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.     

Low-loss offset feeds for electrically large symmetric dual-reflector antennas

Circularly symmetric, dual-reflector, high-gain antenna systems often require feeds placed off the system's axis because of the need for multiple feeds to use the reflector antenna. Also, the constraint requiring the hyperboloid or shaped subreflector to remain circularly symmetric is sometimes added. In a Cassegrainian system, the subreflector and feed may be rotated off-axis around the paraboloid focus and retain main reflector focusing. However, substantial spillover results in considerable noise in a high-gain/low-noise temperature system. In a shaped system, the tilt of the shaped subreflector and feed together results in substantial defocusing as well as spillover noise. If the subreflector is tilted approximately one-half the angle of the feed tilt in either the Cassegrainian or the dual-shaped reflector antenna, it is found that spillover and noise are substantially reduced with tolerable defocusing. An extensive numerical analysis of these effects was conducted to determine the characteristics of a 70-meter, dual-shaped reflector versus Cassegrainian antenna and to gain some understanding of the cause of the observed effects.     

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.     

Feed-reflector design for large adaptive reflector antenna (LAR)

A novel feed-reflector system for large Cassegrain antennas of radio astronomy and deep-space communication applications is investigated. This feed-reflector is used to illuminate a hyperboloid subreflector with a 5-10 m diameter located 500 m above the ground. Because the subreflector is located in the near field of the feed-reflector antenna, a theory based on the near field focusing properties of paraboloid reflectors is established. The focusing at near distance is formed by moving the feed horn away from the focal point of the feed-reflector. In this theory, the properties of axial defocused paraboloid reflectors at near distance are investigated in more detail. By using equivalence path law, the subreflector shape is obtained. It is found that the hyperbola can approximate the subreflector well. A detailed ray tracing is performed on the entire system which reveals that the feed system uses some part of the subreflector three times. The gain, side lobe level, cross polarization, and aperture distribution are calculated for different feed horn locations and taper at the edge of the feed-reflector and also for different sizes and eccentricities of the subreflector. Peak efficiency in excess of 74.8% and side lobe level around -20 dB are obtained for an unshaped system. The performance of the system over the operating band (1-22 GHz) is also studied and shown that the lower-frequency limit is dependent on subreflector and feed-reflector sizes     

Equivalent hyperboloid (ellipsoid) and its application

Analogous to the equivalent paraboloid, the equivalent hyperboloid (ellipsoid) for dual quadratic surface reflector antennas is derived. The condition that the equivalent reflector is center fed is also derived. The numerical example shows that a reflector geometry that satisfies this center fed condition is a good initial antenna geometry to design a shaped dual reflector antenna     

Subreflector shaping to improve multiple-beam performance of a Cassegrain antenna

A method of designing multiple-beam antennas based on shaping the subreflector of an offset Cassegrain antenna is described. It is applied to a compact system having a diameter of 300? that is required to produce beams up to 4-6°either side of boresight. The performance is shown to be only slightly less than that obtainable by also shaping the main reflector, indicating that efficient multiple-satellite-access antennas can be designed without specially shaped main reflectors.     

Surface accuracy of Cassegrain antennas

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.     

Characteristics and design of the LAR offset system with feed-reflector

A novel approach for analyzing the quasi-optical offset large adaptive reflector (LAR) Cassegrain system is described. In this system, a feed-reflector is used to illuminate a reconformable hyperboloid subreflector with 5-10 m diameter, located 500 m above the ground. An exact equation for the offset LAR surface is given. To scan the beam up to 60/spl deg/ which is one of the LAR requirements, the concept of the dual offset LAR with feed-reflector is introduced. In this design, the cross polarization is eliminated by a proper orientation of the subreflector. The parameters of the configuration are obtained by utilizing generalized Gauss-Laguerre beam modes and matrix representation of the beam mode transformation factor. The blockage effect due to the feed-reflector is totally removed.     

Comparisons between a shaped and nonshaped small Cassegrain antenna

An X-band 8.5-ft brassboard antenna system was designed and developed which required a 70% total antenna efficiency and a 25 dB isolation between the circularly polarized transmit and receive ports. To maximize the aperture efficiency, a shaping technique was used to generate a specially contoured subreflector and main reflector. To reduce cost, a configuration was chosen such that the shaped main reflector could be matched with negligible phase error using a commercially available paraboloid. The antenna gain of this shaped system with an electrically small subreflector (10.7 λ) is 0.75 dB higher than that of a conventional system using the same paraboloid and a matching hyperbolic subreflector. Measured results demonstrated that even for a small system the antenna performance can be appreciably improved at low cost by using a shaped subreflector     

Designing classical offset Cassegrain or Gregorian dual-reflector antennas from combinations of prescribed geometric parameters

This paper proposes a simple procedure for the design of classical offset Cassegrain or Gregorian dual-reflector antennas from combinations of prescribed geometric parameters. This procedure has already been applied to classical Cassegrain and Gregorian antennas, to classical displaced-axis Cassegrain and Gregorian antennas, and to classical offset Dragonian antennas. The antenna systems can be fully characterized by 21 parameters, of which only five need to be provided by the antenna designer, as the remaining 16 parameters can be derived in closed form using the procedure described here. In this paper, we assume that the main reflector has a circular aperture, while the subreflector has an elliptical aperture All the antenna geometries presented satisfy the Mizugutch condition (1976), which is the geometric-optics condition for zero cross-polarized radiation. This procedure is very close to the one used for offset Dragonian systems, but all the relevant information is repeated here for completeness.     

