Showing papers on "Cassegrain antenna published in 1991"

FLAPS: conformal phased reflecting surfaces

Abstract: A technology applicable to the reflector antenna field is introduced. FLAPS (flat parabolic surface) allows the use of low-cost fabrication techniques to perform various collimating reflector functions, independently of any specific geometric shape, such as would be required with conventional reflectors. The various elements of control afforded by FLAPS technology allow versatile operation in a variety of simultaneous applications not normally compatible with conventional reflector designs. >

Beam diffraction by planar and parabolic reflectors

Abstract: In the complex source point technique, an omnidirectional source diffraction solution becomes that for a directive beam when the coordinates of the source position are given appropriate complex values. This is applied to include feed directivity in reflector edge diffraction. Solutions and numerical examples for planar strip and parabolic cylinder reflectors are given, including an offset parabolic reflector. The main beams of parabolic reflectors are calculated by aperture integration and the edge diffracted fields by uniform diffraction theory. In both cases, a complex source point feed in the near or far field of the reflector may be used in the pattern calculation, with improvements in accuracy in the lateral and spillover pattern lobes. >

Dual reflector scanning antenna system

Abstract: A fixed feed dual reflector scanning antenna system 10 having a low moment of inertia is disclosed herein. The inventive dual reflector antenna system 10 includes an antenna feed structure 16 for emitting electromagnetic radiation. The antenna system 10 further includes a subreflector 12 for redirecting the emitted radiation. The subreflector 12 is intersected by a subreflector longitudinal axis Ls at a rotation point proximate a vertex 20 of the subreflector 12. A main antenna reflector 14 circumscribing a main longitudinal axis Lm projects radiation redirected by the subreflector 12 as an antenna beam. A mechanical arrangement 22 rotates the subreflector 12 about the rotation point so as to vary the angular orientation between the subreflector longitudinal axis Ls and the main longitudinal axis Lm. In this manner the antenna beam is scanned relative to the main longitudinal axis Lm.

Array fed reflector antenna for transmitting & receiving multiple beams

Abstract: An array fed reflector antenna includes a reflector and a distributed feed array. The reflector has a portion with a dual parabolic shape. The distributed feed array transmits and receives a plurality of electromagnetic energy beams simultaneously, and is positioned adjacent the reflector so that the reflector reflects the transmitted and received electromagnetic energy beams. The distributed feed array is offset from the reflector so that a plane wave formed by the transmitted electromagnetic energy beams reflected by the reflector will not substantially impinge the distributed feed array. The antenna also has a beam switching network which is a hybrid network for selectively actuating separate but overlapping portions of the distributed feed array to produce two transmit elevation beams.

Earth station antenna system

Abstract: A small aperture satellite ground station communications antenna is disclosed. A rectangular section of a curved parabolic reflector is provided having first and second rectangular dimensions. An azimuth elevation pedestal for positioning the reflector is provided such that the antenna bore sight is aligned with a geostationary satellite. The reflector is supported by the pedestal to permit rotation of the reflector about a third polarization axis for the antenna. The rotation along the polarization axis permits the reflector to be optimally positioned such that its long dimension is aligned with an orbital arc of a geostationary satellite. Angle scales are provided on all three axes to facilitate repositioning to other satellites based on an initial satellite location, which serves as a reference for the scales.

Lens-Type Compact Antenna Test Range at MM-Waves

Abstract: A lens-type compact antenna test range (CATR) has been studied both theoretically and experimentally. For a lens with low dielectric constant ?r, the surface accuracy requirement is much less stringent than for a reflector. An experimental CATR with a shaped polyethylene (?r, = 2.32) lens was made at 110 GHz. In order to make the amplitude ripple in the quiet zone small, the use of saw tooth serrations around the lens was studied. Theoretically the quiet zone field is exellent (peak to peak ripple 0.27 dB). Preliminary measured values are promising (peak to peak ripple 2.0 dB).

The design and performance of a reconfigurable mesh reflector antenna

Abstract: The authors present aspects of the design and performance of a reconfigurable mesh reflector antenna, and the first demonstration of electrically controlled reconfiguration is illustrated. These aspects include the influence of boundary constraints on performance when compared to a rigid shaped reflector and the repeatability of practical configurations. The first topic is explored theoretically, while the second involves an experimental study made using an 85 cm diameter mesh reflector antenna.< >

Mode matching analysis of arrays of stepped rectangular horns and application to satellite antenna design

Abstract: Satellite antennas commonly use reflectors with array feeds. In such cases the efficient illumination of the reflector is crucial in the design of the arrays. Methods in use to improve the illumination efficiency of the array include partially loading the horns with dielectric, or employing multi-mode horns. The author describes the analysis of an array of multimode stepped rectangular horns, including the effect of mutual coupling, using the mode matching method. It has already been used in a study of the Hughes Aircraft Company's HS601C satellite Ku-band antennas and some results obtained are outlined herein.< >

