Large Reflector Antennas Research Articles

On distorted surface analysis and multidisciplinary structural optimization of large reflector antennas

The large reflector antenna structural design is a typical multidisciplinary design mainly involving structural mechanics and electromagnetics. Deformation of the antenna reflector affects antenna’s electrical characteristics. In this paper, an analysis is made of the variation of antenna’s performances under different employment conditions by applying the response surface method and the Coons divided-fitting method with presentation of the mathematical formula. Furthermore, three types of optimization models of the antenna structure are discussed and the main structural variables influencing electrical performances are analyzed. Finally, an example of multidisciplinary optimization design of an 8-m antenna is explored to illustrate the accuracy of the methods and the potential of application to improve the electrical performances of reflector antennas.

Hologram-based compact range for submillimeter-wave antenna testing

A hologram-based compact antenna test range (CATR) is being developed to overcome challenges met in antenna testing at submillimeter wavelengths. For the first time, this type of CATR has been built for testing of a large reflector antenna at submillimeter wavelengths. The CATR is based on a 3-m computer-generated hologram as the focusing element. This paper discusses the design and the construction of the CATR, and the verification of the CATR operation with quiet-zone tests done for the CATR prior to the antenna testing. Assembly of the CATR, testing of the 1.5-m reflector antenna at 322 GHz, and the disassembly were all done within two months in 2003. The quiet-zone field measurement results are analyzed in this paper. The CATR was concluded to be qualified for antenna testing. The antenna testing is described in a separate paper.

Characterization of effects of periodic and aperiodic surface distortions on membrane reflector antennas

The focus of this paper is to characterize the effects of periodic and aperiodic surface distortions on the performance of membrane reflector antennas. Since the surface of this class of reflector antennas is very thin, it is susceptible to various types of periodic and aperiodic distortions. The particular antenna dimensions used for this study are similar to the specifications for the JPL/UCLA half scale model of second generation precipitation radar (PR-2) mission reflector. Analytical expressions are introduced to model periodic and aperiodic surfaces and based on these models the effects of distortions on the radiation performance of the antenna are simulated. Aperiodic distortions are more realistic cases of distortions due to the fact that the period of the distortions is not constant through out the reflector surface. For each case, far-field patterns of the reflector are simulated and it is shown that closed-form expressions can then be derived which result in a very efficient computational method to predict some of the unique features of these patterns including location and level of observed grating lobes. Furthermore, based on spatial Fourier analysis of the surface distortion, it is shown that deviation from periodicity in the distortions of reflector surface results in lowering these grating lobes. Parametric studies have been performed to provide design guidelines for acceptable surface behavior for large deployable membrane reflector antennas for future space borne missions.

An active surface for large reflector antennas

We present a solution adopted for the Noto antenna (Sicily) in order to overcome the degradation of antenna efficiency due to the gravitational deformation of the structure of large antennas. This new setup allows a substantial increase in the operating frequency, and eliminates the dependence of the antenna efficiency on elevation.

Hybrid GB and PO analysis of electromagnetic radiation/scattering from large reflector antennas with tapered impedance surfaces

AbstractA hybrid scheme combining Gaussian beam (GB) and physical optics (PO) approaches is presented for an efficient analysis of reflector antennas with tapered‐impedance edge boundaries. The reflecting surface is divided into an interior portion with uniform impedance and a smooth boundary, where the GB approach can be employed, and an exterior portion of tapered impedance, which is analyzed by the PO approach. The high efficiency of the GB analysis potentially accelerates the entire analysis, as compared with the case in which only PO is employed. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 34–37, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20198

Gravitational Deformation of a Reflector Antenna Measured With Single-Receiver Holography

Holographic measurements of deformations in a large antenna reflector are described. The deformations were brought about by changing elevation angle and gravitational loading. A phase-switching arrangement was used, which allowed a single, already installed and operating receiver to be shared by both the main antenna signal and a reference signal. Natural H/sub 2/O masers in our galaxy were used as signal sources, and a reference antenna was attached rigidly to the antenna under test so as to move with it. Observing techniques are described that accommodate changes in source elevation angles and antenna feed position while acquiring a data set.

