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.     

A shaped dual-reflector antenna with a tilting flat subreflectorfor scanning applications

A scanning dual-offset reflector antenna using the simple motion of a flat subreflector to illuminate different parts of a shaped main reflector is presented. A particular case having a 20-degree linear field-of-view (for a DBS application) with a 30λ aperture diameter is explored. The main reflector surface is shaped to minimize the phase aberrations across the field-of-view. Once the main reflector surface is designed, the subreflector parameters are determined to ensure optimum focusing across the field-of-view     

双反射面天线主面模具共享技术及其优化方法

本文提出了通过优化副反射面使同一焦距不同口径的主反射面共用一套加工模具的双反射面天线主面面板模具共享技术。提出了一种快速收敛优化方法,并将其应用于主面共享设计,取得了较为满意的结果。     

双馈源双偏置结构星载通信多波束天线

为了使星载通信天线产生赋形波束的同时,又能辐射具有扫描特性的点波束满足特殊情形下的通信要求,提出了一种以单馈源赋形反射面天线为基础,利用另一个馈源进行副反射面赋形,补偿主反射面口径相位差的方法,从而形成高效率点波束,并通过副反射面及对应馈源横向偏焦实现点波束扫描的目的。为了检验方法的有效性,仿真实验了一副口径为2.2 m 的赋形反射面天线。实验结果表明:赋形波束在服务区内部的最小增益是30.0dBi,点波束在服务区内部及周边区域扫描时具有比赋形波束更高的增益,因此,所提方法是有效的。     

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.     

Zooming and Scanning Gregorian Confocal Dual Reflector Antennas

The zooming and scanning capabilities of a Gregorian confocal dual reflector antenna are described. The basic antenna configuration consists of two oppositely facing paraboloidal reflectors sharing a common focal point. A planar feed array is used to illuminate the subreflector allowing the antenna to scan its beam. The resulting quadratic aberrations can be compensated by active mechanical deformation of the subreflector surface, which is based on translation, rotation and focal length adjustment. In order to reduce the complexity of the mechanical deformation, least squares fit paraboloids are defined to approximate the optimal correction surface. These best fit paraboloids considerably reduce scanning losses and pattern degradation. This work also introduces two different zooming techniques for the Gregorian confocal dual reflector antenna: the first consists of introducing a controlled quadratic path error to the main reflector aperture; and the second is based on reducing the size of the radiating aperture of the feeding array.      

High efficiency and low sidelobe design for a large aperture offset reflector antenna

A design method giving high efficiency and low sidelobes is discussed for large aperture offset reflector antennas. A new reflector shaping technique using the subreflector and the beam waveguide reflector with the parabolic main reflector is proposed to simplify the main reflector manufacturing process. The effectiveness of the technique is confirmed by the model experiments. One problem with this reflector shaping technique is that the subreflector edge level cannot be controlled independently of the main reflector edge level. By investigating the relation between the gain reduction and the subreflector edge level, which affects the wide-angle sidelobe levels, the realizable characteristics of antennas are studied. In order to decrease the subreflector edge level without reducing the aperture efficiency, a technique using an extended reflector is also proposed. Its effectiveness is shown by theoretical and experimental investigations.     

A high-grain trireflector antenna configuration for beam scanning

A trireflector antenna configuration is presented that allows for beam scanning with minimum reflector motion, no feed motion, and high primary aperture utilization for all scan directions. Such characteristics are particularly desirable for geostationary satellite applications. Omnidirectional beam scanning is achieved by rotational movement of a relatively small tertiary reflector about a fixed point such that a simple two-axis gimbal can be employed. The trireflector configuration was specifically designed for high-gain passive remote sensing applications, and therefore the emphasis is placed on high-gain beam scanning; however, the concepts developed are equally valid for low-gain applications and can find use in a broad range of applications that require rapid beam scanning and simple reflector motion     

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     

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     

Design of scanning spherical trireflector antennas with highaperture efficiency

It is frequently desirable to scan the main beam of a large antenna system without moving the main aperture structure. Spherical reflectors have excellent potential in this application. However, they are not commonly used because of poor aperture efficiency and high side lobes in traditional implementations. This work introduces a new dual-subreflector feed system design which does not require oversizing the spherical main reflector to accommodate scan and yet permits a controlled aperture illumination. The design yields high aperture efficiency, low cross-polarization, and low side lobes     

Imaging in a Gregorian antenna from 12 to 30 GHz

A Gregorian antenna with the main reflector illuminated by magnified image of a small horn aperture was built and tested from 12 to 30 GHz. The image is approximately frequency-independent, and the main reflector is illuminated with negligible spillover. Polarization distortion caused by aberration is very small, in excellent agreement with a simple expression derived previously by the author (ibid., March 1987). Spatial filtering by the subreflector causes the far-field sidelobes in the principal plane orthogonal to the symmetry plane to be very low, about 80 dB below the main beam at 16.5 GHz for angles from the axis that are greater than 20°     

Inflection point caustic problems and solutions for high-gaindual-shaped reflectors

