Showing papers on "Cassegrain antenna published in 1982"

Antenna arrangements using focal plane filtering for reducing sidelobes

Abstract: The present invention relates to an antenna arrangement comprising a feed producing a predetermined feed aperture illumination and a plurality of sequentially arranged reflectors including a main reflector forming the aperture of the antenna arrangement. The present antenna arrangement also includes filtering means centered on a real focal point between two reflectors of the antenna arrangement. The filtering means is arranged to pass therethrough the central ray of a beam launched by the feed and for smoothing out discontinuities of the image of the feed aperture illumination in the area of the main reflector normally found without filtering. The main reflector is then made slightly oversize to intercept the smoothened-out image of the feed aperture illumination along a line which produces a predetermined level of edge intensity.

A first-order treatment of aberrations in Cassegrainian and Gregorian antennas

Abstract: The decrease in aperture efficiency caused by small aberrations in a reflector antenna is determined The important case of a Cassegrainian (or Gregorian) antenna with a feed placed in the vicinity of the focal point is treated in detail For this case the various aberration components due to astigmatism, coma, etc, are derived explicitly, their effect on aperture efficiency is shown, and the conditions that optimize performance are given The results are useful for the design of multibeam antennas in ground stations and satellites

Asymptotic analysis of parabolic reflector antennas

Abstract: Dual mode horns employed commonly as feeds for parabolic reflector antennas generate a radiation pattern that can be well-approximated by a Gaussian beam. To determine the far field of the antenna, it has been customary to perform integrations either of the physical optics currents on the reflector surfaces or of the ray optically determined field in the antenna aperture. These time-consuming integrations may be avoided if the Gaussian beam is tracked directly from the feed horn via subreflectors, if any, to the main reflector and then to the far zone. The tracking of such fields may be accomplished either by the complex-source point method or, in principle, by evanescent wave tracking. The former utilizes a complex coordinate space while the latter tracks fields entirely in the physical (real) coordinate space. For a parabolic antenna with an offset beam feed centered at the focus, both methods are examined here and an assessment is made of how the one can best complement the other. Numerical comparisons with results deduced elsewhere by a semi-heuristic procedure, and with experimental data, reveal the accuracy and versatility of the complex ray procedure.

Sunrays focusing apparatus

Abstract: A sunrays focusing apparatus for obtaining highly condensed solar energy. The apparatus includes a first trough-shaped parabolic reflector mounted for rotation about both vertical and horizontal axes. A first elongated refractor is mounted to the first parabolic reflector along or adjacent to the focal axis of the first parabolic reflector. A second trough-shaped parabolic reflector is also mounted to the first parabolic reflector in such a way that focal axes of both first and second parabolic reflectors are disposed at right angles to each other. A second reflector is mounted to the first parabolic reflector along or adjacent to the focal axis of the second parabolic reflector. Mounted on the second parabolic reflector is a sun tracking device responsive to sunrays for directing the first parabolic reflector towards the sun.

Orbital spacecraft having common main reflector and plural frequency selective subreflectors

Abstract: The invention relates to multi-mission orbital spacecraft of the kind comprising a platform and several different payloads including several telecommunication antenna feed systems. The problem is to avoid antenna interference, obtain bigger effective antenna aperture, avoid the necessity for replacing main antennae when replacing payloads, and make maximum use of longer platofrm life. In accordance with the invention, the antenna system comprises a common primary reflector which is a permanent integral part of the platform while the feed systems are mounted on the payloads, which may be launched separately, and are assembled with the platform to cooperate with the common platform antenna system in operation. The invention is mainly applicable to multi-mission satellites.

Multibeam antenna arrangement

Abstract: The present invention relates to a multibeam antenna arrangement comprising a main focusing reflector, a doubly curved subreflector disposed confocally with the main reflector and a plurality of feeds disposed on a doubly curved focal surface of the antenna on which an image of the far field of view is formed. The subreflector is doubly curved in orthogonal directions to introduce a predetermined amount of barrel distortion for transforming a three-dimensional, non-rectangular, matrix in the far field of the antenna arrangement into a substantially rectangular matrix on the doubly curved focal surface of the antenna arrangement. Feeds are aimed such that a central ray from each feed reflected by the subreflector impinges a common point on the main reflector. Beams of a satellite antenna introducing barrel distortion can be re-aimed toward a given set of earth coordinates when a satellite is moved in equatorial orbit by rotating the subreflector about an axis, which is substantially parallel to the axis of the earth, and which passes through the confocal point of the antenna.