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.

Vibroacoustic Loads On Spacecraft Antennae: Analysis And Test Correlations

The acoustic load during launch is one of the dimensioning cases of spacecraft structures, especially demanding for large antenna reflectors. Current specifications rely on the diffuse field idealisation, while tests are actually performed in reverberating chambers, leading to discrepancies between predicted and measured responses. Most of these are due to the limited capabilities of analytical tools for industrial applications and the testing facilities, particularly when dealing with the modelling or control of the acoustic inputs. However, as shown in this paper, some sources of correlation errors are inappropriate signal processing methods, that do not take into account the physical nature of the loads and the fluid-structure coupling. In particular, digital filtering techniques applied to fine resolution power spectral densities are proposed, in order to better retain the effect of cavity modes, instead of the octave-fraction bands approach. This is of interest when analysing the low frequency range, in which the most important contributions to structural stresses usually appear, depending on the size in terms of pressure blocking, the structural impedance, the acceptance of eigen-mode shapes and the radiation coupling. Several industrial examples are studied, providing insight into typical results, the nature of the discrepancies and some possible solutions.

Gaussian-beam analysis of a large adaptive reflector antenna (LAR) using a feed-reflector

A novel approach for analysing the quasi-optical large adaptive reflector Cassegrain system is described. In this system a feed-reflector is used to illuminate a hyperboloid sub-reflector with 5-10 m diameter, located 500 m above the ground. In the proposed method of analysis the feed-reflector aperture field distribution is expanded into a set of Gaussian–Laguerre modes. These modes propagate from the feed-reflector aperture in a simple and well defined way. The feed-reflector near-field radiation pattern is calculated at the subreflector location. The subreflector parameters in this system are found by maximising the LAR aperture efficiency, which includes phase and taper efficiencies, and minimising the LAR spillover loss. This process is computationally more efficient than the physical-optics current-distribution method and more accurate than the ray-tracing approach. It also provides a new insight into the operation of the feed-reflector system.

Application of Gaussian-ray basis functions for the rapid analysis of electromagnetic radiation from reflector antennas

A highly efficient Guassian beam (GB) method was recently developed to provide a relatively rapid analysis of large reflector antennas. This GB method is very efficient because it represents the feed radiation in terms of a set of very few rotationally symmetric GBs and provides an essentially closed form solution for the reflection and diffraction of each GB incidence on the reflector surface. In contrast, conventional approaches evaluate the radiated field via a numerical integration of the physical optics integral over the large reflector surface; such numerical techniques are time consuming for large reflectors. However, the fast GB method becomes less accurate if each GB illuminating spot on the reflecting surface is comparable to the overall reflector size, since its closed-form solution requires a small spot size. This limitation of the GB approach happens particularly in situations where the feed is located relatively far from the reflector, since the GBs are not able to remain narrow in the angular and spatial domains simultaneously, thereby creating too large a spot size on the reflector surface. The above limitation of the GBs is overcome here by introducing a Gaussian-ray basis function (GRBF), a hybridisation of a geometrical optics (GO) ray tube and a GB. Owing to its similarity with a GB in the amplitude variation transverse to the ray, its incorporation into any existing GB code is straightforward. Numerical results are presented to demonstrate the accuracy and robustness of the new GRBF approach.

Near-field line-integral representation of the Kirchhoff-type aperture radiation for a parabolic reflector

A line-integral representation is presented for a linearly polarized Kirchhoff-type aperture radiation from a parabolic reflector antenna. The main purpose of this result is concerned with the acceleration of the numerical integration for calculating the near field of large reflector antennas. The formulation, which is rigorous for a uniform aperture field, is based on the application of the equivalence principle to a projecting surface, which allows the analytical evaluation in a closed form of a twofold surface integral which defines the radiated field at any space point; the extension to a slowly varying primary feed pattern is based on an asymptotic approximation, which is proved to be accurate in the proximity of the aperture to -30 dB of amplitude edge illumination. The present formulation is well suited to be improved by fringe diffraction contributions in the framework of edge-wave theories such as the physical theory of diffraction (PTD) and the incremental theory of diffraction (ITD).

