Showing papers on "Cassegrain antenna published in 1981"

Sidelobe level reduction by improvement of strut shape

Abstract: From the viewpoint of effective utilization of both the geostationary orbit and the frequency spectrum in the satellite communication system, it is strongly required of an earth station antenna to have low sidelobe characteristics in order to minimize the interference from/to the adjacent satellite and the terrestrial microwave links. The reference diagram of the sidelobe gain for a large antenna recommnended by International Radio Consultative Committee (CCIR) is drawn as follows: G = 32-25 \log \theta dBi ( 1 \deg \leq \theta \leq 48\deg ); G = -10 dBi ( \theta > 48\deg ), where G is the sidelobe gain relative to an isotropic antenna and \theta is an offset angle from the main lobe axis in degrees. In the case of an axisymmetrical Cassegrain antenna operating with circular polarization, the level of the cross-polar sidelobe due to diffraction by the subreflector support struts exceeds that shown in the above reference diagram. By introduction of a new type of strut, the sidelobe gain can be suppressed below that shown in the CCIR reference diagram. In this article, a design method based on geometrical theory of diffraction (GTD) for improving the strut shape is presented. The wide-angle sidelobe gain of the standard " A " earth station antenna, installed in Yamaguchi, Japan, being equipped with the improved strut, can meet that shown in the CCIR reference diagram in all directions for both co- and cross-polar polarizations in the 6 GHz band.

Control of reflector antennas performance by rim loading

Abstract: An analysis of the behavior of reflector antennas loaded by a surface impedance along a peripherical rim is carried out in order to ascertain the possibility of improving their performance by such a loading technique. Both single and dual (Cassegrain) reflector antennas are considered. It is shown that an effective control of the radiated field can be achieved by the proper choice of the loading characteristics. In the case of Cassegrain antennas, subreflector loading is sufficient to enhance copolar or crosspolar overall performance.

Scan performance of dual offset reflector antennas for satellite communications

Abstract: The basic numerical steps of a method to analyze dual offset reflector antennas are reviewed. The technique employs an alternative approach to calculating the secondary pattern of an arbitrarily shaped offset dual reflector antenna, and is based on the application of the geometrical theory of diffraction from the subreflector and physical optics (PO) scattering from the main reflector. The PO integral is evaluated using the Jacobi-Bessel series expansions, and numerical results are presented for the off-axis scan performance of an offset clear aperture Cassegrain antenna. Results indicate that the equivalent paraboloid concept can be used to determine the location of displaced feed and define the local optimal focal plane, and the scan performance of the Cassegrain reflector can be improved if the feed is located on the true opimal surface.

GTD analysis of the near-field patterns of a prime-focus symmetric paraboloidal reflector antenna

Abstract: The uniform geometrical theory of diffraction (UGTD) has been applied successfully to analyze the near-field patterns of a prime-focus paraboloid. In order to establish the validity of the analysis, near-field amplitude and phase patterns have been computed over the principal planes at several observation distances for a typical prime-focus paraboloid. These calculations compare very favorably with the corresponding results obtained numerically with the aid of Silver's near-field aperture integration formula.

Antenna arrangement for providing a frequency independent field distribution with a small feedhorn

Abstract: The present invention relates to an antenna arrangement which uses an imaging reflector (10) combined with a small feed or horn (14) which is capable of launching or receiving a spherical wavefront (18) to obtain a nearly frequency independent field distribution over a large antenna aperture. The antenna arrangement comprises a parabolic main reflector (10) disposed confocally with one focal point of a subreflector means (12) and a feed (14) disposed with the apex of the spherical wavefront at the other focal point of the subreflector means and the aperture of the feed centered on the image of the main reflector. If the rim of the feed aperture corresponds to an image of the edge of the main reflector, spill-over is substantially eliminated. Generally, any feed arrangement comprising a feed and subreflector means which transforms a spherical wavefront from the feed into a spherical wavefront emanating from the focal point of the parabolic main reflector can be used.

Frequency spatial filter for two frequency microwave antenna - comprising double sandwich of metallic grids and dielectric sheets

Abstract: The filter can either separate or combine two electromagnetic waves of determinate mean angle of incidence in different frequency bands. A central sheet (2) of low-loss dielectric material (e.g. polyphenylene) carries on both faces (4,5) identical metallic grids of rectangles symmetrical about the plane of incidence (6). The central sheet is sandwiched between two others (1,3) of the same material or of other materials having the same characteristics. The combination reflects quasi-optically a beam of W-band (94 GHz) waves, and transmits without deflection a beam of X-band (10 GHz) waves arriving at the same angle of incidence on the opposite face. Also claimed are a Gregory offset antenna and a Cassegrain antenna, each incorporating such a filter.

