Showing papers on "Cassegrain antenna published in 1987"

Steerable beam antenna system using butler matrix

Abstract: A steerable beam antenna system for use in satellite communication systems and including a main reflector and an antenna array having a plurality of feed elements is disclosed. The antenna array is positionable at or near at least one focal point of the main reflector, and its feed elements can receive microwaves from or transmit microwaves toward the main reflector. A Butler matrix having multiple input ports and multiple outputs is connected to the array of feed elements and substantially performs a spatial Fourier transformation on a generated set of signals to be transmitted which have a predetermined phase relationship between the signals, which is necessary to create the steerable beam. The Butler matrix also can perform an inverse spatial transformation on a set of incoming signals focused on the array by the reflector and received by the feed elements. By using a Butler matrix, the antenna system of the present invention is able to communicate with multiple ground stations simultaneously and with high gain using many virtual spot beams, each oriented to a distinct location on the earth that is dependent on the relative frequency of the beam.

Gaussian mode analysis of Cassegrain antenna efficiency

Abstract: Gaussian mode analysis is a convenient way to characterize long focal length systems, such as the Cassegrain antenna at the secondary focus. We use multimode Gaussian optics to derive several interesting results concerning the aperture efficiencies of Cassegrain antennas fed by corrugated conical horns. The highest efficiency is obtained when the antenna is illuminated by a wide-band, wide-angle horn which has its phase center at the secondary focus, or equivalently by a narrow-band diffraction limited horn with its phase-center located at a confocal tertiary focus. In both cases this corresponds to placing the horn aperture at a position where the incoming fields are frequency-independent and have limited spatial extent. We generalize these results to show that a true image of the aperture distribution can always be formed on the spherical cap in the aperture of any arbitrary scalar horn, by using a single refocusing tertiary lens or mirror. Since both the incoming fields and the horn aperture fields on this surface are frequency independent, it follows that the aperture efficiency is also independent of frequency with either of these arrangements. We suggest that devices for shaping the aperture illumination, such as shaped lenses or dual shaped subreflector systems, should, in general, be installed near an image of the telescope aperture distribution.

Multiple frequency antenna feed

Abstract: A multiple band antenna feed used with parabolic reflector antennas and the like. The feed is arranged as two coaxially disposed waveguides. A planar array of patch elements is disposed at the end of the coaxial waveguides so the energy in each band radiates from a common phase center. This simplifies the arrangement of associated subreflectors.

Omnidirectional antenna with hollow point source feed

Abstract: An omnidirectional antenna comprises a ring-shaped subreflector surrounding an omnidirectional feed and a ring-shaped main reflector for redirecting radiation from the subsidiary reflector to the target zone. The feed has a focal ring and a hollow centre to accommodate supports, feeders etc which facilitates stacking of antennas.

Short-pulse excitation off reflector antennas

Abstract: The transient build-up and decay of the radiated field produced by a short pulse in a reflector antenna are examined. The transient time is proportional to diameter, and inversely pro-portional to f/D, thus Cassegrain reflectors have better short-pulse response.

Reflector antenna for telecommunications

Abstract: A telecommunications antenna is provided with a reflector having a semi-offset geometry, a feed mounted on a feed source rigidly fixed to the reflector, an antenna pedestal for supporting the reflector and the feed. Positioning of the feed is such that, in respect of a reflector illumination half-angle β as seen from the feed such that 45°<β<55°, the offset angle α (angle between the focal axis ΔFoc and the median ray of the total beam which is emitted by the feed and illuminates the reflector) is 25°<α<35°, the antenna mount being of the azimuth-elevation type known as an az-el mount.

Multiple-beam groundstation reflector antenna system : a prelimary study

Abstract: A wide-scanning multiple-beam reflector antenna, two systems are investigated. A bifocal antenna, designed with the use of an existing method, appears to be unsuitable for wide-angle scanning. A dual-reflector offset torus-antenna showed promising results. As an illustration of its benefit, a possible application is examined: the simultaneous reception of signals from a number (n) of geostationary direct broadcast satellites with mutual distance of 6 degrees. Using this antenna yields advantage when compared, with respect to the total required reflector area, with n separate antennas.