Application of rectangular and elliptical dielcore feed horns to elliptical reflector antennas

Abstract: The pattern characteristics of elliptical reflector antennas are investigated when they are fed by rectangular and elliptical horns partially filled with a dielectric. The bandwidth characteristics of these dielcore horns are superior to those of their corrugated horn counterparts. Representative reflector patterns are computed to properly demonstrate the utility of these feeds for reflector antennas with elliptical apertures. This reflector antenna exhibits high efficiency and low cross polarization, and may be suitable for radar and satellite antenna applications. The antenna configuration may become useful in relatively small antennas where more than 10% cross-polar bandwidth is required. The efficient dielcore horns may also be used as feeds for elliptical nonshaped dual-reflector antennas. >

Design of high efficiency Fresnel zone plate antennas

Abstract: With a family of ideal radiation patterns, the theory for designing high-efficiency phase-correcting FZP (Fresnel zone plate) antennas is presented. The optimum 3-dB beamwidth of the feed pattern as a function of F/D (focal length/diameter) of the plate, the relation between the phase efficiency of the antenna and the number of subzones used in each full-wave zone, and the total efficiency limits of various versions are given. It is found that the maximum efficiency of a practical FZP antenna is usually less than 70%. The theory has been employed in designing a four-layer experimental FZP reflector antenna, and good agreement between the theoretical expectation and the measured value is obtained. >

A dual beam slotted waveguide array antenna for SAR applications

Abstract: The authors describe the design process and test results of a dual beam array antenna model. The objective of this ESA funded program is to study the realization of dual beams in a waveguide slotted array of the ERS-1 SAR type (10 m*1 m). The experimental model is 1 m*1 m and comprises 24 slotted waveguides and a Blass matrix feed network. The synthesis and design of the array and Blass matrix is described. Calculated and measured results of the experimental model are presented. The key component in the Blass matrix is a crossguide coupler. The authors present a novel design of a crossguide coupler which makes it possible to realize very tight coupling. >

Comparison of spillover loss of offset Gregorian and Cassegrain antennas

Abstract: The author discusses comparison of Gregorian and Cassegrain antennas with respect to spillover loss and selection of subreflector optics. It was shown that the Cassegrain antenna with the bottom arm has spillover noise lower by at least 5 K at elevation angles 30 degrees and lower as compared to the Gregorian antenna with the arm at the top. Also, from the viewpoint of maintenance, accessibility to the receivers, the feeds, and the subreflector is easy with the arm at the bottom, with the telescope stowed at low elevation angles. However, the National Radio Astronomy Observatory has chosen the Gregorian antenna for the following reasons: first, with the arm at the top, the elevation bearings of the telescope need to be located at a lower height from the ground and, hence, the elevation tower is shorter, saving $3 M to $4 M in steel. Secondly, with the Gregorian antenna, the subreflector can stay installed permanently on the arm, even when prime focus feeds are used. The excess spillover noise of the Gregorian antenna can be reduced by about 2 K by resorting to a feed taper of -15 dB. >

Evolution of the large Deep Space Network antennas

Abstract: The evolution of the largest antenna of the US NASA Deep Space Network (DSN) is described. The design, performance analysis, and measurement techniques, beginning with its initial 64-m operation at S-band (2295 MHz) in 1966 and continuing through the present Ka-band (32-GHz) operation at 70 m, is described. Although their diameters and mountings differ, these parabolic antennas all employ a Cassegrainian feed system, and each antenna dish surface is constructed of precision-shaped perforated-aluminum panels that are secured to an open steel framework. >

Minimisation of reflection coefficient at feed of radome-covered reflector antenna by chiral device

Abstract: A method is proposed which consists of placing a chiral device in the central region of a radome covering a reflector antenna, to minimise the reflection coefficient at the antenna feed and any possible distortion in the antenna radiation pattern due to the device.

The effects of dry snow on reflector antennas

Abstract: Fading due to snow accretion on the reflector antenna is a major problem in satellite communication in Finland and other northern countries. Dry snow may stay on antennas for many days or even weeks, if the temperature is below 0 degrees C, making fading events longer. Dielectric losses of dry snow are very small and can be normally discounted, but asymmetrically gathered dry snow tilts and distorts the antenna beam due to the different phase delays of the different wave paths. Theoretically, the worst case would be a dry snow layer on the lower half of the reflector causing a phase delay of 180 degrees , when the signal entirely disappears from line of sight. The author expands research in this neglected area. The measured radiation patterns of a 1.8 m reflector antenna are presented for three cases of a parabolic reflector partially covered by dry snow. The measurements were made at 20 GHz in one case and simultaneously at 20 and 30 GHz in two cases.

