Showing papers on "Feed horn published in 1977"

Reflection, transmission, and mode conversion in a corrugated feed

Abstract: Microwave antennas are often required to carry signals simultaneously over a broad range of frequencies–e.g., the combined td-2 and th common carrier bands encompass a total frequency ratio of about 1.8 to 1 as do the combined 18- and 30-GHz bands. To achieve these bandwidths, an efficient broadband feed horn is required. The corrugated (hybrid-mode) horn is a leading candidate, but it is not immune to some cross-polarization coupling, input reflection, and pattern asymmetry. These problems are introduced mainly by two phenomena: variation of the dominant mode shape with frequency and mode conversion along the horn taper and at waveguide transitions at the horn input. Simple formulas for computing the magnitude of these phenomena and their effects on return loss and radiation patterns are given.

On the cross polarization of asymmetric reflector antennas for satellite applications

Abstract: It is well known that focussed, axial symmetrical reflector antennas collimate the co- and cross-polar components of the primary field separately, i.e., the reflector does not create a contribution to the cross polarization of the far-field. By a simple extension of a classical physical argument it is demonstrated that this separability does not depend on the symmetry of the antenna, and that it, therefore, holds even for off-set fed reflectors. A new mathematical formulation of the collimation is derived in which this is shown. Yet the separability does depend on how the co- and cross-polar fields are defined, and the cross polarization of feeds for asymmetric reflectors is discussed in detail in the light of this. It is further suggested how to design low cross polarization feeds for off-set fed antennas. As a consequence of the separate collimation such feeds will lead to low cross-polarization of the secondary fields. Two simple examples are treated. The only limitations of the results are those due to the application of the aperture field version of the physical optics approximation.

Dual mode feed horn

Abstract: A dual-mode feed horn for microwave antennas includes a multi-step microwave transformer having a series of abrupt steps with progressively increasing radial dimensions. At least certain of the steps have dimensions sufficiently large to convert TE 11 mode energy passing therethrough to TM 11 mode energy. The transformer is preferably a binomial transformer, and the axial length of the transformer is preferably about equal to the number of steps therein multiplied by 1/4 of the average wavelength of the microwave energy to be passed therethrough. A pair of waveguides are connected to opposite ends of the transformer for transmitting microwaves through the transformer, and the waveguide connected to the larger-diameter end of the transformer has an inside diameter at least as large as the maximum inside diameter of the transformer and a length sufficient to produce a predetermined phase relationship between the TE 11 mode energy and the TM 11 mode energy.

Ultrasonic antenna assembly

Abstract: In an ultrasonic antenna assembly specifically adapted for use in a material level control system and of the type which includes a parabolic reflector and an ultrasonic energy source mounted at the parabolic focus and directed to radiate energy toward the reflector surface from the direction of the parabolic axis, the improvement comprising a sonic absorber of low-density fiberglass disposed at the reflector surface opposite the energy source to absorb energy directed onto said surface from said source, such that multiple reflections between the energy source and the reflector surface are inhibited. In a modified embodiment the parabolic reflector and energy source are adapted to be mounted in the upper portion of a storage tank such that the parabolic axis is substantially horizontal, and a plane reflector is disposed adjacent the parabolic reflector at an angle of forty-five degrees with respect to the parabolic axis.

Cross-polarization in satellite and earth-station antennas

Abstract: Cross-plarization in axially symmetric reflector antennas can be reduced, theoretically, to zero by use of special feeds like the Huygens' source. Alternatively, paraboloidal reflectors with large f/D ratio do not deteriorate further the cross polarization level relative to the value due to the feed itself. The Cassegrainian optics is equivalent to a large f/D paraboloid. The reflector of linearly polarized off set fed antennas contribute more cross-polarization than symmetrical reflectors fed by the same feed. With symmetrical reflectors the cross-polarized component generated by the reflector vanishes in the principal planes and is confined to four main lobes that have peak values in planes at 45° to the principal planes. In the case of offset fed reflectors cross-polarization vanishes in the plane of symmetry and has its peak in the plane of asymmetry. The reflector generated cross-polarization with offset fed antennas may be reduced by use of small offset angles and large f/D ratios. Feed offsetting has but little effect on the peak level of cross-polarization. This is usually accompanied with an asymmetry in the cross-polarization radiation pattern. Feed offsetting also results in spatial tilt in the copolarized and cross-polarized lobes with the cross-polar minimum always coinciding with the main beam peak. The effect of surface errors on the antenna cross-polarization is to partly fill the cross-polar along boresight. The peak cross-polarization, however, changes but slightly.

Redundant rf system for space application

Abstract: An S-band radio frequency subsystem including two transmitters, two receivers and two antennas and capable of connecting either transmitter or receiver to either antenna while permitting simultaneous operation of a transmitter and a receiver. Circulator switches provide selection of a specific transmitter and receiver for connection to either a high gain or low gain antenna. Transmitter output filters, receiver input filters, and diplexers are combined to prevent radiation or coupling of unwanted transmitter and receiver signals and to provide isolation, permitting simultaneous operation of the transmitter and receiver. The filter elements are designed of constant diameter coaxial elements to meet demanding rejection, loss, power-handling and environmental characteristics.

