Showing papers on "Cassegrain antenna published in 1974"

Cassegrain antenna with improved subreflector for terrestrial communication systems

Abstract: A Cassegrain antenna for use in terrestrial communication systems. The antenna has the conventional subreflector including a central area which provides tapered illumination of the entire main reflector, and a peripheral area which directs spillover from the central area onto the peripheral portion of the main reflector to improve the uniformity of illumination of the main reflector, thereby improving illumination efficiency and reducing gain loss due to spillover. The central and peripheral areas of the subreflector are preferably both surfaces of revolution of conic sections of the same kind, such as hyperboloids, ellipsoids or paraboloids, although the peripheral area may be a frustoconical surface if desired. The virtual focal points of the two conic sections of the subreflector are both coincident with the phase center of the feed horn.

Large lateral feed displacements in a parabolic reflector

Abstract: The radiation patterns of a parabolic reflector with large lateral-feed displacements are computed utilizing both the vector current method and scalar aperture theory, and compared to experimental results. The theory is general enough to include asymmetric primary pattern illumination. The scalar and vector solutions are derived from the same initial equation so that the approximations used in obtaining the scalar solution are clearly displayed. Results from the vector and scalar theories are compared and the range of validity of the approximate analysis is indicated.

Highly efficient antenna system using a corrugated horn and scanning hyperbolic reflector

Abstract: In a horn-reflector antenna system for producing a spherical aperture phase front, a corrugated conical horn illuminates a section of a hyperbolic reflector to produce a spherical aperture phase front which produces a far-field beam with low sidelobes and high beam efficiency. The system is insensitive to frequency and polarization changes, and is also insensitive to orientation about the axis of the conical horn for beam scanning.

Cross-polarized parabolic antenna

Abstract: A cross-polarized parabolic antenna employs a horn fed by two rectangular waveguides having their longer transverse dimension in a common plane, with a 90 DEG polarization rotator in the feed between the connection points of the waveguide.

The French quadristatic incoherent scatter facility

Abstract: The original Nancay-St. Santin incoherent scatter facility based on the radio astronomy antenna at Nancay and a CW transmitter located at St. Santin (300 km south) was improved in many ways between 1965 and 1970: doubling of the transmitted power to 140 kw, lowering of the receiver noise level down to 100 K, monitoring of the antenna and data acquisition through the use of a computer, rejection of the parasitic tropospheric echoes. However, the most important improvement brought to the system is the addition of two new receiving stations 100 km east and west of the transmitter. Each of the new stations is equipped with a three-beam Cassegrain antenna (25-m parabola) aimed to simultaneously intersect the vertical transmitted beam at three different heights. Both new receivers have wide band (38 MHz at 3 db) gaseous helium cooled parametric amplifiers and on-line computers. While one of the two new stations is equipped with filter banks for the purpose of spectral analysis, a different choice has been made for the other station. For the latter the data acquisition will imply multibit correlators, for both the ionic spectrum and plasma line measurements. Most of the improved electronic equipment designed for the new stations has also been implemented in the Nancay receiver. The frequency and time synchronization between the four sites is now accomplished by employing a single 5-MHz frequency standard in each station monitored by standard time and frequency signals from MSF and/or HBG stations. The observation procedure consists of tilting the different receiving antennas so as to gather the signal scattered from the ionospheric vertical column illuminated by the transmitter. For calibration purposes a standard ionosonde is operating at St. Santin. The main objective of the quadristatic system is to determine completely the ion drift vector in order to study the electric fields and the neutral tidal winds. Other capabilities offered by the new system are a determination of the plasma line and a higher data acquisition rate. The first observations with one of the new stations were performed in December 1972.

A new class of medium-size high-efficiency reflector antennas

Abstract: The new class of reflector antennas that is described in this communication yields a higher directive gain than obtained with conventional reflector antennas of the same area. An antenna of this new design consists of a reflecting surface, a peripheral rim, and a feed system in the reflector center. It is analyzed as a combination of two radiating sources whose radiation maxima and mutual phase relations can be simply adjusted for highest directive gain in the axis normal to the reflector. The marked directive gain increase is explained on the basis that there is a virtual extension of the radiating aperture beyond the physical dimensions of the reflector. The two antenna models that are described develop directive gains of almost 3 dB above those of conventional reflector antennas of equal size and comparable pattern characteristics. Optimized for a given frequency, the new design covers a pattern bandwidth of approximately 4:1.