Designs for ATDRSS tri-band reflector antenna

Abstract: An approach to designing a triband reflector antenna is described. In this approach, three separate feeds are used. This allows each feed to be optimized for a single band. The feeds are isolated by means of frequency-selective surfaces (FSS). The FSS will transmit certain frequencies while reflecting others. Conventionally, most FSS are designed to be used in two frequency bands. The challenge is to design an FSS that works for three bands (e.g., transmitting two bands and reflecting a third). Two reflector antenna configurations are presented, an offset single reflector and a symmetric-shaped dual reflector. The offset fed single reflector antenna configuration has been selected for integration into proposed ATDRSS (Advanced Tracking and Data Relay Satellite System) designs. CADAM drawings have been completed for the design concept and a 1/13th scale model has been constructed, using this offset-fed antenna configuration. This design is shown to meet the original ATDRSS single-access antenna pointing requirements. >

Determination of array feed excitation to improve performance of distorted or scanned reflector antennas

Abstract: The performance degradation of distorted and/or scanned reflector antennas is manifested primarily in a decrease in peak gain and in an increase in radiation pattern sidelobes. The author discusses a procedure for determination of the excitation coefficients of the array feed in order to approximate the 'ideal' performance for both gain and sidelobes. Representative results of 6 GHz are presented for a hexagonal array of 1.5 lambda diameter dual-mode horns with center-to-center spacing of 1.5 lambda , to compensate for the surface distortion of one offset aperture of a mesh deployable reflector antenna. >

Combined Radar/ESM antenna system and method

Abstract: A combined antenna system mounted in a nose randome of an aircraft for both radar and ESM signals. A flat plate waveguide antenna, a twist panel, a selective reflector, and a feed are aligned along a longitudinal axis in the randome. The polarized electromagnetic energy is twisted 45 degrees; and the selective reflector passes the twisted electromagnetic energy and reflects energy polarized in planes substantially different than the twisted plane.

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

Abstract: 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. >

Low radar cross section reflector antenna

Abstract: An antenna system includes a reflector grid which reflects signals which are within the operative frequency range of the antenna and passes received signals of higher frequency. An absorber is placed proximate to the reflector grid to absorb signals which are passed by the grid. A second reflector located proximate to the absorber reflects signals back through the absorber such that signals reflected by the antenna system are twice attenuated by the absorber.

Fine pointing system for reflector type antennas

Abstract: A pointing system for reflector type antennas providing a high degree of accuracy with low loss over wide scan areas, including a fixed illuminator retained in fixed position at the focal point of the reflector of the antenna. Surrounding the illuminator is a Cardanic joint which acts as a spherical hinge capable of radial rotation around the illuminator. Attached to the Cardanic joint is a support arm which is connected to the reflector. The reflector is therefore freely movable in a spherical path defined around the illuminator with the illuminator coinciding with the center of the sphere. The illuminator is kept within the focal point of the reflector regardless of the position of the reflector as it spherically rotates around the illuminator. Tension springs in the Cardanic joint and guide wires controlled by motors are provided for finely positioning the support arm and reflector in any given position along the spherical rotative path through which the reflector is able to move. The beam axis of the antenna is continuously variable through the complete range of motion of the reflector, and therefore the antenna system is capable of scanning over large areas. The fine positioning of the reflector and the lack of need for articulated connectors to the feed of the antenna provide for low loss and high pointing accuracy.

Synthesis of the aperture field for a contoured beam

Abstract: This paper presents a method to synthesize the optimum contoured beam obtainable from an aperture of a given size. The practical implementation by means of a multi-feed antenna and a shaped reflector antenna is also considered. >

Focusing of high power millimeter-wave radiation by a quasi-optical antenna system

Abstract: The focusing of millimeter-wave radiation from a high power 60 GHz gyrotron is studied using a two-dimensional ellipso-parabolic focusing antenna system. Fine focusing with an elliptic reflector gives a sheet beam of about 10 mm in thickness even at a high power input of about 100 kW. The addition of a parabolic reflector produces a finely focused and nearly Gaussian beam of about 10 mm in diameter with an energy density higher than lOOkWcm-2. Spatial distributions of beam power are compared with calculations and good agreement is obtained.

