Showing papers on "Cassegrain antenna published in 1983"

Combined E - and H -plane phase centers of antenna feeds

Abstract: The feed efficiency, a first approximation to the aperture efficiency of a paraboloid or a conventional Cassegrain antenna, is used to define uniquely a combined E - and H -plane phase center of the feed pattern. A formula for numerical calculation of the combined phase center is presented, as well as theoretical results of the feed position tolerances and the efficiency loss due to differences in the principal plane phase patterns.

An imaging beam waveguide feed

Abstract: A pseudo-frequency-independent beam waveguide feed has been designed and built to accommodate a new liquid-helium-cooled millimeter wave radio astronomy receiver in the side cab of the Crawford Hill 7-m antenna. This enables the antenna to be tilted without tilting the liquid-filled receiver. Comparison with the old vertex-cab feed indicates very little measured transmission loss through the beam waveguide. The frequency independence is based upon the Fresnel zone imaging principle. Design procedures and practical bandwidth limitations are explained. An explicit expression for the third-order term of an offset reflector surface clarifies the approximation of a lens by an offset reflector.

An improvement in electrical characteristics of a short backfire antenna

Abstract: A short backfire (SBF) antenna is considered to be a favorable option for a compact and high efficiency shipboard antenna in maritime satellite communications. Though the SBF antenna has many attractive characteristics, it has a demerit of narrow frequency bandwidths because of its leaky cavity operation in principle. The electrical characteristics of a conventional SBF antenna have been improved by changing its main reflector from a flat disk to a conical plane and by adding a second small reflector. Frequency bandwidths of 20 percent for VSWR's under 1.5 can be obtained by the improved SBF antenna.

The effects of subreflector diffraction on the aperture efficiency of a conventional Cassegrain antenna--An analytical approach

Abstract: An asymptotic theory is presented with which the reduction in aperture efficiency caused by diffraction from a subreflector edge can be calculated for any dual-reflector system. The theory is applied to conventional Cassegrain antennas, for which approximate analytical effieiency formulas are derived. These formulas show that subreflector diffraction may represent a significant efficiency loss even for subreflector diameters as large as 20 wavelengths. The formulas are used to obtain an optimum subreflector size which represents the best trade-off between losses due to subreflector diffraction and geometrical shadowing.

An offset dual-reflector antenna shaped from near-field measurements of the feed horn: Theoretical calculations and measurements

Abstract: A synthesizing procedure for shaping dual-reflector offset antennas is described. The synthesis is based on geometrical optics and near-field measurements of amplitude and phase from the feed element. The procedure preserves good symmetry in mapping from feed to aperture which results in minimal distortions of the electrical characteristics of the antenna. A 1.8 m antenna has been manufactured and measured at 12 GHz. The radiation pattern is characterized by low sidelobes and cross polarization introduced by the offset geometry below -50 dB.

Bicollimated offset Gregorian dual reflector antenna system

Abstract: A dual reflector antenna having two reflectors concave to each other and specially shaped so that beams incident on the antenna from two directions are reflected therefrom in collimated beams in two other directions.

Polarizer reflector and reflecting plate scanning antenna including same

Abstract: A polarizer reflector includes a reflecting layer backing a meander-line polarizer effective to convert the incident beam from linear polarization to circular polarization during the propagation of the beam forwardly through the polarizer, and to reconvert the beam reflected from the reflecting layer from circular polarization to linear polarization but rotated at a predetermined angle, preferably at a right angle, with respect to the polarization of the incident beam. Also described is a reflecting plate-type scanning antenna including a front collimating paraboloid reflector with the above-described polarizer reflector serving as the back reflector, which arrangement has been found to substantially increase the frequency range of the scanning antenna.