The singular nature of the UTD (uniform geometrical theory of diffraction) subreflector scattered field at the vicinity of the main reflector edge (for a high-grain antenna design) is investigated. It is shown that the singularity in the UTD edge-diffracted and slope-diffracted fields is due to the reflection distance parameter approaching infinity in the transition functions. While the GO (geometrical optics) and UTD edge-diffracted fields exhibit singularities of the same order, the edge slope-diffracted field singularity is more significant and is substantial for greater subreflector edge tapers. The diffraction analysis of such a subreflector in the vicinity of the main reflector edge has been carried out efficiently and accurately by a stationary phase evaluation of the φ-integral, whereas the &thetas;-integral is carried out numerically. Computational results from UTD and PO (physical optics) analysis of a 34-m ground station dual-shaped reflector confirm the analytical formulations for both circularly symmetric and offset asymmetric subreflectors. It is concluded that the proposed PO_&thetas;GO_φ technique can be used to study the spillover or noise temperature characteristics of a high-grain reflector antenna efficiently and accurately     

Cassegrain Antenna With Hybrid Beam Steering Scheme for Mobile Satellite Communications

A hybrid antenna (HA) with a modified beam steering method is proposed. This antenna has a Cassegrain structure composed of two reflectors and a feeder. The parabolic-shaped main reflector is designed for high gain, while the subreflector is rotational and flat. The feeder is a phased array with arbitrary shaped aperture and 20 element antennas. The HA is capable of two-dimensional beam steering by means of two operations: rotation of the subreflector and phase control of the feed array. The subreflector is small in size and weight, so it can provide rapid beam scanning. Designed to be loaded in vehicles, the HA and can communicate with satellites on the move by tracking the beam control of the feed array. A prototype of the HA is fabricated with aluminum using a machining center operated by computerized numerical control. The prototype is operated at Ka-band for TX and K-band for RX with gains of 47 dBi and 44.4 dBi, respectively, at a steering angle of 0 $^{circ}$. The two-dimensional beam steering within $pm 2^{circ}$ with respect to 45$^{circ}$ elevation is realized by the subreflector and feed array. All radiation patterns in the beam steering zone meet ITU-R s.465-5 regulations.      

Analysis of near-field Cassegrain reflector: plane wave versuselement-by-element approach

A near-field Cassegrain reflector (NFCR) is an effective way to magnify a small phased array into a much larger aperture antenna for limited scan applications. Traditionally the pattern wave approach, i.e. the field from the feed array incident on the subreflector is approximated by a truncated collimated beam with planar phase and tapered amplitude distribution. This approach simplifies the computation tremendously, but fails to provide design information about the most critical component of the whole antenna system, namely, the feed array. With the help of today's computers, it is now feasible to calculate the pattern of a NFCR by a more exact element-by-element approach. Each element in the feed array is considered individually and the diffraction pattern from the subreflector is calculated by the geometrical theory of diffraction (GTD). The field contributions from all elements are superimposed at the curved main reflector surface, and a physical optics integration is performed to obtain the secondary pattern     

A transform technique for computing the radiation pattern of prime-focal and Cassegrainian reflector antennas

An efficient numerical method based on the use of the fast Fourier transform (FFT) algorithm is developed for computing radiation patterns of aperture antennas with given aperture distributions. The method is also readily applicable to the problem of computing the radiation pattern of paraboloidal reflector antennas when the induced surface currents on the surface of the reflector are known. Using an efficient launching and scanning scheme for subreflector analysis, the method is extended to a Cassegrainian reflector antenna system.     

Field correlation diffraction theory of the symmetrical cassegrainian antenna

The received field as focused by the parabolic main reflector of a Cassegrainian antenna at the surface of an arbitrary profile subreflector is calculated by a spherical wave expansion. This facilitates the application of the field correlation principle and leads to an expression for aperture efficiency taking into account diffraction effects. A comparison is made with numerical results previously published or obtained by other methods. The potential advantage of the technique is the speed of computation and the capability for synthesis as well as analysis of reflector shapes.     

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     

Focused aperture antenna using a spherical main reflector with (Cassegrain) convex feed

A focused aperture antenna has been realised using a (Cassegrain) convex subreflector with a spherical main reflector. The design was based on geometrical optics, and good agreement was obtained between the measured and calculated field in the focal region.     

Electromagnetic scattering by arbitrarily shaped reflectors: Subreflector efficiency

A general expression for the electromagnetic scattering by an arbitrarily shaped reflector is developed and applied in a computer model of offset hyperboloid reflectors. The computed scattering is shown to be in excellent agreement with scattering measurements made on an offset reflector of projected diameter = 24 cm, at frequencies of 10.35 GHz, 20.7 GHz, and 44.5 GHz. Next, using the computer model, the efficiency of the subreflector in a dual-reflector antenna is calculated as a function of subreflector diameter and for two values of illumination taper. For subreflectors truncated at the ray-optics boundary the calculated efficiency is 0.83 and 0.91, respectively, for truncation diameter of7.7 lambdaand30.4 lambda, and with 5 dB of illumination taper; these respective efficiencies increase to 0.91 and 0.95 with 12 dB of illumination taper. However, subreflectors of diameter about two wavelengths larger than the ray-optics diameter have very nearly unit efficiency.