Subreflector shaping to improve multiple-beam performance of a Cassegrain antenna

Abstract: A method of designing multiple-beam antennas based on shaping the subreflector of an offset Cassegrain antenna is described. It is applied to a compact system having a diameter of 300λ that is required to produce beams up to 4-6°either side of boresight. The performance is shown to be only slightly less than that obtainable by also shaping the main reflector, indicating that efficient multiple-satellite-access antennas can be designed without specially shaped main reflectors.

Offset spherical reflector with a low sidelobe radiation

Abstract: This paper presents a design study of an offset antenna which employs a spherical main reflector and phase correcting subreflector. Detailed predictions are compared with the measured performance of a prototype design. The antenna provides a sidelobe envelope well within the US Federal Communications Commission specification for small earth stations.

Spacecraft Antenna Design using Optimisation

Abstract: The design of shaped-beam antennas suitable for satellite broadcasting and satellite communication is described. The method employs an optimisation search routine in which the reflector shape is varied and the antenna far-field is compared at sample points to the desired far-field. Three examples are given corresponding to the CONDOR, EUTELSAT 2 and AFSAT Satellites. In the latter it is found that two feeds are needed in order to meet the specification and their excitation is included in the search in addition to the reflector shape. As far as is known this is the first time a shaped reflector and a small array have been used together to meet intricate pattern requirements.

Radiation-pattern control of Cassegrain antennas

Abstract: Using the results of a theoretical analysis of the radiation pattern of Cassegrain antennas, the paper discusses the requirements imposed by the most recent Consultative Committee on International Radio (CCIR) recommendations for the maximum-allowable sidelobe level applicable to earth-station antennas with D/? > 150 (D being the diameter of the main reflector). Also considered are the requirements of smaller antennas, and a possible sidelobe level response for D/? < 150 is suggested. Although the analysis explicitly considers axisymmetric configurations, the results have been extended to embrace the radiation characteristics of offset geometries. This permits the indication of when it is preferable and sometimes necessary to employ the offset configuration.

Aperture efficiency enhancement in a Cassegrain system by means of a dielectric lens

Abstract: We have succeeded in boosting the overall aperture efficiency of a Cassegrain radio antenna by introducing a dielectric lens into the beam. The gain amounts to 0.7 dB, or about 17%, at millimeter wavelengths. The lens, of weak optical power, but strongly aspherical figure, is placed a short distance in front of the feed. It serves to redistribute the field in the aperture of the telescope, making the amplitude more uniform, with a sharp cutoff at the edge. The aberrations (defocusing and spherical) introduced by the lens are balanced by slightly displacing the secondary mirror. The lens can be adapted to many existing telescopes employing classical paraboloid-hyperboloid optics, at minimal expense.

Low-loss offset feeds for electrically large symmetric dual-reflector antennas

Abstract: Circularly symmetric, dual-reflector, high-gain antenna systems often require feeds placed off the system's axis because of the need for multiple feeds to use the reflector antenna. Also, the constraint requiring the hyperboloid or shaped subreflector to remain circularly symmetric is sometimes added. In a Cassegrainian system, the subreflector and feed may be rotated off-axis around the paraboloid focus and retain main reflector focusing. However, substantial spillover results in considerable noise in a high-gain/low-noise temperature system. In a shaped system, the tilt of the shaped subreflector and feed together results in substantial defocusing as well as spillover noise. If the subreflector is tilted approximately one-half the angle of the feed tilt in either the Cassegrainian or the dual-shaped reflector antenna, it is found that spillover and noise are substantially reduced with tolerable defocusing. An extensive numerical analysis of these effects was conducted to determine the characteristics of a 70-meter, dual-shaped reflector versus Cassegrainian antenna and to gain some understanding of the cause of the observed effects.