Cylindrical and three-dimensional corner reflector antennas

Abstract: Two reflector antennas are proposed. The first is constructed by adding a cylindrical reflecting surface of suitable radius to the V -shaped corner reflector antenna. The feeding element is a half-wavelength dipole. The resulting cylindrical corner reflector provided a 2 dB increase in gain, minimum sidelobe level, low input reactance, and uncritical dependence of performance on frequency. The second antenna is constructed by adding a cylindrical surface to the three-dimensional corner reflector. This extension provided an increase in gain of at least 6.5 dB, an input resistance compatible with the commercially available 50- or 75-\Omega coaxial cables, low input reactance, and uncritical dependence of performance on frequency. A grid-type cylindrical corner reflector antenna, and a three-dimensional corner reflector antenna with a cylindrical subsurface of finite reflecting surfaces were designed, and the measured input impedances, gains, and field patterns showed excellent agreement with the theoretical results for both antennas.

Defocusing loss for a long periodic-fed reflector

Abstract: Log periodic designs are a well established wide-band antenna technology, but one in which the phase center travels along the structure as the frequency varies. When a log periodic antenna is used as a feed for a reflector antenna, the phase center location cannot be maintained at the reflector focus over the frequency range and defocusing results. The purpose of this analysis is to quantify the defocusing loss for a log periodicfed reflector. The analysis presented compares favorably with measured results.

Efficient computation of the far field of parabolic reflectors by pseudo-sampling algorithm

Abstract: The newly developed pseudo-sampling representation is applied for computing the far field of an offset parabolic reflector with a cluster feed illumination. A new comb-type fast Fourier transform (FFT) algorithm is used in a computer program. Both precision and computational time are analyzed, demonstrating the excellent performance of the method.

Imaging system at 94 GHz using tapered slot antenna elements

Abstract: A 94 GHz Imaging System using linearly tapered slot antennas with integrated beamlead diodes as receptor elements has been developed. The elements have sufficient gain to directly illuminate the sub-reflector of a Cassegrain antenna. The one-foot diameter of the main reflector determines a beam-width of 0.7 degrees for each beam. The measured spacing between adjacent beams in a seven element array is 0.64 degrees. A four element array has also been used for tracking.

Multi-beam antenna and its configuration process

Abstract: This invention relates to a multi-beam antenna and a method of configuring the same, where the antenna consists of a main reflector, a plurality of horns for exciting the main reflector, and separate sub-reflectors for correcting phase errors of respective beams caused by reflection at the main reflector, or an integrated sub-reflector which is substituted for said separated sub-reflectors.

Fixed aperture, rotating feed, beam scanning antenna system

Abstract: A beam scanning antenna system that uses a small, rapidly rotatable feed enna to illuminate a large, fixed secondary collimating device, such as a reflector or electromagnetic lens, which in turn illuminates a large, fixed primary collimating device. The primary collimating device forms a narrow collimating beam that is reflected or transmitted into space. Rotation of the small feed antenna causes the beam to scan.

Radome-covered reflector antennas

Abstract: An antenna assembly (10) comprises a paraboloidal reflector (11) having a microwave signal feed assembly (40) including a feeder (16). A radome (20) secured to the reflector (11) comprises a first parabioloidal portion (22) of annular configuration surrounding a second paraboloidal portion (24), portion (24) being of shorter focal length than portion (22) such that the focus (25) of portion (24) is no nearer the reflector (11) than the feeder (16). The focus (27) of portion (22) is also noncoincident with feeder (16) and a body (28) of radiation absorbing material is located at focus (25) to absorb reflected radiation from portion (24). Body (28) may be mounted on the radome (20) or on the assembly (40). As shown the arrangement comprises a Cassegrain antenna with a convex reflector 41. Other arrangements are envisaged.

