Beam efficiency limitations of large antennas

The beam efficiency of an antenna may be defined as the ratio of the power radiated within the main beam to the total power radiated. The beam efficiency is derived for ideal rectangular and circular apertures, as a function of the edge-to-center amplitude ratio. Random phase errors are assumed to exist across the aperture. Various types of feeds for parabolic reflectors are also considered in relation to the fraction of power which the feed directs into the paraboloid. A primary limitation on the beam efficiency of a paraboloid is shown to be produced by the surface roughness.     

Beam squint in axially symmetric reflector antennas with laterallydisplaced feeds

Numerical calculations have been carried out and the beam squint for circular polarized excitation has been measured using a 100 m telescope. The telescope was operated in the Gregorian mode, where the equivalent focal length equals 387.5 m. The feed horn was laterally displaced by 1.364 m from the optical axis at the system focus. Good agreement was obtained between the numerical calculations and the experimental results. The authors found a shift of the two radiation patterns of ≃2 arcsec. The orientation of the beam squint in the configuration with a laterally displaced feed is different from the orientation in offset reflector antennas     

Beam squint determination in conic-section reflector antennas withcircularly polarized feeds

The beam squint phenomenon in circular-polarized-feed paraboloidal, Cassegrain, and Gregorian antennas is characterized. The geometrical optics/aperture field method is used to derive generalized squint-predicting formulas, the accuracy of which is verified by computer simulations based on diffraction analysis. From these formulas, the squint-free conditions are developed. An extension of these formulas is suggested to predict the squint angles for general multireflector antennas     

A simple formula of current in dipole antennas

A simple and quantitatively accurate representation of the current distribution in a dipole antenna is derived. Numerical data are given and are found to be in good agreement with the experiment whenh geq O.15lambda.     

Rim loaded reflector antennas

A general theory of reflector antennas loaded by surface impedances is presented. Spatial variation of primary illumination is taken into account using a generalized slope diffraction coefficient. The theory is experimentally checked on surface loaded square plate scatterers and then used for computing the radiation diagram of parabolic and hyperbolic dishes. Computer programs and computed diagrams refer to the case of focal illumination and negligible tapering of primary illumination.     

Offset parabolic reflector antennas

The paper provides a tutorial review of a number of offset parabolic reflector configurations including both single and double-reflector geometries. The author commences by describing some basic techniques which can be applied to predict the vector radiation fields and provides some indication of the validity of these methods. The formulation of a relatively simple analytical model for the offset reflector antenna is described based upon the physical-optics approximation. The electrical performance of the single-offset reflector is examined by comparison of predicted and measured data. The particular problems arising from the choice of polarisation and reflector dimensions are highlighted, and some practical applications involving multiplebeams, shaped and contoured beams, monopulse tracking and low sidelobes are briefly reviewed. Practical primary-feeds for offset-reflector antennas are discussed and the matched-feed concept is outlined, the matching of the electric fields in the primary-feed aperture to the reflector focal fields being illustrated. The advantages and disadvantages of dual-reflector antennas are then examined, with particular emphasis upon the open Cassegrainian configuration and the optimised doubleoffset configuration which offers, in principle, both freedom from blockage and low levels of cross-polarised radiation.     

Astigmatism in reflector antennas

The characteristics of the astigmatic phase error in large parabolic reflector antennas are described. A procedure for focusing an antenna and diagnosing the presence and degree of astigmatism is given.     

Diffraction-optimised shaped-beam reflector antennas

An iterative method is presented to find the reflector profile which minimises the mean-square error between a scattered and a desired field, thereby achieving pattern synthesis. This technique is applied to 3-dimensional circulary symmetric systems for either field- or power-pattern synthesis. Theoretical and experimental results are given for the problem of synthesising a uniform-amplitude sector beam between 20° and 66°.     

ICARA: induced-current analysis of reflector antennas

The aim of this work is to present the ICARA (induced-current analysis of reflector antennas) software, which is able to predict the behavior of reflector antennas using the physical optics method. The software offers different options for antenna configurations, single and array feed models, and far-field or aperture-field analysis.     