Large scan antenna with fixed main reflector and fixed feed, particularly for use at ultrahigh frequencies, carried on board a satellite and a satellite equipped with such an antenna

Abstract: A large scan antenna, particularly for embarked use on board a satellite and for operation in the ultrahigh frequency range, of the type formed of a feed and a main fixed secondary reflector, the antenna being readily foldable into a launching configuration with a minimum of component elements, and comprising at least one guide G each formed of a first and a second parabolic secondary reflectors (12, 13) aligned optically along the axis of the wave guide G between the feed (10) and the main reflector (15), said wave guide G being on the one hand variable in length by moving at least one (13) of said secondary reflectors with respect to the other (12) along the axis (14) of the wave guide G, and on the other hand, rotatable about the axis (11) of the feed (10).

Electronic scanning antenna having a small number of radiating and phase-shifting elements, and a restricted scanning angle

Abstract: This antenna includes an electronic scanning array DELTA of dimensions DELTA and of scanning angle alpha 0, and a focusing system itself including an objective D of diameter D and focal length f and an eye piece D' of focal length f and f' and of magnification gamma such that gamma = f/f', which are placed on the axis of the array so that the array is situated in the focal plane of the eye piece and the eye piece in the focal plane of the objective, the assembly being equivalent to an electronic scanning antenna of dimensions D such that D/ DELTA = gamma and of scanning angle beta 0 such that beta 0 DIVIDED alpha 0 = 1 DIVIDED gamma . The objective D consists in particular of a Cassegrain antenna comprising a main reflector MP and an auxiliary reflector MA. Application to electronic scanning antennas.

Use of imaging with spatial filtering in reflector antennas

Abstract: The classical problem of illuminating efficiently the aperture of a reflector antenna is discussed. We combine a small feed with an ellipsoidal reflector producing, directly on the main reflector, a magnified image f(x, y) of the feed aperture distribution f_{0}(x_{0}, Y_{0}) . The transformation relating the feed distribution to its image is approximately frequency-independent, since it is given accurately by the imaging laws of geometric optics. Thus the antenna is efficient over a wide range of frequencies. Furthermore, since the entire image distribution f(x, y) is reflected by the main reflector with negligible spillover, the antenna far field is approximately a replica of the focal plane field and, therefore, its sidelobes can be reduced substantially by using spatial filtering in the focal plane.

Comparative study of reflector antennas for small earth stations

Abstract: For a small earth station, a detailed comparative study of three types of dual offset reflector antenna is presented. These are the shaped dual-reflector antenna, the sector-shaped paraboloidal reflector antenna and the spherical offset reflector antenna. The efficiency, the radiation patterns and the complexity of fabrication are some of the aspects considered.

94 GHz Search And Track Antenna

Abstract: An offset Cassegrain modified focal-pivot-scan monopulse antenna was designed for use with a 94 GHz search and track radar system which would be mounted on an armored vehicle. The focal-pivot-scan design provides for scanning of the main reflector about its focal point while the feed horn and subreflector remain fixed. This permits rapid beam scanning in azimuth for target acquisition while minimizing the mass that must be scanned (and the required scan motor capacity) and precludes the necessity for rotary joints with their attendant losses and possible mechanical failure. The elevation search function and tracking in both azimuth and elevation are accomplished using separate drive motors.

Optical method for searching for, and suitability testing of, locations for satellite receiving antennas

Abstract: The method is used by the erectors of parabolic reflector antennas to find the correct location for an antenna, especially for those for private domestic reception from direct-broadcast radio and television satellites. Depending on the apparatus design, it is thus also possible to align the parabolic reflector to the satellite. The apparatus enables an aimed sectional view of the location (which is known in terms of direction and inclination angle) of the invisible satellite and thus indicates possible obstructions.