Compact frequency reuse antenna

Abstract: A compact antenna system that permits orthogonally polarized frequency reuse operation is achieved by two overlapping parabolic reflectors. Each of the reflectors has a reflecting surface comprised of parallel, reflecting, conductive elements with the reflecting elements of one reflector polarized orthogonally to the reflecting elements in the other. Each reflector has an associated feed copolarized with the reflecting elements of the particular reflector. The two reflectors are overlaid without coinciding their respective focus points so that cross-polarized fields generated by the parallel elements in the surface of each reflector from its associated feed are scattered away from the copolarized beam of each reflector.

An improved antenna for microwave Radio systems consisting of two cylindrical reflectors and a corrugated horn

Abstract: A broadband antenna is described with an elliptical beam suitable for efficient illumination of the United States from a satellite in synchronous orbit. The antenna is also suitable for use in terrestrial radio systems above 10 GHz. It consists of a corrugated feed and two parabolic cylinders that efficiently transform the circularly symmetric beam radiated by the feed into an elliptical beam. Depolarization of the incident beam by the two cylinders is very small and essentially independent of the angle of incidence, which can therefore be chosen as large as required to avoid aperture blockage. The performance is described of an antenna with a 5.8° × 2.9° elliptical beam at 18.5 GHz. For any input polarization, the cross-polarized component of the far field remains over the entire beam at least 33.5 dB below the main component on axis. This cross-polarized component is due in part to imperfections in the corrugated feed and in part to some aperture blockage by the feed and depolarization by the cylinders. A first-order analysis of the frequency dependence shows that the beamwidths vary little with frequency for an antenna using a properly designed feed and cylinders of sufficiently large apertures. As the frequency is increased from 18.5 to 29 GHz, the measured horizontal and vertical beamwidth variations are +1.7 and −14 percent, respectively.

360{20 {0 Azimuth scanning antenna without rotating RF joints

Abstract: A 360* scanning radar antenna has a plurality of primary focusing-structures arranged in a circular fashion illuminating corresponding secondary focusing-structures which in turn are arranged about, and directed toward, a rotating multi-sided halfwave plane reflector. Radar energy is switched to radiate from a given primary focusing-structure during the time when the plane reflector is in position to reflect all of the energy collimated by the corresponding secondary focusing-structure. The secondary focusing-structure may be made to appear transparent to the beam reflected by the plane reflector.

Variable beamwidth antenna

Abstract: An antenna system for single or plural beams providing continuously variable beamwidth selectively in one or both of two orthogonal senses, i.e., azimuth and elevation, for either communications or angle-tracking. The system includes two parabolic cylindrical reflectors, which are respectively a main reflector and a sub-reflector; the reflectors are positioned with the focal axes thereof orthogonally. A point or multibeam (e.g., monopulse) feed is mounted adjacent the main reflector on the focal axis of the sub-reflector in the Airy disc of the system. Beamwidth is controlled using telescoping sections on the main and sub-reflectors to control the size of the surface areas thereof. Simultaneous operation of the telescoping sections of the reflector and sub-reflector provides bidirectional zooming of the beam without distortion while individual operation of the sections of the reflectors permits unidirectional zooming. Bidirectional zooming of beams from the multibeam feed requires rotating the position of the multibeam feed as the Airy disc changes during adjustment of the telescoping sections.