Coaxial feeds for high aperture efficiency and low spillover of paraboloidal reflector antennas

Coaxial feeds produce an approximate sector-shaped pattern, an almost optimum pattern of a feed for high aperture efficiency and low spillover of paraboloid antennas. Such a coaxial feed consists of a central circular waveguide which is surrounded by one or more conductors with circular cross sections. Theoretical and experimental investigations on coaxial feeds excited by H11modes have shown that the first ring yields the highest increase in the aperture efficiency of paraboloid antennas illuminated by them. Measurements performed on paraboloid antennas illuminated by a coaxial feed with only one ring yielded aperture efficiencies of 68 to 75 percent for angular apertures of the paraboloidal reflector of100degto160deg. Circularly symmetric patterns in conjunction with almost linearly polarized aperture fields can be achieved by multimode coaxial feeds. The values for the aperture efficiency, which are calculated for paraboloid antennas illuminated by multimode coaxial feeds, nearly reach the theoretical optimum. The measured values are 68 to 80 percent. In addition, the multimode feeds produce very little cross polarization.     

Double spherical cassegrain reflector antennas

There is a need for high gain antennas that are capable of rapid scanning and multibeam operations. This paper presents a study of the double-spherical Cassegrain reflector to ascertain whether this antenna is practical for these purposes. Aperture blocking, spread of the field, and phase and amplitude distributions are examined using geometric optical techniques. Some radiation patterns are calculated to show the effects of illumination taper, aperture blocking, and phase errors.     

Aperture blockage in reflector antennas

The aperture blockage effects on both the copolarized and the cross polarized components of the front-hemisphere pattern of a typical quadrupod supported primary-feed paraboloidal reflector antenna are evaluated. A detailed numerical model of the scattering process which involves the feed, the feed supporting struts, and the reflector itself is employed to compute the blockage effect.     

Polarization losses in reflector antennas

Various definitions for polarization-loss efficiency of Cassegrainian and front-fed reflectors are compared. The effects of flare angle, feed taper and the feed pattern asymmetry on the polarization-loss efficiency are investigated. The definitions based on aperture fields are shown to be inadequate and far fields must be used for calculating the polarization losses.     

Depolarization properties of offset reflector antennas

The cross polarized radiation for linearly polarized excitation and the beam displacement for circularly polarized excitation have been investigated for offset reflector antennas. Numerical calculations are given to illustrate the dependence upon the angletheta_{0}between the feed axis and the reflector axis as well as upon the half-angletheta_{c}subtended at the focus by the reflector. In the casetheta_{0} = theta_{c} = 45deg, measured results have been obtained for both linearly and circularly polarized excitations with a dual mode feed illuminating an offset paraboloid. The cross polarized radiation of horn reflector and open Cassegrainian antennas rises sharply to rather high values off the beam axis; however, in general, the maximum cross polarized radiation of offset reflector antennas can be made small by using a small angle between the feed and reflector axes. The cross polarization caused by offset is compared with that caused by an unbalanced feed pattern. The effect of the longitudinal current distribution and of departure of the surface from a paraboloid on cross polarization are also examined. The clarification of these cross polarization properties is found to be valuable in the design of reflector antennas.     

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     

Astigmatism in reflector antennas: measurement and correction

Investigates astigmatism is Cassegrain antennas from cross scans through in-and-out-of-focus diffraction images of extended radio sources. The use of extended sources is of interest for large (mm-wavelength) telescopes where the bright planets subtend a substantial fraction of the beam. The experimental and theoretical results presented refer to the IRAM 30-m telescope and measurements at 7.3 mm-1.3 mm wavelength; however, extrapolation to other telescopes is evident. The authors report on an experiment to correct the astigmatism (phase) at a position close to the focal plane of the telescope, i.e. the flat surface of the corotating Nasmyth mirror     

Analysis of the radiation patterns of reflector antennas

The development and application of a numerical technique for the rapid calculation of the far-field radiation patterns of a reflector antenna from either a measured or computed feed pattern are reported. The reflector is defined by the intersection of a cone with any surface of revolution or an offset sector of any surface of revolution. The feed is assumed to be linearly polarized and can have an arbitrary location. Both the copolarized and the cross polarized reflector radiation patterns are computed. Calculations using the technique compare closely with measured radiation patterns of a waveguide-fed offset parabolic reflector. The unique features of this technique are the freedom from restrictive feed assumptions and the numerical methods used in preparing the aperture plane electric field data for integration.     

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.