Designing axially symmetric Cassegrain or Gregorian dual-reflectorantennas from combinations of prescribed geometric parameters

A procedure to design axially symmetric Cassegrain or Gregorian dual-reflector antennas from various combinations of prescribed geometric parameters is presented. From these input parameters, the overall geometry of the antenna is derived in closed form. This procedure can be used as the starting point of a synthesis procedure, where both main reflector and subreflector are shaped to create the desired aperture field distribution     

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.     

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。     

Preliminary study on offset scan-corrected reflector antenna system

Preliminary study on offset shaped dual reflector antenna systems has been carried out to assess the feasibility for multibeam satellite applications. The two-dimensional offset shaped reflector antenna geometry is generated by first creating the nodal points according to a bifocal condition and then connecting the nodal points by smooth curves to form the profiles of the main and subreflectors. The three-dimensional geometry is created by body revolution. The offset geometry is obtained by properly tailoring the three-dimensional geometry. This offset shaped reflector antenna system has an inherent astigmatism which can be either fully or partially compensated. For applications requiring a scan range in azimuth more thanpm 5beamwidths, the offset shaped dual reflector antenna systems offer better scan performance (in terms of peak gains) than offset Cassegrain geometries at the expense of the performance of the on-axis beams. In elevation with a 16 beamwidth scan range, the shaped design provides 0.3 dB less scan loss than the Cassegrain design.     

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.     

Scanning properties of large dual-shaped offset and symmetricreflector antennas

The scanning properties of shaped reflectors, both offset and circularly symmetric, are examined and compared to conic section scanning characteristics. Scanning of the pencil beam is obtained by lateral and axial translation of a single point source feed. The feed is kept pointed toward the center of the subreflector. The effects of power spillover and aperture phase error as a function beam scanning are examined for several different types of large reflector design including dual-offset, circularly symmetric large f/D, and smaller f/D dual reflector antenna system. It is shown that the Abbe-sine condition for improved scanning of an optical system cannot, inherently, be satisfied in a dual-shaped reflector system that is shaped for high gain and low feed spillover. The gain loss, with scanning, of a high-gain shaped reflector pair is demonstrated to be due to both aperture phase error loss and power spillover loss     

Offset Cassegrain Earth Station Antenna for the Japanese Domestic Satellite Communication System

Offset reflector antennas have advantages for communication systems because they are not severely subject to blocking. Difficulties mainly arising from structual asymmetries have inhibited the realization of an offset reflector antenna with a large aperture for commercial use. This paper describes the design of an offset Cassegrain earth station antenna for the Japanese domestic satellite communication system. Antenna measurements showed 76 and 69 percent aperture efficiencies at 20 and 30 GHz, respectively, less than -20 dBi wide angle directivity and an 18 K noise temperature in operating conditions. Performances are far superior to conventional axisymmetrical earth station antennas. The antenna was reassembled on a telephone office building after the measurements. The antenna gain was reconfirmed there, using the sun as a radio frequency source. Experiments show that the earth station antenna and a terrestrial antenna can be placed on the same building without serious interference.     

Design of high-efficiency antennas employing dielectric-cone feeds

A design procedure based on the principles of geometrical optics, for any arbitrary amplitude and phase distributions, is described for antennas employing dielectric-cone feeds. The procedure can be used for dual-reflector antennas, reflector antennas with a subreflector supported with dielguides and reflector antennas using dielectric-cone feeds with a shaped reflecting surface of the cone material.     

一种多功能反射面天线的设计

介绍了一种多功能反射面天线的设计方法,利用同一反射面实现跟踪和干扰的功能。跟踪采用副面为栅条形式的卡塞格伦天线,干扰采用前馈抛物面天线。2种天线正交极化,通过理论计算、仿真优化结合合理的结构设计,保证2种天线的电性能及相互之间的隔离度。     

Designing axially symmetric Cassegrain or Gregorian dual-reflectorantennas from combinations of prescribed geometric parameters. 2.Minimum blockage condition while taking into account the phase-center ofthe feed

For pt.1 see ibid., vol.40, no.2, p.76-82 (1998). An easy procedure to design classical Cassegrain or Gregorian dual-reflector antennas has been presented in Granet (1998). This procedure allows the antenna designer to fully define the antenna geometry with different sets of input parameters, depending on the requirements of the antenna size and its performance. In this paper the conditions derived in Granet are extended to take into account the phase-center position of the feed, to achieve the minimum-blockage condition. This procedure can be used as the starting point of a synthesis procedure, where both main reflector and subreflector are shaped to create the desired aperture-field distribution