Two-band microwave antenna with nested horns for feeding a sub and main reflector

Abstract: A two-band multimode microwave source for an antenna of a low-elevation-tracking radar comprises a higher-frequency section nested in a lower-frequency section, the two sections having E-planes perpendicular to each other. The lower-frequency section includes two outer pairs of waveguides separated by a block which convergingly projects beyond their output ends and is bisected by the E-plane of that section. The higher-frequency section includes two inner pairs of waveguides disposed within that block and separated by an obstruction lying in the last-mentioned E-plane. The higher-frequency wave emitted by the inner waveguides is made planar by a lens disposed at n output aperture of the structure which is transparent to the lower-frequency wave. In a Cassegrain-type radar antenna the lower-frequency wave emitted by the source is returned by a semitransparent intermediate reflector toward a main reflector provided with a grid which rotates its plane of polarization to let it pass out through the intermediate reflector along with the higher-frequency wave which, passing unattenuated through the intermediate reflector, is returned by a solid outlying reflector to the main reflector.

Polar mount antenna satellite tracking apparatus and method of alignment thereof

Abstract: A polar mount for a parabolic satellite tracking antenna insures close tracking of the antenna to a synchronous satellite track sector bearing multiple satellites within the earth's equatorial plane. The apparatus tilts the polar pivot axis of the antenna and the dip of the antenna boresight to cause an elliptical antenna track which better approximates the track of the synchronous satellite. The antenna is provided with a removable Polaris telescope alignment fixture to permit selective adjustment of the tilt angle at a sine setting derived from specific formula prior to aligning the antenna mount for true north and with further ajustment for dip angle or declination to achieve alignment accuracy within thirty arc-seconds of the antenna boresight to the satellite track sector bearing the satellites whose signals are to be received. A zero backlash linear actuator selectively drives the antenna about the polar axis to sweep the satellite track sector, with exacting alignment, from satellite to satellite.

VHF parabolic cylinder antenna for incoherent scatter radar research

Abstract: A parabolic cylinder reflector antenna has been designed for high-power radar observations of signals scattered from the ionospheric plasma at a frequency of 224 MHz. The antenna is constructed with an offset feed system to reduce aperture blockage. Efficient illumination for circular polarization using crossed dipoles is achieved by means of primary beam forming, which amounts to the construction of a polarization sensitive corner reflector. The measured aperture efficiency is 64%, and the gain 43.6 dB. The antenna can be mechanically steered through 90° in the meridian plane, and the beam can be offset from this plane by ±21.3° by manual phase steering of the 128-element line feed. The width of the aperture is 40 m, and the length 120 m. The reflector is constructed in four 30-m-long sections which can be individually steered. The RF distribution system is such that the two antenna halves can be operated and pointed completely independently. The paper describes the design considerations, the construction, and the testing of the performance of the completed antenna.

Contoured-beam synthesis for array-fed reflector antennas by field correlation

Abstract: A method is described for calculating the feed excitations of an array-fed reflector antenna for the production of a desired contoured beam. It is based on field correlation at the nearest reflector between the surface currents from selected stations in the field of view and the field radiated by the array. Mode coupling in the array is included in the formulation. The feed excitation is derived by satisfying constraints on the envelope of the copolar and crosspolar correlation functions through a minimax optimisation. Some preliminary results are presented for a simple contoured beam, which is realised using an open Cassegrain antenna and a 19-element circular-waveguide feed cluster.

A transform technique for computing the radiation pattern of prime-focal and Cassegrainian reflector antennas

Abstract: An efficient numerical method based on the use of the fast Fourier transform (FFT) algorithm is developed for computing radiation patterns of aperture antennas with given aperture distributions. The method is also readily applicable to the problem of computing the radiation pattern of paraboloidal reflector antennas when the induced surface currents on the surface of the reflector are known. Using an efficient launching and scanning scheme for subreflector analysis, the method is extended to a Cassegrainian reflector antenna system.