Focal finder for parabolic reflector antenna

Abstract: A compact, easily manipulable measuring device is provided, particularly useful for properly positioning the feed horn of a parabolic reflector antenna, or the like, relative to the satellite reflecting dish of the antenna. When installing or maintaining an antenna, it is important for optimum signal reception to position the feed horn at or near the focal point of the satellite dish. Such feed horns typically present a central axis, which if aligned with the center point of the satellite dish approximately positions the feed horn at the focal point of the satellite dish. The device of the present invention presents a telescopic rod structure coupled to a support head. The support head presents structure for attachment to the feed horn such that the rod structure preferably lies along the feed horn central axis. With the rod structure extended to a position adjacent the satellite dish, it is readily apparent whether the feed horn central axis is aligned with the satellite dish center point or if adjustment of the feed horn is necessary.

Inverted Cassegrain antenna for multiple function radars

Abstract: An inverted Cassegrain antenna utilizing polarization rotation. The polarizer-reflector or mirror utilized comprises two or more polarizer-reflector elements joined together around hinges perpendicular to the widening direction required. A sheet comprising parallel metal wires covers the whole of the elements. For each element, a metal film is separated from the sheet by a first dielectric layer. Each hinge is formed, on one side, by the flexible sheet of metal wires which straddles the gap between elements. The other side of the hinge includes a second dielectric layer backed by a rigid metal strip, also straddling the gap. One end of the metal strip is fixed to the metal film of one adjacent element while the other end of the metal strip is in moveable electrical contact with the metal strip of the other adjacent element. The antenna is particularly applicable to multiple function radars.

Narrow multibeam satellite ground station antenna employing a linear array with a geosynchronous arc coverage of 60°, part II: Antenna design

Abstract: A dually polarized narrow ( ) beam antenna which provides a geosynchronous arc coverage of 60\deg is proposed and analyzed. To track the geosynchronous arc accurately, the properly oriented antenna produces a conically scanned beam by means of a linear array of feed horns with bias cut apertures illuminating a pair of parabolic cylinder reflectors in an imaging arrangement. This design, with reduced size array and singly carved reflectors, is relatively simple to construct. Calculations for a 0.35\deg beamwidth Ku band earth station antenna show a 0.05\deg pointing accuracy with scan loss due to aberrations less than 1 dB and little pattern degradation throughout the scan region. For multiple beam capability, a Rotman lens is proposed and analyzed as a means of exciting the feed array. It allows communication with multiple satellites spaced as close as 1\deg .

A Beam Waveguide Launcher for High Power ECRH Heating in Large Tandem Mirror Machines

Abstract: We explore the possibility of applying a quasi-optical beam waveguide of the Cassegrain type as a launching system for RF heating of a fusion plasma. The plane wave required for proper operation of the Cassegrain antenna can be obtained from the azimuthally polarized TE0n gyroton output by means of, e. g., a horn reflector with a stepped-twist reflector mounted in its throat. Our launcher has the potential to keep the loss level extremely low, continue the outputs of a number of gyrotrons, and direct the radiation on the plasma in specific directions and polarization states.

New monopulse tracking radar

Abstract: A tracking radar which employs a new frequency-comparison monopulse technique by means of sum and difference frequencies is described. A Cassegrain antenna with a five-horn feed transmits and receives two carrier frequencies, and tracking errors due to target glint and thermal noise are reduced by using a wideband FM technique.

A Circularly Polarized Offset Reflector Antenna for Direct Broadcasting Satellites

Abstract: The design objectives and electrical performance data of a circularly polarized offset reflector antenna for the illumination of the FRG are presented. The requirements are based on the WARC regulations settled for direct broadcasting satellites /1/. The antenna consists of an offset paraboloid reflector with an elliptical corrugated feedhorn with RF-sensor. Measured radiation pattern, gain figures and RF-sensor performance data are presented. Pattern degradations caused by reflector tilting for beam fine pointing and thermal deformations are considered.

