Showing papers on "Cassegrain antenna published in 1968"

Antenna array at focal plane of reflector with coupling network for beam switching

Abstract: An antenna system for steering plural beams from a secondary reflector is formed from a primary array of linear feed elements, which can be arranged in either a one- or two-dimensional configuration. By controlling which of the radiators are simultaneously excited with a switching matrix, the position of the primary beam is varied as a function of the axis of the reflector forming the secondary beam.

Vehicle headlamp with two light sources

Abstract: A main parabolic reflector has an iodine lamp located slightly in front of the reflector focal point. A masking element below the lamp defines a plane below which no light from the lamp strikes the reflector. An auxiliary reflector having an aperture substantially one half that of the main reflector has a second iodine lamp at its focal point. The vertical axis of the auxiliary reflector is spaced from the vertical axis of the main reflector, and the aperture of the auxiliary reflector is tangent to the plane defined by the mask.

Aperture blockage by a subreflector in a Cassegrain antenna system

Abstract: The problem of aperture blockage by a subreflector in a Cassegrainian antenna system is discussed. The Lorentz reciprocity theorem is used to derive an expression for the far field radiated by the Cassegrain system. It is suggested that the field scattered by the subreflector can be computed using Keller's geometrical theory of diffraction.

Broad band antennas having spiral windings

Abstract: A broad band antenna or feed for a parabolic reflector having a pair of spiral windings skewed with respect to the axis of a conical support surface on which they are wound in alternating relationship. Two such supports are rotatably mounted with their axes offset at an angle from the reflector axis so as to conically scan the target area. The feed achieves maximum illumination of the control portion of the reflector and provides a constant squint angle with respect to the reflector axis over a broad band of frequencies.

Computer-aided analysis of cassegrain antennas

Abstract: A method of analyzing, in detail, the performance of symmetrical Cassegrain antennas has been developed that uses a digital computer efficiently. For a specified antenna geometry and feed excitation, the program will compute and graphically display the amplitude and phase illumination of the subreflector, main reflector, and far-field pattern. These results may be used to optimize antenna performance by changing parameters and observing the effect. Analysis of a Cassegrain antenna with a near-field conical horn feed is discussed as an application of the method. Because the radiation characteristics of the horn are determined by the horn flare angle rather than the horn aperture, broadband performance is obtained. It was indeed found that a 50 per cent bandwidth is achieved with a dual mode TE 11 –TM 11 mode feed, provided the proper phase relationship between the modes can be maintained over the band. For dual mode excitation an aperture efficiency of 70% and a noise temperature due to the power loss at the sub and main reflectors of less than 6.5$dGK was obtained. For a single mode feed (TE 11 ), there was a degradation in the E-plane side lobe levels and a corresponding 10$dGK increase in noise temperature. Excitation in the TM 01 mode was also examined for angle-error sensing purposes. Also, the antenna can be used with reasonable efficiency well below the design frequency in which case it functions as a far-field fed Cassegrain antenna.

Constant-beamwidth antenna development

Abstract: Broad-band microwave receiving and tracking systems generally require medium to high-gain antennas that have a minimum beamwidth requirement dictated by probability of intercept and acquisition time considerations. This paper discusses a technique whereby the secondary pattern beamwidth variation is decreased or eliminated over portions of the frequency band of these antennas. The antennas consist of a parabolic reflector illuminated by a ridge-loaded horn. A brief analysis, based on empirical antenna equations, is presented along with a description of a pencil-beam antenna and a very broad-band conical-scan tracking antenna that were made possible through the use of this technique.

Optimising the blocking efficiency in shaped Cassegrain systems

Abstract: The blocking efficiency of a Cassegrain antenna system can be improved considerably by shaping the main and the subreflector in such a way that the power, which is normally lost by scattering against the blocking obstacles, arrives at the aperture in the correct phase and direction and with the correct amplitude. It is explained that shadows of (nearly) spherical waves on the aperture can be made to disappear entirely. A practical example is described.

