Showing papers on "Cassegrain antenna published in 1980"

Shaped reflector antenna analysis using the Jacobi-Bessel series. [design for space and satellite communication]

Abstract: The physical optics approximation is employed to derive the radiation integral for a doubly curved offset reflector antenna illuminated by an arbitrary source. A novel procedure is presented for expressing the radiation integral in terms of a summation of Fourier transforms of an "effective" aperture distribution which includes the effect of the curvature of the surface. The Jacobi-Bessel series is used to evaluate the Fourier transforms. The vector nature of the far-field pattern is studied by evaluating its three Cartesian components in a unified fashion. The rapid numerical evaluations of the expressions obtained are demonstrated via extensive test cases. In particular, the scattering characteristics of symmetric and offset parabolic, spherical, and shaped reflectors are studied in detail, and comparisons are made with other available data.

Shaped reflector antenna analysis using the Jacobi-Bessel series

Abstract: The physical optics approximation is employed to derive the radiation integral for a doubly curved offset reflector antenna illuminated by an arbitrary source. A novel procedure is presented for expressing the radiation integral in terms of a summation of Fourier transforms of an "effective" aperture distribution which includes the effect of the curvature of the surface. The Jacobi-Bessel series is used to evaluate the Fourier transforms. The vector nature of the far-field pattern is studied by evaluating its three Cartesian components in a unified fashion. The rapid numerical evaluations of the expressions obtained are demonstrated via extensive test cases. In particular, the scattering characteristics of symmetric and offset parabolic, spherical, and shaped reflectors are studied in detail, and comparisons are made with other available data.

Combined radar and infrared scanning antenna

Abstract: An antenna system has a parabolic reflector mounted so that it points generally downwards and a primary aerial is located at the focus of that reflector. Waves from the primary aerial are reflected back by the parabolic reflector to a reflector plate through an aperture in which the primary aerial projects. Scanning in both elevation and azimuth is effected by movement of the reflector plate.

Coverage shaping of contoured-beam antennas by aperture field synthesis

Abstract: A synthesis method for high-gain antennas with beams of elliptical or even more complicated crosssections is developed. The desired beam contour is realised in an offset reflector with circular aperture illuminated by a single feed by optimising the phase of the aperture field. A number of cases are presented in which the desired contour is an ellipse. The results are checked by a reflector antenna analysis program using physical optics.

Circularly polarized feed for cylindrical parabolic reflector antennas

Abstract: The feed design described is a linear array of crossed dipoles above a ground plane. The radiation patterns of the longitudinal and the transverse dipoles are made equal by parallel longitudinal rods, referred to as beam forming rods or beam matching rods. When used as a circularly polarized feed for an offset parabolic cylinder antenna, aperture efficiencies (spillover included) as high as 0.89 are potentially available if the field distribution along the array is uniform. The beam patterns of the feed were computed both by the method of moments and the geometrical theory of diffraction and are compared with patterns measured on a model at 1.5 GHz. A method of matching the impedance for both linear polarizations is proposed using parallel impedance matching rods.

Primary radar antenna having a secondary radar (IFF) antenna integrated therewith

Abstract: A primary radar antenna has a secondary radar antenna or an IFF antenna integrated therewith. Excellent properties with respect to compactness, radiation and frequency dependency are achieved by providing a bilevel pillbox antenna having radiation deflection on a cylindrical parabolic reflector from one interplate space to another. The lower interplate space has, in proximity of the primary radar signal radiator, which is arranged with its radiation center in the focal line of the parabolic reflector, an additional feed for in-coupling of the IFF signal. The antenna is particularly suited as a combined primary and IFF radar antenna for smaller vehicles.

Broadband astigmatic feed arrangement for an antenna

Abstract: The present invention relates to an antenna arrangement capable of correcting for astigmatism over a broadband range, the antenna arrangement comprising a main focusing reflector arrangement (10), such as, for example, a Cassegrain antenna system, a feed (12) and astigmatic correction means (14) disposed between the feed and the main focusing antenna arrangement. The astimatic correction means comprises a first and a second doubly curved subreflector (16, 18) or lens (30, 32) which are curved in orthogonal planes to permit the launching of an astigmatic beam of constant size and shape over a broadband range.