A compact EHF dual-frequency antenna for ASCAMP

Abstract: A compact, high-performance, dual-band antenna has been developed for use with ASCAMP (advanced single-channel antijam man-portable). It consists of a compact feed with an elliptical subreflector which produces the required focus ring for a displaced axis parabolic reflector. The main reflector of the antenna, which weights 2.6 pounds, is made up of six lightweight petals which can be separated and nested for easy portability. Because the petals in the assembled antenna do not mate up perfectly, the aperture illumination efficiency of the antenna is reduced relative to a single-piece reflector by 0.9 dB and 0.2 dB at 44.5 GHz and 20.7 GHz, respectively. Even with this added loss, however, the efficiency of the final design is better than 53% in both frequency bands. The author discusses the considerations which led to the final antenna design, describes the antenna's mechanical configuration, and presents measured performance test results. >

Low sidelobes antenna

Abstract: A radar type center fed antenna comprising a small radiating horn supported at the focal point of a parabolic reflector by three struts which are oriented to minimize the parallel polarization scattering and which have a low scattering ogive cross-section. The horn is mounted at the vertex of the parabolic surface and the intersection of the three struts using a bracket that provides minimal blockage. The struts are attached to the perimeter of the reflector. One strut having a feed waveguide is attached to the top-center of the reflector and the other two are attached at points on either side of the bottom-center at thirty degree angles to the vertical plane. The strut shape and feed-horn supporting and attaching arrangement and the integration of the feed waveguide into one of the struts results in a very low sidelobe antenna that produces a far-field pattern that has very low forward scattering due to feed and strut blockage.

High aperture efficiency, wide angle scanning reflector antenna

Abstract: A microwave single reflector antenna is provided with a large field of view and high aperture efficiency. The antenna exhibits good lateral scanning while preserving excellent focusing capabilities. The high aperture efficiency yields higher antenna performance than a conventional reflector antenna of the same size, or the same performance as a conventional scanning antena of larger size. The antenna has an improved surface configuration defined by a fourth-order profile extended into a three-dimensional focusing surface.

VSWR reduction for large millimeter-wave cassegrain radiotelescopes

Abstract: Multiple reflections in large radiotelescopes used for astronomical spectroscopy cause characteristic modulations of the observed spectrum (“baseline ripple”). For a given mechanism, the magnitude of the effect depends primarily on the reflection coefficient, which for the most important paths is proportional to λ. Although ripple is thus generally negligible at mm-wavelengths, there are some instances where it may still be significant, and a recent experiment at the15 m James Clerk Maxwell Telescope was severely affected. We describe a technique which has proved effective in reducing the ripple to an acceptable level. The purpose of this research note is to draw this technique to the attention of the astronomical community. We calculate the reflection coefficient for a typical instrument and show that it can be greatly reduced over a very broad bandwidth by means of a tapered absorber in the centre of the secondary mirror. The graphical analysis of the tapered absorber also demonstrates that the scattering cones and circular absorbers widely used in radioastronomy donot in fact reduce the reflection coefficient significantly.

Towards a better dielectric rod antenna

Abstract: Dielectric rod antennas (polyrods) have been the subject of considerable analysis and measurement Even so, design philosophy is limited to guidelines, usually aimed at maximising gain, without consideration of those other attributes that would make the polyrod an attractive moderate gain antenna Polyrods have been used as reflector type antenna feeds in the form of short cylindrical rods, as backfire elements, or with waveguide horn launchers However, feeds with sufficient directivity, adequate side lobes, cross polarisation performance etc for offset reflector geometry have not received much attention A simplistic physical model of how the rod works is put forward and from reasoning based on this a polyrod design of good performance is obtained >

Antenna having a circularly symmetrical reflector

Abstract: A microwave antenna has a circularly symmetrical reflector made of a material having a liquid phase and a solid phase, with the reflector being obtained by centrifuging the material while in its liquid phase and then allowing it to pass into its solid phase. This results in a convex paraboloidal reflector face having a very high degree of surface accuracy. When housed inside a microwave absorbing ring connecting the reflector face to a microwave-transparent window and used with a suitable source held at a predetermined focus position between the reflector and the window by means of microwave absorbent triangular struts, the antenna demonstrates improved radio and mechanical properties. Alternately, the centrifugally cast convex paraboloidal reflector may be used as the main reflector in a hybrid Cassegrain optical system in which the subreflector has a "conformed" non-paraboloidal surface that is machined from a solid piece of material. This results in an antenna that is easy to manufacture, and yet has much of the improved radio performance associated with the use of "conformed" surfaces for both reflectors. By also using a "conformed" lens at the aperture of the source, performance of such a hybrid system is further enhanced.

Electronically controlled reconfigurable reflector antenna

Abstract: Results are presented showing antenna pattern reconfiguration using an electronically controlled surface. Spot-beam and shaped-beams have been generated and excellent agreement was found between predicted patterns and those measured at 10 GHz with a compact antenna range. The antenna comprises an 85-cm-diameter offset reflector fed from a 30 degrees corrugated horn. As an example of a typical shaped-beam, a template was chosen corresponding to a scaled version of an INTELSAT requirement. >