Surface-current analysis of distorted reflector antennas

Abstract: An important requirement in satellite antenna design is the prediction of the antenna performance when it is placed in the space orbital environment. An accurate estimate of its performance deterioration, caused by antenna distortions which are induced by the temperature extremes encountered, should be obtained. A method suitable for this type of analysis of reflector antennas is presented. It involves the division of the reflector surface into triangular elements whose distortions are computed using a structural analysis program (NASTRAN). The currents flowing on each distorted elemental surface are then used to give the diffracted fields. The computed results of an offset fed reflector are given. A way is suggested, which makes use of monopulse tracking, to minimise the performance degradation caused by the surface distortion.

Array-fed reflector antenna design and applications

Abstract: One phase of the spacecraft antenna research carried out at the Jet Propulsion Laboratory for deep space and near-earth communications is described. Array-fed reflector antenna designs have been elaborated for beam scanning, power amplification, and multiple beam applications. The antenna designs derive from similar optical concepts. Each antenna comprises a relatively small plane-wave array feeding a near-field dual-reflector system. The main reflector and the the subreflector are confocal paraboloids having identical focal length to diameter ratios. The array feed is positioned in such a way that the subreflector is in the near-field (collimated beam) region of the feed. The field distribution of the feed aperture is thereby recreated approximately in the main reflector aperture. The detailed design concepts of the antennas (beam-scanning, high-power, and mutliple-beam) are discussed, and the calculated and measured RF performances of the antennas are presented.

Microwave antenna with radiation scattering support member elements

Abstract: An antenna, such as Cassegrain or parabolic antenna, intended for use in the microwave or millimeter wavebands having improved wide angle radiation characteristics. At least one edge portion, in first embodiments, of a supporting post structure which supports a primary radiator or subreflector is modified to randomly scatter waves so that the field strength of scattered waves due to the presence of the supporting post is decreased.

Jemcy conical receiving antenna

Abstract: An antenna having high gain and narrow beamwidth over broadband VHF and UHF portions of the electromagne-1ic spectrum comprises a biconical radiator/receiver (2) mounted substantially at the focal point of a parabolic reflector (3). Typically, the biconical radiator/receiver and the parabolic reflector are mounted on a supporting rod (1). Director rods (4,5) may also be provided on the supporting rod, the director rods being mounted on the side of the biconical radiator/receiver opposite to the reflector. The invention provides a low-cost high-gain antenna particularly suitable for television reception.

30/20 GHz Band Small Earth Station for ISSDN Experiment

Abstract: The system design and operational results of a 30/20 GHz band small Earth station for integrated services satellite digital network (ISSDN) experiments are presented. The Earth station has a high efficiency offset Cassegrain antenna with elliptical beam, a high power klystron amplifier with 1 kW output power and an uncooled parametric low noise amplifier with 200 K noise temperature. Experiments were performed using the medium capacity communications satellite for experimental purposes (CS). Performance on the Earth station was high. Effective radiation power (ERP) was greater than 88.1 dBW, and the receiving figure of merit (G/T) was larger than 30.6 dB/K. Good transmission characteristics were obtained through the satellite link for demand assigned time-division multiple access (DA-TDMA) with 20 Mbit/s bit rate.

Frequency band shared antenna

Abstract: PURPOSE:To obtain the wide-angle radiation characteristics, using a metallic rod having the small unevenness to the wavelength at a low frequency band and the unevenness larger than the wavelength at a high frequency band for a VHF band antenna element that is provided on the secondary reflector of a Cassegrain or Gregorian antenna. CONSTITUTION:A parabola antenna which receives the feed through a linear antenna 4 provided on a secondary reflector 2 with the frequency of the VHF band and is reflected by a main reflector 1 is also used as a Gregorian antenna which receives the feed through a primary horn antenna 3 with the frequency of the microwave band or millimeter wave band and is reflected by the reflectors 2 and 1. Dipoles 5a and 5b plus reflectors 6a and 6b are set to the antenna 4 with <=45 deg. angle to the travelling direction of the electric wave of a high frequency band. At the same time, a metallic rod having the unevenness which is small and large to the wavelength of the low and high frequency bands respectively are provided to the antenna 4. Thus the scattered waves caused by a linear antenna of the electric wave at a high frequency band are radiated within a specific range to obtain the wide-angle radiation characteristics in other ranges.

Near field cassegrain antenna - has lobe widened using extra reflector or dielectric lens in path of primary radiation

Abstract: The antenna consists of a primary radiator (1)(eg a horn), an auxiliary reflector (2), a collecting reflector (3) and a main reflector (4). The auxiliary reflector is located behind a central hole (5) in the main reflector. A dielectric lens (6) or an extra reflector (14)is used to widen the radiation lobe and is located behind the central hole (5) in the main reflector. The lens or extra reflector has its focus arranged such that solely a part of the collecting reflector, and therefore also a part of the main reflector, is irradiated. The advantage lies in avoiding using moving parts other than those needed to insert the lens or extra reflector into the radiation path.

