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.      

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     

Computer-aided design of reflector antennas: the Green Bank RadioTelescope

This paper presents an evaluation of the electrical performance of the Green Bank Telescope (GBT) reflector antenna, operating as single- and dual-offset configurations, as well as a general overview of the GBT system. The GBT dual-offset Gregorian configuration is designed for low cross polarization (XPOL) using the dual-offset reflector antenna (DORA) synthesis package code developed by the authors. The procedure implemented in DORA to upgrade an existing main reflector to a low cross-polarized dual-offset Gregorian reflector antenna is also described in this paper. All computed patterns were obtained with the parabolic reflector analysis code (PRAC) program, also developed by the authors, and with the commercial code GRASP7. The GBT radiation patterns and performance values, which include original data not available anywhere else as far as the authors know, indicate that low XPOL performance can be achieved with a dual-offset configuration, provided that a low XPOL feed is used. The GBT configuration is employed as a case example for the aforementioned procedure. However, an effort is made to present the main conclusions as generically as possible     

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.     

A design procedure for classical offset dual reflector antennaswith circular apertures

A geometrical optics procedure for designing electrically optimized classical offset dual reflector antennas with circular apertures is presented. Equations are derived that allow the size and spacing of the main and subreflectors of the antenna system, along with the feed horn subintended angle, to be used as input variables of the design procedure. The procedure, together with these equations, yields an optimized design, starting from general system requirements. The procedure is demonstrated by designing both an offset Cassegrain and an offset Gregorian antenna, and is validated by analyzing their radiation patterns using physical optics surface current integration on both the main and subreflectors     

Designing classical Dragonian offset dual-reflector antennas fromcombinations of prescribed geometric parameters

This paper proposes a simple procedure for the design of classical offset-Dragonian dual-reflector antennas from combinations of prescribed geometric parameters. This procedure has already been applied to classical Cassegrain and Gregorian antennas, and to classical displaced-axis Cassegrain and Gregorian antennas. We provide a list of 20 parameters from which the antenna system is fully characterized, but only five of these parameters need to be provided by the antenna designer, as the remaining 15 parameters can be derived in closed-form using the procedure described. We consider that the main reflector (MR) has a circular aperture, while the subreflector (SR) has an elliptical aperture     

An offset-fed reflector antenna with an axially symmetric main reflector

A design method for an offset-fed, dual reflector antenna (Cassegrain type or Gregorian type) system with an axisymmetric main reflector is presented. Geometrical optics (GO) and the geometrical theory of diffraction (GTD) are used to find the surface-current density on the main reflector. A modified Jacobi-Bessel series (JBS) method is used to find the far-field pattern for the physical optics (PO) integral. In the defocused mode of operation, a new technique is developed to find the reflection point on the subreflector corresponding to the defocused feed and a general field point on the main reflector. Two sample systems are designed.     

Focal region fields of dual reflector antennas

A simplified analytical method to predict the field distribution in the focal region of dual reflector antennas having circular symmetry is presented. The method is used to study the field distribution at the subdish and in the focal plane, and hence the ability of the antenna in focusing the incident wave. Experimental results obtained on a 2.6-m spherical Gregorian reflector operating at 9.0 GHz agree well with theoretical ones.     

Leaky-Wave Slot Array Antenna Fed by a Dual Reflector System

A leaky-wave slot array antenna fed by a dual offset Gregorian reflector system is realized by pins in a parallel plate waveguide. The radiating part of the antenna is composed by parallel slots etched on one side of the same parallel plate waveguide. The dual offset Gregorian reflector system is fed by an arrangement constituted by two vias and a grid, also constituted by pins. Also this feed arrangement realizes a leaky type of radiation, this time inside the parallel plate waveguide. A prototype of the antenna has been designed, manufactured and successfully tested. The low profile, low cost and high efficiency of the antenna render it suited for a variety of radar or telecom applications.     

A dual chamber Gregorian subreflector system for compact rangeapplications

A dual-chamber compact range configuration is proposed wherein the main reflector and target zone are located in the main chamber and an oversized Gregorian subreflector and associated feed assemblies in the other. The chambers are isolated by an absorber fence except for a small coupling aperture which is used to transmit signals between them. The absorber fence prevents diffraction by the subreflector and spillover by the feed from illuminating the main reflector and target zone. System performance is analyzed with and without the absorber fence to show how the coupling aperture should be shaped to minimize diffractions     

A Gregorian corrector for spherical reflectors

The inherent spherical aberration of a spherical reflector antenna is corrected by using an auxiliary Gregorian reflector feed system that rotates about the center of curvature of the reflector. Tests at bothX- andK-band frequencies demonstrate feasibility of the design for wide-angle scanning.     

Surface tolerance loss for dual-reflector antennas

A formula is derived to evaluate the relative influence of subreflector and main reflector surface errors on overall tolerance loss for a dual-reflector antenna. Comparisons are made with the results of random-number simulation of surface errors in a Gregorian system.     

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     

Shaped Circularly Symmetric Dual Reflector Antennas by Combining Local Conventional Dual Reflector Systems

This paper presents a geometrical optics (GO) shaping method for circularly symmetric dual reflector antennas. The method is a one step procedure obtaining a local dual reflector system including both reflector surfaces and caustic simultaneously, such that a given feed power is transformed through a corresponding local reflector system into a desired aperture distribution. A shaped reflector system consists of an electrically small section of distinctive local reflector surfaces and its own caustic. Each caustic location varies and explains how the power is redistributed through the shaped reflector system. Each local reflector system is found iteratively with a previously known local dual reflector system. Several nonlinear algebraic equations are formulated based on GO principals and geometrical properties of a dual reflector system. The method reduces the solution to one simple non-differential equation with one unknown in order to find a local dual reflector system. Physical Optics (PO) analysis is used to verify the solution.      

Texas 5-m antenna aperture efficiency doubled from 230-300 GHz witherror compensating secondary

A study to upgrade the high-frequency performance of the University of Texas 5-m millimeter-wave reflector antenna established surface tolerance of the reflector as the limiting factor. The prime focus antenna was converted to a folded Gregorian geometry. The resulting trireflector system was measured holographically at 113 GHz. A machined secondary reflector was fabricated on a highly accurate computer-controlled milling machine. The inverse of the measured surface perturbations of the primary was machined into the secondary reflector. The modification of ray path lengths effectively reduced the surface tolerance of the antenna. Radiometric measurements using a remote transmitter and planets as sources demonstrated an increase in antenna aperture efficiency by more than a factor of two over the frequency range of 230-300 GHz     

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°     

Ray tracing method for doubly curved reflector surfaces

Shaped reflector surfaces designed for microwave antennas are often defined by a regular grid of discrete points. A ray tracing procedure useful for computing aperture and power distributions produced by an arbitrarily shaped reflector surface is described.     

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。     

Synthesis of offset dual reflector antennas transforming a given feed illumination pattern into a specified aperture distribution

The problem of transforming a given primary feed pattern into a desired aperture field distribution through two reflections by an offset dual reflector system is investigated using the concepts of geometrical optics. A numerically rigorous solution for the reflector surfaces is developed which realizes an exact aperture phase distribution and an aperture amplitude distribution that is accurate to within an arbitrarily small numerical tolerance. However, this procedure does not always yield a smooth solution, i.e., the reflector surfaces thus realized may not be continuous or their slopes may vary too rapidly. In the event of nonexistence of a numerically rigorous smooth solution, an approximate solution that enforces the smoothness of the reflector surfaces can be obtained. In the approximate solution, only the requirement for the aperture amplitude distribution is relaxed, and the condition on the aperture phase distribution is continued to be satisfied exactly.