Multifrequency reflector antenna

Abstract: of the Disclosure A multifrequency antenna includes a main reflector for signals in a low frequency band. One or more portions of the main reflector are deformed to provide one or more auxiliary reflectors for signals in one or more higher frequency bands. The feed for the low frequency band is located axially of the main reflector, while the feeds for the higher frequency bands are offset to the side of the low frequency feed. The lateral and axial offsets of the vertex of an auxiliary high frequency reflector from the vertex of the main low frequency reflector and the focal length of the auxiliary high fre-quency reflector are selected to minimize the degradation of the per-formance in the low frequency band.

Loss-free scanning antenna

Abstract: A system for, and a method of, scanning a collimated beam of electromagnetic radiation with a very large primary reflector of an antenna system without loss (ie, with zero phase error) and without moving the primary reflector The system is a two dimensional scan, loss-free, Cassegrain antenna system which comprises a stationary, parabolic-shaped, primary reflector, and a smaller, movable, parabolic-shaped subreflector, and also a movable, plane wave, electromagnetic radiation beam source interposed between the two reflectors It is shown that, without moving the primary reflector, the beam can be scanned over wide angles with no phase error in the plane of scan, provided that the source and the subreflector are rotated about the focal point of the antenna in a specific manner A unique angular relationship between the source and the subreflector is required for each scan angle, and is taught The method includes the step of using this antenna system in scanning the beam from the primary reflector, without any phase error and without moving the primary reflector

High efficiency and low sidelobe design for a large aperture offset reflector antenna

Abstract: A design method giving high efficiency and low sidelobes is discussed for large aperture offset reflector antennas. A new reflector shaping technique using the subreflector and the beam waveguide reflector with the parabolic main reflector is proposed to simplify the main reflector manufacturing process. The effectiveness of the technique is confirmed by the model experiments. One problem with this reflector shaping technique is that the subreflector edge level cannot be controlled independently of the main reflector edge level. By investigating the relation between the gain reduction and the subreflector edge level, which affects the wide-angle sidelobe levels, the realizable characteristics of antennas are studied. In order to decrease the subreflector edge level without reducing the aperture efficiency, a technique using an extended reflector is also proposed. Its effectiveness is shown by theoretical and experimental investigations.

Evaluation of focal fields and radiation characteristics of a dual-offset reflector antenna

Abstract: A receive-mode analysis of a dual-offset reflector antenna consisting of an offset paraboloid and an offset hyperboloid is presented. In this analysis the field scattered from the offset paraboloid is determined using a g.t.d. formulation involving two edge points and a reflection point, if one exists. A method is described for deciding whether there is a reflection point from the gradient of the path length at the edge points. The focal-region fields and the radiation characteristics are computed by numerical integration of the physical optics current on the offset hyperboloid. An experimental antenna operating at 35 GHz is described Results of measurements of radiation patterns and of the power coupled into a circular waveguide array feed are compared with theory. In each cae reasonable agreement is achieved for copolarised directed components. However, the experimental antenna produced higher crosspolarisation than expected.

Dielectric-loaded trifurcated horn for H-plane stacked reflector feed array

Abstract: A waveguide horn is described which improves the efficiency of H-plane stacked horn array for reflector feeds. It makes use of dielectric loading for sharpening the H-plane pattern and trifurcation to reduce the large E-plane sidelobes. As a result, there is less energy loss in the spill over and more energy available for the secondary beam. Expressions for the horn radiation patterns are given. The computed patterns show good agreement with those measured.

Paraboloidal reflector spatial filter

Abstract: An antenna system is disclosed comprising a paraboloidal reflector and a feed array having several substantially identical feed elements. The antenna produces a coverage pattern of transmitted radiation in the form of a shaped beam. The present invention permits the use of a smaller number of feed elements for a given size of the reflector's aperture by means of attenuating the sinusoidal ripple that is present in the shaped beam radiation pattern and caused by the size of the feed elements. A plurality of disks is placed in the focal plane of the paraboloidal reflector. The disks are selective with respect to one spatial frequency of the emitted radiation (which is at a constant electromagnetic frequency). The disks may be fabricated of a material which reflects the radiation, a material which absorbs the radiation, or a dielectric material of a certain thickness stipulated herein which changes the phase of the radiation by 90°. By this technique, ripples are substantially removed from the beam pattern. The principles of the invention can be applied to antenna systems which are used to receive electromagnetic radiation.