Portable parabolic reflector for microwave energy

Abstract: A PORTABLE PARABOLIC MICROWAVE REFLECTOR GENERALLY IN THE FORM OF A CROSS COMPRISES A SERIES OF MALE AND FEMALE ALUMINUM CASTINGS JOINED TOGETHER BY MEANS INCLUDING AN ALIGNMENT PIN AND SPRING-LOADED BALLS. ASSEMBLED REFLECTOR SECTIONS ARE SUPPORTED BY WELDED, TUBULAR A-FRAMES THAT BECOME ATTACHED TO THE REFLECTOR BY MEANS OF A SERIES OF MALE AND FEMALE ALUMINUM CASTINGS IN CONJUNCTION WITH AN ALIGNMENT PIN, LOCKED IN PLACE BY SPRING-LOADED BALLS. THE A-FRAMES ATTACHED TO WHATEVER SUPPORT STRUCTURE IS REQUIRED TO SUPPORT THE ANTENNA.

The influence of the scattered field on the reradiation pattern of reflector arrays

Abstract: This communication reports the results of an experimental investigation of a new log-periodic (LP) antenna. Tbe antenna consists of an LP array of radial waveguides excited by a two-wire transmission line. The antenna exhibits a vertically polarized omnidirWtiOM1 radiation pattern which is essentially constant over the designed bandwidth. Much progress has been made in the development of frequency-independent antennas. The main features of this class of antennas are directional radiation patterns and impedance which are constant over a very m-ide bandwidth. These antennas are generally not capable of omnidirectional operation. Dyson [ 1 ] has shown that a logspiral antenna of the proper configuration would have an approximately Omnidirectional radiation pattern which is circularly polarized. The discone antenna, which does not belong to the log-periodic (LP) class, has an omnidirectional vertically polarized radiation pattern over an approximate 6 to 1 bandwidth [2], [3]. The purpose of this investigation was to determine whether the log-periodic principle could be applied to a structure to give omnidirectional and vertically polarized characteristics over a wider range of fiequencies than is now possible. The basic element chosen for such a structure was a pair of parallel circular disks which are closely spaced compared to the wavelength. Such a basic element, when excited at the center of the plates, operates as a radial waveguide in the lowest E-type mode [4], thus producing the desired far-field radiation patterns. The LP structure is composed of an array of such disks with the diameter of the plates and the spacing between them varying in a log-periodic manner. The elements are alternately connected to a narrow two-wire transmission line running through the center of the disks. The LP antenna is shown in Fig. 1. Tests were made on three models of the structure having values of the LP parameter 7 of 0.90, 0.96 and 0.98. The ratio of the plate spacing to the diameter u was held constant at 0.115. The antennas were designed to operate over a 3: 1 bandwidth (500 MHz to 1500 MHz). The diameter of the largest plate was approximately 0.4 wavelength and the height was approximately 0.93 wavelength at 500 MHz (T =0.96). Measurements of impedance, far-field radiation patterns, and polarization were made. For all models, the impedance was relatively constant over the designed frequency quency range, being within an SWR of 2: 1 relative to I5+,20 ohms. The far-field pattern measurements showed that the structure was omnidirectional within 3 dB over most of the designed bandwidth. Better omnidirectionality was obtained with r closer to 1 than when T was a lower value. Fig. 2(a) shows the typical azimuth variation. The eleManuscript received March 13, 1968; revised April 1, 1968 and July 8. 1968. This work was supported in part by the Small Industries Research Program through The Pennsylvania State University, and in part by National Science Foundation Traineeship Grant GE7919.1. Fig. 1. The log-periodic parallel plate antenna. vation patterns showed that the angle of maximum radiation occurred between 45 to 75 degrees with the axis of the antenna and scans with frequency. A typical elevation pattern is shown in Fig. 2(b). The polarization measurement, shown in Fig. 2(c), was made by changing the orientation of a linearly polarized transmitting antenna with respect to the axis of the test antenna. Zero degrees on Fig. 2(c) represents the point where the polarization of the transmitted wave was parallel to the axis of the test antenna, i.e., vertically polarized. Ninety degrees represents the point where the polarization of the transmitted wave was perpendicular to the axis of the test antenna, i.e., horizontal polarization. Thus the structure is, indeed, vertically polarized. More details of the experimental investigation may be found in [5]. The investigation indicated that the structure is capable of vertically polarized omnidirectional radiation and an impedance characteristic which are both relatively constant over a very wide bandwidth. However, the deviations from the desired omnidirectional characteristics indicate that the lowest E-type mode may not be the dominant one on the antenna at some frequencies. Near-field measurements indicate that higher-order modes are present. The nonpropagating modes will not affect the far-field radiation characteristics, but can account for the large reactive component of the input impedance. A comparison of the LP disk antenna with a discone antenna of the same bandwidth (3 to 1) indicates that the LP antenna is considerably larger in size. For a lower cutoff frequency of 500 MHz, the largest diameter of the LP antenna is 9 inches whereas the largest diameter of the discone antenna is 5.5 inches. The height of the LP antenna is 20 inches compared to 5 inches for the discone antenna. Theoretically, the LP disk antenna should be I capable of operation over a wider bandwidth than is obtainable with a discone antenna. L Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on October 2, 2009 at 07:12 from IEEE Xplore. Restrictions apply.