Three-dimensional corner reflector antenna

Abstract: A reflector antenna utilizing three planar reflectors and a 3/4 λ-unipole radiator is proposed. Calculations based on image- and EMF-methods predict that the gain is about 5 dB greater than that of the two-dimensional corner reflector antenna, and that the input impedance is roughly between 50 and 75 Ω. Experiments conducted on the three-dimensional corner reflector antenna with rectangular corners confirmed the predicted properties and also showed the sidelobe in the electric plane being suppressed by the effect of diffraction. This effect is explained by the calculated radiation pattern of 3/4 λ-unipole on conducting half plate

Integrated radiometric seeker gyro

Abstract: A radiometer and gyroscope are integrated in a single unit by coupling part of an antenna directly to the spinning inertial mass or rotor of the gyroscope. In the preferred embodiment a Cassegrain antenna is coupled to a dynamically tuned, wide-angle, free-rotor gyro by connecting the smaller secondary reflector to the gimbal-mounted rotor. A stationary antenna feed and waveguide are arranged inside of the hollow drive shaft. While the rotor, primary and secondary reflectors are spinning, the rotor and secondary reflector attached thereto can be tilted off the spin axis, to which the primary reflector, however, is constrained.

Large reflector antenna pattern computation using moment methods

Abstract: A technique, whereby the radiation patterns of large size reflector antennas can be approximated by computers using a moment method without excessive computer time or memory, is described. Computational time increases only linearly with antenna size. There is no limit on the minimum reflector size (a single wire reflector can be analyzed). An example is discussed showing the radiation patterns of a truncated parabolic reflector excited by feeds displaced from the parabola focus.

Realisation of generalised far fields by reflector synthesis

Abstract: A method of designing reflector surfaces by the geometrical-optics approximation is introduced, in which a given generalised far field is realised when the reflector is illuminated by a point source. The problem is shown to be reduced to solving a nonlinear 2nd-order partial differential equation of the Monge-Ampere type. A computed example is given.

A dual-reflector optical feed for wide-band phased arrays

Abstract: This paper contains a description of the fundamental performance characteristics of a new space feed suitable for wideband operation of phased-array antennas. A unique aspect of this feed is the combination of an active time-delayed small source array and a confocal passive two-reflector system in an offset Gregorian configuration to provide wide-band compensation to a scanning phased array antenna contaning only radiators and phase shifters. A two-dimensional model is analyzed using the plane wave spectrum representation of the near field. Calculations for bandwidths of up to 20 percent of center frequency are given. The good electrical performance of the feed as a function of bandwidth and scan angle establishes this system as an attractive candidate in space feed applications and as a tradeoff competitor or alternative to the conventional corporate feed.

Windshielding structure for an antenna

Abstract: A windshielding structure for a large-diameter high-frequency dish-type antenna includes a flexible membrane extending from the edge of the antenna dish to a fixed rigid member mounted on a base plane. The membrane permits controlled movement of the antenna dish in azimuth and elevation, but minimizes the effect of wind approaching the dish from the front direction (head-on wind) by preventing the wind from interacting with the antenna dish edge to cause Von Karman vortices and associated mechanical oscillations of the antenna. The enclosure formed behind the antenna dish by the membrane, the antenna dish, the fixed rigid member and the base plane can be used for protection of equipment and personnel. The fixed rigid member may have a lip protruding from its surface to divert from the dish and membrane, wind approaching the antenna dish from other directions.

Antenna reflector support

Abstract: A slotted cylindrical electrically conductive support for a parabolic micro-wave transmitter antenna reflector coaxial with and surrounding a conical antenna reflector whereby omnidirectional rays of energy from said conical antenna pass from the inside of the cylindrical support radially to the outside of said support, because the slots in the support are of such shape and dimensions that each of these slots can be considered as a radiating element. A non-conducting weather protecting film may surround the cylindrical support.

Antenna feed system

Abstract: An antenna feed system is disclosed which is formed by successively combining a phase transformer element providing no beam waist and a phase transformer element providing beam waist, so that deviation in beam width due to frequency fluctuations is compensated by the combined transformer elements. This apparatus may be used in the input of a reflector antenna system, such as Cassegrain antenna, for example, which is operated at high gain over broad frequency bands.

Vertical plane pattern calculations for shaped beam reflectors with multiple feeds

Abstract: The calculation of approximating vertical plane radiation patterns for doubly curved shaped beam antenna reflectors illuminated by a multiple feed is discussed. Separability of the vertical and horizontal plane patterns is assumed. A calculated example for a two-element feed is compared with measured pattern data. Farfield pattern shaping behavior with multiple feed excitation is described in terms of vector superposition of the patterns due to the individual feed elements.