Antenna with two orthogonally disposed parabolic cylindrical reflectors

Abstract: An antenna set-up comprising a first and a second reflector in the form of substantially parabolic cylinder surfaces and a feed source, directed to the first reflector and placed on the focal line of that reflector and in which set-up the directrix of the cylindrical wave front obtained by exposure of the first reflector coincides with the focal line of the second reflector and the focal line of the first reflector is not parallel to the symmetry plane of the second reflector.

Reflex feed system for dual frequency antenna with frequency cutoff means

Abstract: A reflex feed system for a dual frequency antenna which may be, for example, one which transmits and receives both S and X-band signals, uses, in place of the dichroic plate normally employed for directing X-band radiation away from the X-band horn a flange about the opening of the X-band horn.

Focal region fields of dual reflector antennas

Abstract: A simplified analytical method to predict the field distribution in the focal region of dual reflector antennas having circular symmetry is presented. The method is used to study the field distribution at the subdish and in the focal plane, and hence the ability of the antenna in focusing the incident wave. Experimental results obtained on a 2.6-m spherical Gregorian reflector operating at 9.0 GHz agree well with theoretical ones.

Near-Field Gain Correction for Transmission between Horn Antennas

Abstract: When the gain of a horn antenna is to be measured from the power transmission loss between it and a standard horn, the Friis transmission formula must be corrected if the horn separation is not considerably greater than 2d2/?. Expressions for the correction between a pyramidal horn and a conical horn, and between dissimilar pyramidal horns, are presented with sample results. A method of applying these corrections to minimize the errors in the horn gain measurements is described.

Shaped-Beam Antenna with low cross polarization

Abstract: It is well known that low cross polarization may be obtained in a conventional high gain antenna with rotational symmetry when a Huygens source is used to illuminate the reflector surface. In satellite communication links it is, however, often desireable to have an elliptic beam with extremely low cross polarization. The present paper describes a method by which such a beam may be realized.

Near-field distribution of a paraboloidal reflector

Abstract: A major problem in communication antenna design is to obtain maximum gain and efficiency from large reflector antennas; few paraboloidal or dish antennas have efficiencies greater than 70 percent. The basic problem is the spreading of power by the source feed located at or near the focus of the reflector, which causes a concentration of power near the center of the aperture. In order to determine what kind of field distribution is required in the feed region to produce a uniform distribution over the reflector the authors have studied the inverse problem of a plane wave incident normally upon a paraboloidal reflector

Parabolic horn antenna with Cassegrain supply - has reversing system of two auxiliary reflectors between feed horn and main reflector

Abstract: The antenna is intended for very short electromagnetic waves. Its reflector is in the form of a parabola sector. A feed horn has a single deflection, contains the feed point and extends up to the reflector. The antenna feed supply is of Cassegrain type, in which between the feed horn (5) and the main reflector (3) is provided a reversing system of two auxiliary reflectors (6, 7) for double beam reversing. The feed horn, the reversing system and the main reflector are in the form of a compact, enclosed module which has an output aperture (4). The auxiliary reflectors may be in the form of parabola sectors. The first auxiliary reflector (6) focus lies in the feed point (F2), while the focus of the second auxiliary reflector (7) coincides with that of the main reflector.

Degradation in the efficiency of medium-size parabolic antennas with propagation conditions

Abstract: The results of a simple, short-term experiment suggest that significant errors may arise in the overall design and costing of a communication system where parabolic antennas are employed at the receiver terminal. Under high signal conditions the relative gains of two parabolic antennas of different diameters are in close agreement with the theoretically predicted value. Under low signal conditions, when a larger diameter antenna might be preferred in practice, there appears to be a considerable reduction in the proportion of power collected by a large antenna in relation to the proportion collected by an antenna of significantly smaller diameter.

High-efficiency spherical reflector antenna--A case study

Abstract: With spherical antennas, the basic problem is to eliminate the effect of spherical aberration. Although various methods are available, this paper discusses the use of field matching techniques to design high-efficiency corrugated feeds for a 9-ft spherical dish. Both single and dual-reflector antennas are considered, but the emphasis is on the single-reflector system.

80 GHz Radiometer Calibration System

Abstract: A calibration system for an 80 GHz radiometer has been designed and constructed. During observations, calibration is obtained from a noise valve, the noise of which is fed to the radiometer via a beam switch. The calibration signal is controlled by a mechanical waveguide switch. It has been confirmed by experiment that, at millimetre wavelengths, noise valves need careful calibration. For this purpose, one can use the temperature difference of boiling argon and nitrogen. Alternatively, one can use a calibrated thermometer to measure the temperatures of two thermal radiators. In both cases the absolute calibrator is placed before the feed horn. The reference beam observes radiation coming from a piece of microwave absorber at ambient temperature. The positioning of this piece is crucial.