Subreflector shaping for antenna distortion compensation: an efficient fourier-jacobi expansion with GO/PO analysis

Technological demands have brought a renewed interest in the application of large reflector antennas with steadily increasing operating frequencies and antenna dimensions. The high surface accuracy of the main reflector required by these antennas can often not be achieved with available manufacturing technologies. The utilization of a shaped subreflector for main reflector-distortion compensation is considered an effective measure to enhance the overall radiation performance of an antenna system. In the process of evaluating the suitability of the subreflector shaping, however, it is crucial to accurately assess the most suitable subreflector shape within a reasonable amount of computational time. This is especially true for electrically large reflectors, where simple analysis of the radiation characteristics already creates a serious computational burden, moreover, since reflector shaping is a synthesis process that necessitates repeated computation of the radiation characteristics. In this paper, the development of an efficient computational tool for subreflector shaping is presented. The subreflector shaping is performed through a combination of geometrical optics (GO) and physical optics (PO) on the subreflector and the main reflector, respectively. To significantly limit the number of parameters subject to optimization, the subreflector surface is parameterized by the coefficients of a global, orthogonal Fourier-Jacobi set (related to Zernike polynomials), which allows us to accurately represent a surface with only a small number of coefficients. The incorporation of this surface expansion into a GO/PO synthesis technique is detailed, representative results are given for a computationally challenging reflector configuration, and the tolerances for the shaped subreflector surface are studied.

Passive intermodulation on large reflector antennas

In this work, an analytical model for the study of passive intermodulation (PIM) on large reflector antennas is presented. Passive intermodulation, in the scattered field, arises when the scatterer is nonlinear, or when it presents junctions connecting linear materials. Its presence causes a degradation of some antenna parameters and, especially, the cross-polar level, which may rise by several tens of dB. It would then be useful to develop a technique to predict its influence, in order to take appropriate steps during antenna design. A heuristic model for the junction problem has previously been derived and validated with measurements in a time-domain physical optics (TD-PO) framework. These results are applied here to a TD-PO analysis of reflector antennas, in particular, for a satellite-communication antenna and for a radio-astronomy antenna.

Finite difference analysis of electrically large parabolic reflector antennas

A reflector antenna is analyzed using the finite-difference method (FD). The induced current densities on an axially symmetric parabolic reflector are rigorously calculated. The measured equation of invariance (MEI) is used to terminate the FD mesh very close to the reflector surface. To take advantage of the axial symmetry, the theory of coupled-azimuthal potentials (CAPs) is employed. Illustrative results are obtained for reflector antennas with different aperture dimensions. Results by physical optics (PO) approximation are also included for comparison. The purpose of this paper is not to replace ray optics (RO) and PO in the design of reflector antennas, but to demonstrate the advancement in the FD method, which hitherto was limited to low-frequency and closed-boundary regime. The calculated surface current densities of a reflector antenna do show that the normal component of the current densities at the edges exhibits high standing waves which are missing in PO, and which we know should be there. The standing wave of current densities may not affect the main beam, but certainly will have an effect on side lobes and have a major impact in estimating the loss of the antenna.