New approach to compact measurements on reflector antennas

Abstract: The letter describes a novel scheme for very compact measurements on reflector antennas. The measurement configuration involves a plane reflector located just beyond (and parallel to) the aperture plane, so that radiated power is reflected and refocused back into the feed. Measurement of return loss can then give information on antenna efficiency and gain. The basic concept is described and its performance confirmed by means of experiments on a 10 ft (3.048 m) diameter reflector. Possible methods of extending the performance of the scheme are briefly discussed.

Collapsible antenna reflector

Abstract: According to the invention, this reflector comprises an articulated structure (11, 13) on which a reflective cap (15) can be mounted, the shape of which is adjustable thanks to tension links (16). Equipment of artificial satellites.

Generation of Highly Contoured Beams by Shaping the Surface of an Offset-Fed Single Reflector

Abstract: The surface of an offset reflector illuminated by a single balanced feed is shaped so that the modified phase of the aperture field generates the desired contoured beam The deviations from the paraboloidal surface are so small that the amplitude distribution in the aperture plane remains essentially undistorted and only negligible cross polarization is generated

An antenna pointing mechanism for large reflector antennas

Abstract: An antenna pointing mechanism for large reflector antennas on direct broadcasting communication satellites was built and tested After listing the requirements and constraints for this equipment the model is described, and performance figures are given Futhermore, results of the qualification level tests, including functional, vibrational, thermovacuum, and accelerated life tests are reported These tests were completed successfully

Scan Properties of the Schwarzschild Antenna

Abstract: A theoretical investigation of the defocused Schwarzschild antenna system is described. Scan properties have been compared with those of the Cassegrain antenna system. It is shown that both the focused and defocused radiation patterns of the two systems are almost identical.

Primary radiator for duplex reflecting mirror attenna

Abstract: PURPOSE:To enable to supply the plane waves having symmetrical liminous intensity distribution from the location independently of the rotation of the antenna, by specifying the location and shape of the reflecting mirror provided with the flat mirror, between the subreflecting mirror and the cone horn for the earth station of satellite communication. CONSTITUTION:In the primary radiator 1'', the ellipse reflecting mirror E1 and reflecting mirror F4 are inserted between the parabola reflecting mirror P1' and core horn L, and the flat mirror F3 is provided between the reflecting mirror P1' and the subreflecting mirror 2 of a Cassegrain antenna. The relation of the angle alpha between the rotating shaft S of said reflecting mirror P1' and the rotating shaft T, the angle beta between the rotating shaft T and the mirror image U to the flat mirror F4 located at the center axis of the horn L, and the eccentric rate of the reflecting mirror E1 is restricted so that the geometrical and optical assymmetry between the reflecting mirror P1' and E1 can be cancelled. Thus, the flat wave plane 5 having axially symmetrical luminous intensity distribution in which the cross polarized wave component is suppressed, can be obtained. Said horn L is fixed independently of the rotation of the antenna with the rotation mechanisms R3, R4.

Near field Cassegrain antenna - has lobe widened by moving region of auxiliary reflector between horn radiator and collecting reflector

Abstract: The antenna consists of a primary radiator (1),(eg a horn), an auxiliary reflector (2), a collecting reflector (3) and a main reflector. The auxiliary reflector is located behind a central hole (5) in the main reflector. A movable concentric region (6) is located on the auxiliary reflector. The magnitude of the movement is chosen so that the reflected wave undergoes a phase shift amounting to 120 to 180 degrees at the antenna's aperture depending on the diameter of the main reflector. The radiation lobe of the antenna can thereby be widened without mechanically affecting the collecting reflector and without defocussing the exciter.

A 12m Cassegrain Antenna with "Off-Set" Feed System for Two Frequency Ranges and Monopulse Tracking with Bearing Error Correction

Abstract: A 12m Cassegrain antenna for the space monitoring service is described, which is designed for an overall bandwidth of 1.5 to 13 GHz. With the present receive equipment it can be operated alternatively in three different frequency bands (1.52-1.79 GHz, 2.1-2.3 GHz, 10.95-11.8 GHz). A novel offset feeding principle makes it possible to switch between these bands within < 1 min, the retrofitting of additional bands is planned in the future. The accuracy of the monopulse tracking (TE21-modes) is reduced by the influence of certain interference modes which are internally generated by the feed system. A special passive network is finally discussed which compensates the resulting pointing errors.

Near field Cassegrain antenna - has collecting reflector with circular segment steps movable by flap device to produce 180 degree phase shift

Abstract: The antenna consists of a primary radiator (1) (eg a horn), an auxiliary reflector (2), a collecting reflector (3) and a main reflector (4). The auxiliary reflector is located behind a central hole (5) in the main reflector. The collecting reflector has quarter-wave steps to introduce a phase shift of 180 degrees. The steps are formed by segments (12) of a circle centered on the collecting reflector and moved by a flap device (13-16). When in operation, the stops lie on the collecting reflector, and when not in operation they lie outside the paths of the radiation between radiator and collecting reflector and between collecting reflector and main reflector.