicted from mechanical measurements when the nature of such errors across the surface is know. It is possible to predict that relatively simple mechanical readjustments of the reflector and subreflector may raise its efficiency to 30 percent.     

基于最小二乘法的天线变形反射面的拟合

面天线变形反射面表达式的确立是计算其电性能的重要基础 ,提出的拟合方法就是用来精确确定变形反射面的数学方程的。其基本原理是通过分析天线结构 ,根据反射面节点位移 ,最小化整个曲面上采样型值点的轴向误差的均方根 ,从而求出用来确定变形抛物面几何位置的 6个参数 ,同时也就得到了基于最小二乘法的变形反射面方程。该拟合方法简单实用 ,易于程序实现 ,能够对天线机械结构设计人员提供了理论指导与帮助。     

Surface accuracy measurement of a deployable mesh reflector byplanar near-field scanning

Using a near-field antenna measurement facility, it is possible to simultaneously evaluate the surface accuracy of a reflector antenna as well as the far-field pattern of the antenna for a short time. The surface errors of a 2-m deployable mesh reflector for satellite use were measured by a planar near-field system. As a result, the influence of periodic structures, due to the antenna ribs, is clearly observed. Also, the surface accuracy obtained with the near field scanning technique coincides well with that obtained by an optical measurement technique     

反射面天线馈源喇叭边缘照射电平的分析

反射面天线设计的关键就是通过选择合适的喇叭馈源边缘照射电平以追求天线的高性能（增益,旁瓣电平, 效率等）。本文主要利用实例验证了标准反射面天线馈源喇叭的优选边缘照射电平,分析了赋形反射天线馈源喇叭的优选 边缘照射电平,给出了标准反射面和赋形反射面在设计中各自馈源喇叭的优选边缘照射电平。从而可以在设计工作中对 于不同形式的反射面天线有针对性的设计馈源喇叭的边缘照射电平。     

Antenna with multibeam applications

The design of a dual-reflector antenna that focuses an inclined incident plane wave to a point away from the axis of symmetry of the reflector is discussed. A useful application of such a system would be as a multibeam antenna for use in communication via satellites. Curves of path-length errors are presented for the feed positioned on the axis of symmetry, and the effects of refocusing are shown.     

Analysis of the effects of water on the ACTS propagation terminal antenna

The NASA advanced communications technology satellite (ACTS) propagation experiment was designed to observe the attenuation produced by rain on Earth-satellite paths operating in the Ka-band. Unwanted effects of water on the antenna reflector surface were noted. Wet-antenna attenuation could be attributed to the combined effect of a water layer on the reflector surface and water wetting the feed window surface. A model was developed to calculate the antenna reflector and feed surface water layer thickness values as a function of position on each surface. The thickness values were used to calculate the additional attenuation produced by the water layers as a function of rain rate on the antenna. The wet-antenna-attenuation prediction model was verified by sprayer tests. The goal of the ACTS propagation experiment was to obtain path attenuation statistics, statistics that represent the effects of rain on the Earth-satellite path but not on the antenna itself. The wet-antenna attenuation prediction model was used to remove the effects of water on the antenna from the combined antenna-plus-path attenuation statistics produced by the experiment. The overall efficacy of the model was demonstrated by comparing the corrected path loss statistics from two ACTS propagation experiment sites with earlier COMSTAR path loss measurements made at or near those sites. The empirical distribution functions from both the ACTS and COMSTAR experiments were identical within the expected uncertainty of an empirical annual distribution of attenuation by rain.