Beam scanning characteristics of front fed offset cassegrain–type multibeam antenna

Abstract: This paper shows theoretically and experimentally that the front-fed offset Cassegrain (FFOC) antenna is suited as a multi-beam antenna to be borne on an international satellite. First, the problems are identified for conventional reflector configurations. Then the beam deflection characteristics are shown numerically in the case where an FFOC reflector surface is used as a multibeam antenna. The performance of the test antenna is confirmed experimentally. From the measured results of a test antenna of 120 wavelengths, it is found that the reduction of the gain in the beam deflection within a circle of the visible radius of 10° is less than 1.8 dB which agrees well with the computed results. The cross-polarization level is less than-34 dB. Because the cross-polarization component generated by the primary radiator is −40 dB, the cross polarization generated by the reflector system is quite small.

A dual polarized line source feed for cylindrical reflector antennas

Abstract: A line source feed for an offset cylindrical parabolic reflector antenna is described. The line source consists of two transverse and longitudinal dipole arrays over a ground plane. The elements of each array are fed separately from a feed waveguide through a coaxial line which is connected to a probe inserted into the waveguide. Within each waveguide these probes form a tapered array. The probes are treated as an N-port network to determine their penetration depth for a required Taylor distribution along the line source. The line source is then constructed and tested for both aperture field distribution and the input impedance. Its performance as a feed for an offset cylindrical parabolic reflector antenna is also studied.

Radar reflector with bidirectional communication capability

Abstract: A radar reflector with bidirectional communication capability is presented. The reflector consists of a Luneberg lens and a reflecting plate combining a microstrip antenna and a switching circuit, which is placed at the focus of the lens. The reflector is capable of transmitting information from a reflector site as well as receiving signals from a radar station. Characteristics of the reflector are analyzed by the moment method using a wire-grid model. The results are confirmed by experimental measurements. An experimental model of the reflector yields an antenna gain of 26.5 dB and a modulation ratio of reflectivity of 13 dB. A passive telemetry system covering a wide area can be built with a radar station and a group of these reflectors. Alternatively, a Luneberg lens having many reflecting plates can respond to plural radar stations simultaneously and independently.

Cassegrain antenna for the microwave band

Abstract: The invention relates to a Cassegrain antenna for microwaves, especially millimetric waves, having a supply device, subreflector and main reflector. The main reflector consists of a plurality of circular rings (paraboloid sections) which are curved in a concave manner, have different external diameters and are arranged concentrically and offset in a stepped manner, have a common focus, and whose focal lengths differ by an integer multiple of half the microwave wavelength.

Cylindrical corner reflector antenna with different feeding configurations

Abstract: Consider a corner reflector antenna which consist of two planar surfaces at an angle 2X/N, where N is an integer. The third surface is a part of a circular cylinder whose axis coincides with the line of the two planes as shown in Fig. 1 and whose radius is "a". The antenna is driven by using M dipoles, for example, each with length "I" and the seperation between any two of those dipoles is "d". The distance between those dipoles and the z-axis is "R". Employing the principle of images C33 yields I'M" stacked, two dimensionalcircular arrays. Each one of them consist of two groups, one with radius "R" and the other with radius

Parametric study of shaped Cassegrain antennas

Abstract: The effect on gain and the first few sidelobes of Cassegrain antennas due to axial feed and subreflector shifts is investigated. The shaped and classical Cassegrain designs are the objects of study, and the motivation is the concern over possible compromise introduced by the maximum G/Ta design philosophy on a radio telescope antenna. The result of the investigation is a set of general design rules for shaped Cassegrain antennas, based on the examples studied.

Low sidelobe earth station antenna using symmetric parabolic reflector

Abstract: INTRODUCTION. Axi-symmetric parabolic reflectors are easy and cheap to make. A large number are already in use as TVRO earth stations. The advent of the more stringent sidelobe specifications adopted by the FCC, INTELSAT and EUTELSATnhas generally been taken to imply that the specifications can only be met.with offset reflectors because blocking needs to be removed. This paper shows that it is possible to synthesise a shaped subreflector with a standard parabolic main reflector which meets the 29 251og0 sidelobe specifications for earth station antennas and also maintains a relatively high efficiency.