Novel Gaussian beam method for the rapid analysis of large reflector antennas

A relatively fast and simple method utilizing Gaussian beams (GBs) is developed which requires only a few seconds on a workstation to compute the near/far fields of electrically large reflector antennas when they are illuminated by a feed with a known radiation pattern. This GB technique is fast, because it completely avoids any numerical integration on the large reflector surface which is required in the conventional physical optics (PO) analysis of such antennas and which could take several hours on a workstation. Specifically, the known feed radiation field is represented by a set of relatively few, rotationally symmetric GBs that are launched radially out from the feed plane and with almost identical interbeam angular spacing. These GBs strike the reflector surface from where they are reflected, and also diffracted by the reflector edge; the expressions for the fields reflected and diffracted by the reflector illuminated with a general astigmatic incident GB from an arbitrary direction (but not close to grazing on the reflector) have been developed in Chou and Pathak (1997) and utilized in this work. Numerical results are presented to illustrate the versatility, accuracy, and efficiency of this GB method when it is used for analyzing general offset parabolic reflectors with a single feed or an array feed, as well as for analyzing nonparabolic reflectors such as those described by ellipsoidal and even general shaped surfaces.

Integrated structure-electromagnetic optimization of large reflector antenna systems

A novel multiparameter optimization method is developed for use with terrestrial and space reflector antenna electromechanical systems and other metallic and composite engineering structures. To satisfy extremely high design requirements, the proposed approach incorporates the objectives from various structural and electromagnetic (EM) performances of the system at many working/loading cases simultaneously. A finite element method is used for structural analysis. Optical ray tracing, spline function aperture field interpolation, geometric optics aperture integration, and FFT techniques are employed to analyse the EM performances of distorted reflector antennas. A systematic method is used for parameter profile analysis of the system. The optimization involves member size, structural geometric and material design variables. Various terrestrial and orbital working environments and loading cases which affect antenna performances can be included in the optimization model. The optimization of an 8 m antenna system, as an example, is discussed and the results are given.

高精度大形メッシュ反射鏡の設計と評価

A large antenna reflector with a continuous mesh surface has the advantage of higher electrical gain and lower side-lobe level characteristics. We designed a hybrid cable network for a continuous and highly accurate mesh surface to realize a surface accuracy of 1.0mmRMS for a 10m class antenna reflector. This cable network consists of both a triangle-shaped network and a cobweb-shaped network. One segment model with a diameter of 2.8m was manufactured to confirm the surface adjustment capabilities. A partial model consisting of seven segments and with a diameter of 7m was also manufactured to verify the surface accuracy and the interface conditions between the mesh surface and the truss structure. The surface accuracy was confirmed to within 0.96mmRMS after three courses of adjustments and measurements. The surface accuracy repeatability was confirmed to within 1.0mmRMS. The new design concept of the highly accurate mesh surface was also verified by those experiments.

Uniform asymptotic solution for electromagnetic reflection and diffraction of an arbitrary Gaussian beam by a smooth surface with an edge

A closed form solution is obtained to describe, in a physically appealing manner, the reflection and diffraction of a general astigmatic Gaussian beam which is incident on an arbitrary smooth, electrically large, slowly varying curved, perfectly conducting screen (or reflector). This closed form solution is obtained via an asymptotic evaluation of the radiation integral for the fields scattered from the reflector, to within the physical optics approximation that remains valid for the present situation. The analysis developed here is particularly well suited for the fast analysis of electrically large reflector antennas by representing the feed illumination by a relatively small set of Gaussian beams launched from the feed plane. Each of these Gaussian beams after being launched undergoes reflection and diffraction at the reflector; the expressions for the reflected and diffracted fields are developed in this paper and utilized by Chou [1996] to compute the radiation pattern of large reflector antennas in a matter of a few seconds as compared to the conventional numerical physical optics integral method which takes hours on the same computer.

Piezoelectrically adjustable array for large reflector antenna surface distortion compensation

Piezoelectrically adjustable array for large reflector antenna surface distortion compensation

Piezoelectrically adjustable array for large reflector antenna surface distortion compensation

A novel piezoelectrically adjustable array of mirrors is proposed for large reflector antenna surface distortion compensation. As compared to the conventional approach with variable phase shifters and amplitude feed array, significantly lower rf loss, mass, and cost reflector antennas can be achieved with this innovative approach. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 14: 221–224, 1997.