Conical scan process in a radar antenna, radar antenna implementing such a process and use of such an antenna in a tracking radar

Abstract: The present invention relates to a conical scanning process in a radar antenna. The antenna comprises a principal parabolic reflector (1) and a primary rear-feed source (2) fed by a circular waveguide (3) and displaced with respect to the focus of the parabolic reflector (1). The conical scanning is produced by the rotary movement of the parabolic reflector (1) about the source (2), the said source (2) and the circular guide (3) remaining fixed. The present invention is applicable to tracking radars operating in the millimeter waveband.

An experimental broadband imaging feed

Abstract: Imaging with Fresnel diffraction taken into account is applied to the design of a broadband narrow pattern feed. This feed is not only a basic building block of an imaging beam waveguide, but also essential for an offset reflector antenna of large effective F/D ratio. Furthermore, it can be used as a constant beamwidth radiometer antenna for multifrequency remote sensing. We have built and tested a practical example that consists of an offset ellipsoidal reflector and a corrugated horn. Measured amplitude and phase patterns agree with calculated results, which include truncation effects. Systematic design procedures are obtained for a given feed horn and the required reflector illumination. Necessary and sufficient conditions of the thin lens model are translated into design parameters of an offset ellipsoidal reflector with projected circular aperture. Geometrical relations of the offset ellipsoid and calculations of radiation patterns are described in the appendices.

Reflecting plate antenna including a polarizer reflector

Abstract: A polarizer reflector includes a reflecting layer backing a meander-line polarizer effective to convert the incident beam from linear polarization to circular polarization during the propagation of the beam forwardly through the polarizer, and to reconvert the beam reflected from the reflecting layer from circular polarization to linear polarization but rotated at a predetermined angle, preferably at a right angle, with respect to the incident beam. Also described is a reflecting plate-type scanning antenna including a front collimating paraboloid reflector with the above-described polarizer reflector serving as the back reflector, which arrangement has been found to substantially increase the frequency range of the scanning antenna.

Reduction of sidelobe level of reflector antennas by covering feed struts with a periodic structure

Abstract: A periodic structure has been investigated and optimised which can be used to cover the struts of reflector antennas in order to reduce the far sidelobes caused by scattering from the struts.

RF design and predicted performance for a future 34-meter shaped dual-reflector antenna system using the common aperture X-S feedhorn

Abstract: The Networks Consolidation Program (NCP) will utilize 34-meter shaped dual-reflector Cassegrain antennas. The shaping calculations, the X/S-band feedhorn to be used, and the predicted RF performance of this antenna system are discussed.

Monopulse radar system

Abstract: This invention is a description of a tracking radar system which uses a frequency-comparison monopulse technique by means of sum and difference frequencies. A Cassegrain antenna is employed and it consists of a parabolic reflector with a five-horn feed A, B, C, D and E. Two different carrier frequencies (fc + f1) and (fc - f2) are transmitted from transmitter Tx via a duplexer 6 and after reflection from a distant target, the signals received are separated by sideband filters 4, 5, 7 and 8. Each FM receiver 12 or 15 extracts a difference frequency signal (f1 - f2) which yields target information in azimuth and elevation respectively, and an AM receiver 14 extracts a sum frequency signal (f1 + f2) for providing AGC bias and an output for range measurement. Tracking errors due to thermal noise and target glint are reduced by employing a wideband FM technique and a ratio detector (Fig. 3, not shown) in each FM receiver. Target tracking in azimuth, elevation and range is achieved by using closed- loop control systems in two orthogonal planes.

Inverse cassegrain antenna for a multifunction radar

Abstract: The present invention relates to a Cassegrain antenna inverted polarization rotation. The polarizer mirror used consists of two or more reflector-polarizer elements articulated around hinge perpendicular to the desired widening direction. A sheet comprising N parallel metal son (100) overlies the set of elements. For each element (e 1, e 2) a metal film (105, 104) is separated from the layer N by a first dielectric layer (103, 102) and a metal tongue (108), secured to the film (105) and in electrical contact with the film (104) by a second dielectric layer (107, 106). The present invention is applicable to multi-function radar.