Inverse cassegrain antenna for multiple function radar

Abstract: Inverse Cassegrain antenna making it possible to use on the one hand the qualities of a conventional fine beam for look-out and tracking functions and making it possible on the other hand to have a widened beam either in the elevation plane or in the bearing plane. According to one embodiment, this antenna is provided with a mirror constituted by two reflector - polarizer elements joined to one another by a hinge which permits the articulation thereof, a remote control device regulating their relative orientation.

Radar antenna with rotating linear polarization designed to reduce jamming

Abstract: A radar antenna of the Cassegrain type comprises a paraboloidal principal reflector confronting a hyperboloidal or flat auxiliary reflector of smaller radius directing linearly polarized radiation from a source on their common axis toward the principal reflector along lines passing through the focal point thereof to produce an axially extending outgoing beam whose central part is intercepted by the auxiliary reflector The latter consists of an array of linear conductors generally parallel to the direction of polarization while the principal reflector is formed by another conductor array generally inclined at 45° to that direction, the latter array being backed by a solid mirror separated therefrom by a distance smaller than a quarter wavelength at the midfrequency of the emitted radiation The composite wave leaving the principal reflector has an elliptical polarization which it retains in an annular portion of the outgoing beam bypassing the auxiliary reflector; only a linearly polarized component of that wave rotated through 90° from the original direction of polarization, however, passes the array of the auxiliary reflector in the central beam portion intercepted thereby

Rotating dual frequency range antenna system

Abstract: Disclosed is an antenna system designed to operate in two frequency rangesimultaneously, namely S-band (1660 to 1700 MHz) and X-band (8500 to 9600 MHz). The system comprises two separate antennas which are conically scanned and share a common parabolic reflector within a radome. The S-band antenna is adapted for passive angle tracking and reception of radiosonde data by means of a balun fed dipole feed system which includes an offset hemispherical reflector which is rotated by a scan motor to provide conical scanning. The X-band antenna comprises an active feed system which includes a stationary feedhorn and a tapered dielectric lens which is coupled to the S-band hemispherical reflector and is rotated therewith about an axis through the vertex of the parabolic reflector. The tapered lens tilts the constant phase front of the X-band radiation pattern thereby producing a displaced phase center near the focus of the antenna to implement its respective conical scanning operation.

Analysis of radiation pattern and G/TA for shaped dual-reflector antennas

Abstract: An accurate calculation of the radiation pattern from shaped Cassegrain-type dual-reflector antennas is given. In the forward axial region this is achieved by using the classical methods of physical optics and aperture field. Elsewhere we use the g.t.d. in the analysis, utilising a recent solution for the double knife-edge diffraction phenomenon between the two reflector edges. The result is used to compute the antenna temperature TA at a number of frequencies in the 1–100 GHz region commonly used for radio astronomy and space communications. The use of shaped reflector profiles to maximise G/TA consistent with other requirements such as acceptable sidelobe levels is discussed.

A low-blockage dipole array reflector antenna feed for the lower microwave frequencies

Abstract: An antenna is described which was devised as a feed for a small axisymmetrical paraboloid reflector at an operational frequency of just over 1 GHz. The antenna consists of a broadside array of two dipoles on a printed circuit board (PCB), joined by a common transmission line. Each dipole is backed by a small strip reflector. With this simple array, which has one central feed point, the H -plane radiation pattern can be varied independently of the E -plane pattern. In addition, the aperture blockage of the feed is small.

Offset Cassegrain Earth Station Antenna for the Japanese Domestic Satellite Communication System

Abstract: Offset reflector antennas have advantages for communication systems because they are not severely subject to blocking. Difficulties mainly arising from structual asymmetries have inhibited the realization of an offset reflector antenna with a large aperture for commercial use. This paper describes the design of an offset Cassegrain earth station antenna for the Japanese domestic satellite communication system. Antenna measurements showed 76 and 69 percent aperture efficiencies at 20 and 30 GHz, respectively, less than -20 dBi wide angle directivity and an 18 K noise temperature in operating conditions. Performances are far superior to conventional axisymmetrical earth station antennas. The antenna was reassembled on a telephone office building after the measurements. The antenna gain was reconfirmed there, using the sun as a radio frequency source. Experiments show that the earth station antenna and a terrestrial antenna can be placed on the same building without serious interference.

