Showing papers on "Cassegrain antenna published in 1984"

Scanning beam antenna with linear array feed

Abstract: The present invention relates to a scanning beam antenna comprising a doubly-curved main reflector, a doubly-curved subreflector and at least one linear array feed. The main reflector is doubly-curved in two planes to produce a separate first focus for each feed in a first plane in front of the subreflector and a separate second focus for each feed in a second orthogonal plane behind the subreflector. The subreflector is curved to refocus each second focus of the main reflector to coincide with the image line of a first axis on the reflecting surface of the main reflector in the first plane. Each feed array is disposed at a separate secondary focus of the combination of the main reflector and subreflector. The resulting antenna provides very little phase error due to aberrations as each beam is linearly scanned.

The current state of the reflector antenna art

Abstract: The current state of the reflector antenna art is presented in the form of a review article, intended primarily for engineers who are new in the field. The properties of paraboloidal antennas are related to aperture efficiency, surface tolerance, feed defocusing, and aperture blocking. Other reflector shapes considered are offset-fed paraboloids, classical and offset Cassegrain systems, shaped-dual reflectors, and scanning reflectors. Properties of contoured-beam systems are reviewed. Structures and materials are reviewed for ground stations and satellite-borne reflectors. Emphasis is placed on specific hardware examples for each principle or concept. Near-field probing, dichroic surfaces, and techniques of surface metrology are presented as examples of promising areas for new developments.

Meniscus-lens-corrected corrugated horn: a compact feed for a Cassegrain antenna

Abstract: The physical size of a corrugated conical horn feed having narrow beamwidth can be reduced considerably by correcting the spherical phase front in the aperture using a meniscus lens. Measured and calculated radiation characteristics of such lens-corrected scalar horns are presented. It is shown that the horns can provide efficient aperture illuminations when used as feeds for conventional Cassegrain antennas, for example radiotelescopes. Further, it is shown that the high crosspolarisation, which is normally present in lens horn designs, can be reduced to that of the horn itself by corrugating the lens surfaces with holes. Improved design formulas for such holes are given.

Scalar horn with shaped lens improves Cassegrain efficiency

Abstract: It is shown that the aperture efficiency of a classical Cassegrain antenna can be considerably improved if the radiation pattern of the feed is optimally shaped. The corresponding optimum field distribution Over the aperture of the feed consists of a circular main lobe which is surrounded by concentric sidelobe rings. This optimum field distribution with one sidelobe ring included is realized by shaping of a dielectric lens which is positioned in the aperture of a corrugated horn antenna. The design can provide a theoretical aperture efficiency of 90.5 percent when subreflector diffraction and aperture shadowing are neglected, i.e., an improvement of 0.4 dB compared to an optimum conventional feed. Measurements of a model at 22.8 GHz gave a practical result of 85.6 percent. If the model is used to feed a 30 m radiotelescope the overall antenna efficiency becomes about 71 percent.

Phase center calculations of reflector antenna feeds

Abstract: A method of determing the optimum phase center location of antenna feeds employed in reflector systems is described. The phase center of the wide-band scalar horn used in prime-focus and dual-reflector antennas has been calculated numerically and the results are discussed.

Aperture efficiency and line feed phase center of parabolic cylindrical reflector antenna

Abstract: Formulas for calculation of the aperture efficiency of a parabolic cylindrical reflector antenna fed by a line feed along the focal line are presented. The efficiency is factorized into a number of subefficiencies which include contributions from line feed end diffraction, and from blockage and reflections from line feed supports and from diffraction by gaps in the reflector surface. One of the subefficiencies is used to define a phase center for the line feed as well as to obtain a formula for calculating it.

Dual gridded reflector structure

Abstract: A dual gridded antenna reflector system and method for constructing the same is disclosed. The reflector system comprises a pair of reflector dishes each having a grid of parallel conductors. One of the reflector dishes is mounted over the other reflector dish by linear support ribs therebetween such that the conductors of the one reflector dish are perpendicular to the conductor of the other reflector dish. The linear support ribs are placed perpendicular to or parallel to the conductors of the overlapped reflector and are placed substantially outside of the high field region across the aperture of the overlapped reflector.

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

Abstract: 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.

Bicollimated Gregorian reflector antenna

Abstract: A bicollimated Gregorian reflector is structurally similar to a classical confocal Gregorian reflector, but its surfaces are shaped to have better scan capability. A geometrical optics procedure is used in designing the reflector surfaces. A three-dimensional ray tracing procedure is used in analyzing the aperture phase errors as the beam is scanned to different angles. The results show that the bicollimated configuration has about 45 percent greater angular scanning range than the equivalent confocal Gregorian reflector antenna.