Design studies for 440-foot-diameter radio and radar telescope

Abstract: : Studies conducted by the Cambridge Radio Observatory Committee during the past two years show that a large steerable telescope can be constructed at lower cost and with higher precision if the antenna is protected from the environment by a radome. These studies also indicate that the extremely high pointing precision (10 arc-sec) needed to make effective use of a large aperture at wavelengths as short as 5 cm can be obtained with a radome-protected antenna. The proposed 440-foot antenna employs a deflection-compensated vertical truss reflector support structure to maintain a 0.09-inch rms tolerance in the primary reflector under all operating conditions. The low cost of the antenna results from the ability to use lightweight reflector panels, an efficient computer-optimized reflector support structure, a small-diameter azimuth bearing, and a low-power control system. The estimated cost of this novel antenna design and the radome is substantially lower than the cost of alternative exposed antennas. The equivalent aperture and gain of the proposed antenna-radome system are made equal to that of a 400-foot antenna by increasing the antenna diameter by 10 percent. This report also describes the 520-foot-diameter space-frame radome and the planned electronic instrumentation and control facilities for a proposed radio and radar observatory.

Offset spherical reflector aerial with a line feed

Abstract: The present trend in satellite communication systems is towards large geostationary satellites which are accurately maintained on station. A fixed, offset, spherical reflector with a mechanically steered line feed is proposed as an economic design for a ground-station aerial. The distribution of electric field along the axis of the reflector is calculated for the case of an incident circularly polarised wave, and it is shown that the optimum distribution for a transmitting feed follows closely the axial field in the receiving case. Because of the asymmetry of the system, an entirely numerical technique has been developed, using a digital computer to calculate the axial field. A similar technique is used to compute the performance of an aerial using an offset spherical reflector and a line feed of crossed current elements. The aerial is shown to have a high theoretical efficiency (81%), with low spillover and a good sidelobe structure. The limitation of bandwidth with this type of feed is discussed, and proposals are made for an improved, design of line feed.

Pressure-formed parabolic reflectors for millimeter wavelengths

Abstract: An empirical investigation of metallic diaphragms deformed by uniform normal pressure to produce accurate parabolic reflectors is discussed. 30-cm diameter ( f/D =0.4 ) reflectors produced by this technique were measured mechanically and found to have maximum deviations of only 0.5 mm or \lambda/20 at 30 GHz from a true paraboloid within a circle of 14-cm radius. Electrical measurements at 30 GHz are discussed.

A study of an asymmetric Cassegrainian- antenna feed system and its applications to millimeter-wave radio-astronomical observations

Abstract: Design and evaluation of nodding hyperboloidal subreflector in Cassegrainian antenna system used for millimeter wave radio astronomical observations

Coupling in reflector arrays

Abstract: In order to reduce the space occupied by a reflector array, it is desirable to arrange the array antennas as close to each other as possible; however, in this case coupling between the array antennas will reduce the reflecting properties of the reflector array. The purpose of the present communication is to demonstrate that this is true for a Van Atta reflector consisting of four half-wave dipoles arranged on a line. Theoretical results obtained by using the methods described in references are compared with experimental results. In contrast to previous experimental results which have been presented as normalized, the results described are absolute

Computed radiation patterns of a thermally distorted 30-foot ATS F and G parabolic reflector antenna

Abstract: Radiation patterns for thermally distorted, 30 ft Applications Technology Satellite parabolic antenna computed using digital techniques