Pattern shaping with a metal plate lens

Abstract: The metallic parallel plate lens is a viable alternative to both the shaped reflector and the array antenna for beam shaping. A C -band antenna using this lens is designed to provide a csc^{2} \theta (cos \theta)^{1/2} power pattern over the angular limits of 60\deg-80\deg below horizontal. The Woodward-Levison synthesis technique is applied to determine the required amplitude and phase distribution across the aperture. Initial plate positions and lengths of the lens are determined from geometrical optics and infinite waveguide theory. The positions and lengths of the lens plates are adjusted experimentally until a satisfactory pattern is obtained.

Reflector antenna profile measurement using ultrasound

Abstract: A method for the measurement of reflector antenna surface profiles requiring no mechanical contact is described. The technique consists of placing an ultrasonic transmitter at the reflector focus and measuring the phase over the whole of the reflector aperture. Profile errors of order ±0.1 mm have been measured.

Comparison of G/T between dual-reflector and primary-focus antennas

Abstract: Cassegrain operation of moderate-sized antennas at low frequencies often poses difficulties because of the large feed horn required. Primary-focus operation using a two-hybrid-mode feed horn is shown to give a G/T ratio close to that provided by classical Cassegrain operation using a single-hybrid-mode feed.

A European Contoured Beam Reflector Antenna Development

Abstract: This communication provides the present results of an ongoing development, sponsored by the European Space Agency, of design software and of a feasibility model for a contoured beam reflector antenna (COBRA), to cover Europe from a geostationary satellite The design software, feed array elements and feed network components are described Present computed and experimental results are given New results include the use of original power synthesis algorithms, the combination of GTD and of Whittaker's pattern reconstruction technique for fast analysis of contoured beam reflector antenna performances, data on circularly polarised hexagonal array feed elements and design techniques for high phase and amplitude accuracy feed distribution networks

Reflection coefficient optimisation at feed of parabolic antenna fitted with vertex plate

Abstract: An expression giving the reflection coefficient at the feed of a parabolic antenna fitted with a vertex plate is minimised. The parameters open to optimisation are the vertex plate radius and its distance from the centre of the antenna. Graphs of the reflection coefficient as a function of frequency for some optimisation frequencies are given and compared with the corresponding graphs obtained from another method and from the case where the vertex plate is missing.

Bifocal reflector antenna and its configuration process

Abstract: The present invention provides a bifocal reflector antenna with no aberration and a configuration process thereof wherein the two-dimensional Ray Lattice Method is extended to the three-dimensional method. By setting the central section curve of a subreflector or main reflector as an initial condition, the surface curves of the sub- and main reflectors are determined so that a ray from the focus to the antenna aperture can satisfy the reflection law and path length condition. The aberration over the antenna aperture is perfectly eliminated by using the new method.

Orientable beam antenna for telecommunications satellite

Abstract: The orientable beam antenna comprises a fixed source, a fixed first reflector and a mobile reflector. The axis of the source is offset with respect to the focal aspect of the fixed reflector, and the mobile reflector connected to the elements through a universal joint device having two orthogonal axes, enabling the mobile connections to be limited to commercially available bearings.

New technique for efficient pattern computation of aperture and reflector antennas

Abstract: A new and efficient technique for computing secondary patterns of antennas with known aperture distributions is described. The method is extendable to parabolic reflector antennas with given surface current distribution. The fast Fourier transform (f.f.t.) algorithm is used to compute the coefficients of expansion of a series representing the radiation pattern.

Center-fed parabolic reflector antenna cross polarization due to slightly right-left asymmetric feed patterns

Abstract: Slightly right-left (R-L) asymmetric feed pattern effects on a center-fed parabolic reflector antenna cross polarization have been described with some numerical computation examples, noting that measured feed patterns, both in the E plane and in the H plane, are generally not completely R-L symmetric and have a slightly different R-L field intensity which produces asymmetrical reflector surface currents. It is shown from the numerical computations that even a slight feed asymmetry, in both amplitude and phase, gives rise to a cross polarization in the secondary prinicpal planes and that, in particular, phase R-L asymmetry incurs two-dimensional movement of principal- and cross-polarized patterns resulting in the rapid increase of principal-plane cross polarization as well as the maximum cross polarization, principal-pattern beamshift, and the sidelobe variations.