Diffraction corrections to the cylindrical wave radiated by a linear array feed of a cylindrical reflector antenna

Abstract: A phase-steered linear array feed for a parabolic cylindrical reflector antenna is considered. The nearly cylindrical wave radiated from this line feed in the Fresnel zone is expressed in terms of the isolated-element pattern. The correction to this wave due to diffraction from the endpoints of the line feed and from the grating formed by the array elements is also derived.

Synthesis of offset dual shaped subreflector antennas for control of Cassegrain aperture distributions

Abstract: 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.

Aperture blockage in reflector antennas

Abstract: 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.

Reflector antenna having sidelobe suppression elements

Abstract: Suppression of selected sidelobes in a reflector antenna is accomplished by the positioning of two or more metallic disks on the reflector surface at specific distances above the reflector surface.

A new method of analysis of the near and far fields of paraboloidal reflectors

Abstract: An analytical technique for predicting accurately the near (electric and magnetic) fields as well as the far fields of a reflector antenna with a pencil beam is presented. The technique proposed involves the near-field geometrical theory of diffraction (GTD) analysis of reflector antennas developed earlier and spherical vector mode functions. The proposed technique does not place any restriction on the range of polar angles or radial distances of the observation point. It is demonstrated that the technique proposed can predict the fields radiated by the reflector with greater accuracy by comparing the calculated results with the available measured results. A few important applications of the analysis proposed are also highlighted.

A beam-steerable antenna with an offset spherical reflector for earth station

Abstract: This paper proposes a newly devised beamsteerable antenna which may suggest a future large scale earth station antenna for satellite communications. This is an offset type spherical reflector antenna with two specific subreflectors and a trireflector beam-waveguide feeder. It can steer its radiation beam without moving the main reflector and the feed horn. An experimental model antenna of 2 x 3 m at 52/34 GHz corresponding to a 20 m class antenna at 614 GHz has demonstrated its excellent electrical performance over wide scanning angles.

Performance of a rectangular-waveguide-excited short backfire antenna employing a conical main reflector

Abstract: The letter presents experimental results at X-band for a short backfire antenna employing a conical reflector, which is excited by a rectangular waveguide. It is shown that the use of a conical main reflector can lead to a compact antenna having a directive gain of 16.7?18.3 dB, sidelobe level below 13.8 dB and input VSWR better than 2.5 over a 20% bandwidth from 9?11 GHz.

Optimum location of the wobble axis of secondary mirrors in cassegrain-type telescopes

Abstract: The best position for the rotation axis of the wobbling secondary mirror in a Cassegrain type of antenna is through the focal point of the main mirror: the prime focus. Compared to the usual situation, where the axis of rotation contains the vertex of the secondary mirror, this means a considerable improvement of beam quality, especially at large wobble angles.

Scanning dual reflector antenna

Abstract: An antenna system, on a spacecraft for example, comprises a movable reflector dish so that the antenna pointing direction can be varied or scanned and a fixed feed. To reduce separation of the feed from the focal point of the dish during such scanning, the feed is positioned at the point about which the dish is movable and is directed towards a convex auxiliary dish fixed to and movable with the main dish and producing a virtual image of the feed at the focal point of the main dish.

Parabolic dual reflector antenna with offset feed

Abstract: A dual polarized mechanical scan antenna capable of rapid beam scanning includes a parabolic reflector having an elliptically shaped solid metal central section with conductive parallel grids extending therefrom to the perimeter of the paraboloid to form two outer grided sections. The axis of the parabolic reflector and the radiation axis of the feed antenna, positioned at the focus in alignment therewith, is positioned at a predetermined angle from a reference axis of the system. A polarization twist reflector is positioned at the focus for pivoting about the perpendicular to the reference axis. Dual polarization is realized when the twist plate is positioned such that the antenna system receives signals along paths that clear the parabolic reflector.

A study of the diffraction and blockage effects on the efficiency of the Cassegrain antenna

Abstract: The diffraction and blockage efficiencies of the conventional Cassegrain antenna are calculated by using the uniform geometrical theory of diffraction and physical optics. The various subefficiency terms corresponding to different diffraction fields are tabulated for a typical antenna and the results are discussed. Design curves that provide the aperture efficiency value as a function of main reflector diameter, subreflector diameter and the feed illumination taper are presented.

Comments on "Synthesis of offset dual shaped subreflector antennas for control of Cassegrain aperture distributions"

Abstract: In the paper by V. Galindo-Israel and R. Mittra it is claimed that a dual shaped subreflector (DSS) feed can be used to upgrade the efficiency of classical Cassegrain antennas. In this comment it is shown that upgrading is not possible unless the diameter of the DSS feed is extremely large. In fact, if the Cassegrain antenna at Goldstone, CA, used as an example in the above paper, is fed by the DSS feed, the efficiency in X -band will be about the same as or less than if it is fed directly by an optimized scalar horn. The low efficiency is caused by diffraction. Instead, the efficiency can be upgraded by using a scalar horn with shaped lens.