Optimum design of horn feeds for reflector antennas

Abstract: A method of determining the optimum dimensions of a horn feed for a parabolic reflector using the power transferred to the feed as a criterion is described. To reduce the computation time, the focal plane and feed-horn aperture field distributions were expanded into finite power series whose coefficients were determined using collocation techniques. The paper extends previous work to include horns with flare angles greater than 10\deg and contains useful design curves.

Directional cassegrain microwave antenna - has stepped parabolic reflector or stepped secondary convex reflector to give sector radiation pattern

Abstract: The directional Cassegrain microwave antenna has a secondary radiator located at the focus of a stepped parabolic reflector to produce a sector-like radiation pattern. The secondary radiator consists of a concave reflector facing the primary radiator which radiates through a hole in the parabola along the axis of the latter. If the main reflector is stepped, then the secondary reflector is smooth. If the main reflector is smooth, then the second reflector is stepped. The advantage of the antenna lies in its giving a good approximation to a sector-like radiation pattern for very few components and low cost.

Comprehensive Measurement of Aperture Fields, Near-Fields and Far-Fields of a Large Reflector Antenna from a Microwave Hologram

Abstract: A microwave holographic technique is used to obtain quantitative data on the amplitude and phase of the principal and cross-polarised fields in the aperture of a large reflector antenna. The hologram formation process utilises the existing scanning system of the antenna. Data is processed computationally using the FFT and is visually displayed in hard copy with 32 levels. The lateral and longitudinal positioning of the E- and H-plane phase centres of the feed is located and the r.m.s. value of the reflector surface profile errors obtained. Accurate prediction of E- and H-plane radiation patterns in any section of the near and far fields can be obtained from the reconstructed aperture distributions. The technique provides a total description of the antenna.

Reflector antenna with focusing spherical lens

Abstract: A reflector antenna providing an asymmetrical beam focusing for shortwave radar transmitting and receiving operation, which essentially consists of a multiple curved reflector which rotates at a constant angular velocity, as well as a primary radiation or beam source. Between the reflector and the stationarily constructed primary radiation or beam source, a spherical lens having a relatively large frequency band width which rotates with the reflector, and wherein the beam extends in circularly edged transverse planes intersecting the rotational axis. The spherical lens may be constructed to serve as a rotary coupling for the transfer of the beam power, and to concurrently serve as a condenser for illumination of the elliptical reflector aperture.

Coupling between crossed-dipole feeds

Abstract: Various effects of coupling between crossed-dipole antennas are analyzed and by using an arbitrary feeding network some generality is preserved With one cross excited and another cross acting as a parasitic loaded antenna, coupling losses and gain and polarization losses are presented as a function of orientation and feeding network properties The antennas are used as feeds for a parabolic reflector, and the effect of coupling on the secondary fields is analyzed Especially significant is the polarization loss and it may, to some extent, be reduced by a proper choice of feeding network

Approximate method of calculating the radiation pattern of a corner-reflector antenna

Abstract: The known surface currents in a corner reflector having infinite-length sides are used as approximations to the surface currents in a finite corner reflector, to calculate a radiation pattern. It is shown that this approximation results in nearly zero tangential electric field at the surface of the reflectors, so that the assumed surface currents can be expected to be good approximations to actual currents. Experimental and calculated radiation patterns for a particular corner reflector are presented, and show excellent agreement.

Lightweight 3.66-meter-diameter conical mesh antenna reflector

Abstract: A description is given of a 3.66 m diameter nonfurlable conical mesh antenna incorporating the line source feed principle recently developed. The weight of the mesh reflector and its support structure is 162 N. An area weighted RMS surface deviation of 0.28 mm was obtained. The RF performance measurements show a gain of 48.3 db at 8.448 GHz corresponding to an efficiency of 66%. During the design and development of this antenna, the technology for fabricating the large conical membranes of knitted mesh was developed. As part of this technology a FORTRAN computer program, COMESH, was developed which permits the user to predict the surface accuracy of a stretched conical membrane.