Peak gain of a Cassegrain antenna with secondary position adjustment

Abstract: For an enclosed Cassegrain antenna, the loss of peak gain and beam deviation due to structural deformations of the primary reflector and rigid body displacements of the secondary reflector and of the feed are computed from the combined changes in the radio frequency (RF) path length. As the antenna moves in elevation, the position of the secondary reflector may be adjusted mechanically to minimize the loss of peak gain; a general method for the computation of the magnitude of such adjustments and of their effects on the gain and pointing of the system is presented. Numerical results are obtained for a particular case of a 45-ft diameter antenna designed for operation at 95.5 GHz RF for which the computed peak gain of the antenna varies significantly with the elevation angle. The results indicate that the loss of peak gain as the antenna moves in elevation can be substantially reduced by mechanical adjustment of the position of the secondary reflector.

Surface tolerance loss for dual-reflector antennas

Abstract: 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.

Antenna element having non-feed conductive loop surrounding radiating element

Abstract: The directivity as well as the gain of an antenna element associated with a finite length reflector, which includes a feed antenna such as a dipole antenna and a reflector having a finite length of several wavelengths or less, are improved by disposing a non-feed loop element having a peripheral length of about 2 wavelengths along a common imaginary plane with the feed antenna so as to surround the feed antenna. Preferably, the non-feed loop element is disposed in parallel to the reflector, and more preferably it is disposed nearly symmetrically with respect to the center of the feed antenna.

Diagonal-conical horn-reflector antenna

Abstract: A horn-reflector microwave antenna has a reflector plate which is a section of a paraboloid, and a flared feed horn for supplying microwave signals to the reflector plate The horn has a conical section forming a circular aperture at the wide end, which is the end closer to the reflector plate, and a pyramidal section forming a square aperture at the narrow end, which is the end farther away from the reflector plate Microwave signals are supplied to the feed horn with the electrical field extending along a diagonal of the square aperture

Low sidelobe, high efficiency mirror antenna with twist reflector

Abstract: A mirror antenna employing a continuous mirror without a hole. A feed hornnd an electromagnetic lens, located behind a rotatable twist reflector (the mirror), collimates a beam toward a polarized reflector located near the twist reflector and tilted to aim energy toward the twist reflector. Energy reflected back toward the polarized reflector from the twist reflector passes through the polarized reflector to free space.

Thermo-mechanical design and analysis system for the Hughes 76 inch parabolic antenna reflector

Abstract: This paper describes the methods and materials used to control the temperatures on a dichroic, multibeam antenna ref lector for a communications satell i te. It i l lustrates the combination of analysis and test programs in both thermal and mechanical d isc ip l ines used to predict thermal distortions and verify the final design.

An experimental investigation of the changes of VLBI time delays due to antenna structural deformations

Abstract: Structural deformations primarily occur as functions of antenna elevation angle due to gravity loading. For a Cassegrain antenna, one of the major effects of structural deformation on measured VLBI time delays are those delay changes associated with axial subreflector displacement from its nominal position. Two types of time delay changes that occur when the subreflector is axially defocused are: a change which is a linear function of subreflector defocus position; and a cyclical change caused by multipath. Test results show that for the 64-m DSN antenna, the linear change is 1.8 times the subreflector defocus position, while the peak-to-peak change in cyclical variation is about + or - 3 cm when a spanned bandwidth of 38 MHz at 2290 MHz is used.

Quasi-optical antenna design and applications

Abstract: The symmetrical, prime-focus-fed paraboloid is the most commonly used reflector for medium and high-gain, pencil-beam applications. This reflector is relatively straightforward to analyse, design and fabricate. An extensive literature exists to describe its properties.

Substantially frequency-independent aberration correcting antenna arrangement

Abstract: The present invention relates to an antenna arrangement which uses a large offset spherical main reflector to communicate with several, spaced-apart, remote locations. Large aberrations caused by the main reflector are corrected by a first subreflector forming a small image of the main reflector at a conjugate image surface and a second subreflector which is disposed at the image location and is shaped to correct for the aberrations caused by the main reflector. Such correction is, to a good approximation, frequency independent and provides aberration free operation at feeds adjacent each other and associated with remote locations having small differential angles of incidence on the center of the main reflector.