A variable power dual mode network for reconfigurable shaped beam antenna

Abstract: A three output port variable power dual mode network is described. This network can be useful in driving a three element feed array feed of a communication satellite antenna to provide a dual mode reconfigurable shaped beam pattern.

Antenna with pseudo-toric coverage having two reflectors

Abstract: An antenna with pseudo-toric cover for transmitting and/or receiving a microwave, comprising principally an array transmitting a microwave towards a first reflector in the form of an elliptic or parabolic skull cap, whose concave side is turned towards the array and which reflects the energy towards a second reflector in the form of a concave ring, whose center is occupied by the array and whose concavity is turned towards said first reflector. The reflector is preferably supported by a radome.

Microstrip Array for Reflector Feed Applications

Abstract: The performance of a small microstrip array for use as a reflector feed is reported. Patch arrays incorporating both coplanar and reduced height electro magnetically coupled feed networks have been investigated. Radiation from the feed network is found to be significant and results in some copolar pattern distortion and cross polarisation of about -20dB and -28dB for the two feed types respectively. Cross polarisation of -28dB is observed from a reflector with the coplanar feed. Array analysis based on a magnetic current model and including feed network radiation and diffraction effects is presented and shows good agreement with measurements.

Beam Forming Network for a Multibeam Antenna at 20 GHz

Abstract: A multibeam antenna, which generates 115 Area Beams for European Continuous Coverage and 12 City Beams to cover 12 main ground stations, has been defined. Area and City Beams are generated by the same array of feeds and are isolated through polarization discrimination. On both, frequency reuse, through spatial separation, is applied. Furthermore, the central ground station provides a 27 GHz beacon signal for RF Sensing. For the isolation objectives (30dB) achievement, each City Beam is generated by a cluster of seven feed elements, part of which is shared between two adjacent beams. The feeding of the cluster and the sharing are performed by a Beam Forming Network (B.F.N.), in rectangular waveguide. The paper describes the B.F.N. components and their computed electrical performance, compared with the results measured on a partial B.F.N. breadboard. A good agreement and a flat behaviour of the B.F.N., in more of 10% bandwidth, is shown.

Forward radiation from axially symmetric reflector antennas

Abstract: The fields radiated from focus-fed symmetric reflector antennas in the vicinity of forward axial direction are determined by a series representation of the physical optics integral. The considered feeds have cosine-tapered patterns with different tapers in E - and H -planes. The influence of feed pattern asymmetry, subreflector blockage, feed taper, focal distance to diameter ( f/D ) ratio and the reflector size on the performance parameters of paraboloidal, Cassegrain, near-field Cassegrain, and corresponding Gregorian antennas is investigated. Design curves are presented to predict the performance parameters of the considered antennas.

Preliminary study on offset scan-corrected reflector antenna system

Abstract: 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 than \pm 5 beamwidths, 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.

Systematic design of a dual offset reflector antenna with an elliptical aperture

Abstract: The systematic design of an optically shaped dual offset reflector antenna, capable of illuminating a given elliptical aperture over which the phase is uniform and the power is arbitrarily prescribed when fed by a scalar horn, is described in the paper. A conformal mapping transforms the ellipse to a circle, allowing the present problem to be solved by an existing method. Designs based upon Cassegrainian and Gregorian systems are derived.

Solar powered focusing and directing apparatus for cutting, shaping, and polishing

Abstract: A solar powered cutting, shaping, and polishing device is disclosed. In the preferred embodiment a large concave parabolic reflector is mounted on an X and Y axis rotatable (telescope type) mount and directed towards the sun. A smaller convex parabolic reflector is mounted on an adjustable support structure attached to the large concave reflector such that the two reflectors can be made to share, or nearly share, the same focal point. Mounted between the large and small parabolic reflectors on an additional rotatable mount is a flat reflector. The large and small parabolic reflectors are used to concentrate solar energy to a desired and adjustable focal point. The flat reflector can then be used to direct the concentrated energy across a material in an operator-determined pattern. Depending on certain factors associated with the material and the positioning of the focal point, the material can then be cut, shaped, or polished as the user desires. Also disclosed is the use of lenses to perform the concentrating function.

Power focusing characteristics of ellipsoidal reflector

Abstract: Power focusing characteristics of a prolate spheroidal reflector, the aperture of which is parallel to the line connecting the two focuses, are numerically studied. As a transmitting and a receiving antenna, linear wires with finite length are placed at or near the focuses of the reflector. Reflected near fields from the reflector are calculated by the physical optics technique, and transmitting and receiving antenna characteristics are obtained by the method of moments. Calculated results are given for the power focusing characteristics as a function of the eccentricity, the aperture size of the reflector, antenna directions, antenna lengths, and the receiving antenna displacement. Better power focusing characteristics are obtained when the transmitting antenna becomes less directive, or the size of the aperture becomes larger. Measured and calculated results agree very well.