Flanged parabolic antennas

Abstract: The radiation characteristics of symmetric parabolic antennas with a peripherical flange are examined in detail in order to assess the effectiveness of such a flange in reducing the backward scattered field. It is shown that the best behavior is obtained by using a right-angled flange, which allows achievement af a significant field level reduction in a wide rear angular sector, without affecting the field radiated in the forward directions.

Multiple reflector antenna

Abstract: A multiple reflector antenna is disclosed that comprises a main reflector formed of a portion of a rotatively symmetrical surface with respect to a rotation axis, a sub-reflector, at least one auxiliary reflector, and at least one wave source, said rotation axis being parallel to or slightly deviated from an aperture plane. This antenna is characterized by such construction that said sub-reflector and auxiliary reflector intentionally cause distortion of an electro-magnetic field distribution to cancel the distortion generated at said main reflector.

Antenna in accordance with the Cassegrain principle, having a holder for the subreflector

Abstract: It is intended to specify an electrically and mechanically favourable solution for holding the subreflector (3) on the horn emitter (1) of a Cassegrain antenna. To this end, the holder consists of a hard-foam material body (2) which is provided with a relatively thin, preferably painted-on plastic coating (6). This results in a holder for the subreflector (3), which holder has virtually no influence on wave transmission but is very stable and is insensitive to temperature.

Bicollimated near field gregorian reflector antenna

Abstract: : A bicollimated near-field Gregorian reflector is structurally similar to a classical confocal parabolic 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% greater angular scanning range than the corresponding confocal parabolic dual-reflector system.

Geometrical optical characteristics of the Scwarzschild scanning antenna. Comparison with the Cassegrain antenna

Abstract: Possible geometrical configurations of Schwarzschild antenna systems are investigated. Input parameters to a rapid design procedure are the subreflector subtended angle and the blocking ratio Ds/D. To allow comparison with Cassegrain antennas, simple formulas are developed for the aperture field, the gain factor, co-and crosspolar components of the radiation pattern and the polarisation efficiency of the antenna using geometrical optics (GO). The scan properties are investigated for both lateral feed displacement (as assumed by Schwarzschild) and displacement of the feed over the special locus minimising the weighted phase error in the aperture plane. Comparison with the focused and defocused properties of the Cassegrain antenna leads to the conclusion that, even without leaving the framework of geometrical optics, there appears to be no reason to prefer the more complicated Schwarzschild antenna system.

Beam waveguide feed for antenna.

Abstract: In microwave communication systems it is common practice to use a beam waveguide to transmit energy between a waveguide (2) connected to transmitting and receiving equipment located at ground level and a main antenna reflector mounted above ground level. Hitherto the waveguide (2) has terminated in a very large, accurately machined, horn which is necessary to produce a beam having a small angle of divergence suitable for entry into the beam waveguide (6). The present invention proposes that this large (and very expensive) horn be replaced by a relatively small horn (18) which produces a beam having a relatively wide angle of divergence. By using a concave reflector (22) and a sub-reflector (21) in Cassegrain configuration this widely diverging beam is converted into a more nearly parallel (or a totally parallel) beam which enters the beam waveguide (6) as illustrated.

Beam scanning and focal surfaces for parabolic cylinder reflectors

Abstract: The location of the focal surfaces for parabolic cylinder reflectors with Gaussian tapered illumination is investigated and compared with that obtained by numerical simulation. It is found that an idealized surface that holds approximately in most practical cases is the circle described on the vertex-focus line as diameter. Formulas are derived for the change of gain and beamwidth with scan angle.

Bandwidth capabilities of array-fed parabolic cylinders

Abstract: The bandwidth of hybrid phased-array/reflector antennas, in which a large parabolic cylinder reflector is fed by a relatively small phased array, is investigated. Two types of array feeds are considered: one employs only phase-amplitude matching, the other employs amplitude and time-delay matching. In the case of phase-amplitude matching, the on-axis hybrid array/reflector designs are shown to have substantially higher bandwidth than pure phased arrays. A form of time-delay compensation is described which removes the bandwidth limitations of hybrid array/reflectors, but introduces interfering signals. This interference can be avoided by either locating the feed outside the caustic zone or by using off-axis array feeds.