GB2052875A

GB2052875A - Dipole/reflector assembly

Info

Publication number: GB2052875A
Application number: GB8017313A
Authority: GB
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1979-06-22
Filing date: 1980-05-27
Publication date: 1981-01-28
Also published as: DE2925158C2; NL8002314A; FR2460051B3; DE2925158A1; NO152229B; NO801856L; FR2460051A1; NO152229C

Abstract

A row of crossed dipoles (1 . . . i. . . n) for circular polarisation is mounted upon a flat reflector (11) which serves to reflect horizontal polarisation components and which supports, at equal spacings at each side of the dipole row, conductor elements (21, 22) which are designed for reflection of vertical polarisation components by forming the reflector as a corner reflector. The height of the conductor elements (21, 22) above the reflector 11 is conveniently smaller than one half of the operating wave length; the spacings of said elements (21, 22) are advantageously 0.45 of operating wave length. <IMAGE>

Description

SPECIFICATION Dipole/reflector assembly This invention relates to an assembly comprising a row of crossed dipoles (or aerials or antennae) and a reflector for radiating a circularly-polarised wave composed of two linearlypolarised waves.

Present in the upper atmosphere are various ionised layers which contain free electrons and positive ions in fairly large numbers. The ionisation occurs mostly as a result of photoelectric effects, i.e. as a result of the action of short-wave ultraviolet solar radiation, which is absorbed completely in the upper atmosphere.

Since the air density at these heights is very low, the charges re-combine very slowly, so that a high ionisation equilibrium value occurs. In the highest layer, this disappears so slowly that a residue persists throughout the entire night.

In the daytime, subdivision into three layers, designated D, E and F, is possible. Crucial for the propagation of electro-magnetic waves is the outer F layer, which is designated as the F-2 layer; its ionisation state does not follow a simple law. The course of the ionization during the day differs considerably according to season, degree of longitude, and degree of latitude.

An accurate knowledge of the property of the outer ionosphere and magnetosphere is necessary for radio communications with satellites and space vehicles. For their investigation, an incoherent scatter technique (scattering) is employed. In this method, short-wave radiation is directed into the atmosphere by a high-power transmitter and a strongly-bunching or concentrating antenna. Part of the radiated energy is scattered back by the cloud-like upper edge of the E-layer, after reflection on the high F-2 layer, to the place of transmission. Conclusions regarding the behaviour of the F-2 layer can be derived from the magnitude of the back-scattering. More especially, in this respect, rotation of the direction of polarisation is of importance.For the investigation of the outer ionosphere, therefore, strongly-bunching antennae are necessary, which can transmit and receive with equal efficiency in all of the polarisation directions, i.e. linearly (horizontally and vertical), circularly to the left and circularly to the right.

However, radar equipment having purely circular polarisation is necessary not only to investigate the ionosphere with the aid of radar equipment, but also in other instances, for example to suppress rain echos. Moreover, for radio communications, more especially to satellites and space vehicles, preferably circular polarisation is used.

It is, for example, known to use crossed dipoles for radiation of linearly-polarised or circularly-polarised electro-magnetic waves. If a strongly-bunched or concentrated radiation diagram is intended to be achieved, then a plurality of crossed dipoles have to be arranged in the form of a row in front of a reflector. In this respect, however, the diagrams, radiated by the two dipoles, for longitudinal polarisation (i.e. in the direction of the co-linear dipoles) and the transverse polarisation (i.e. in the plane perpendicular to the colinear dipoles) are not the same. On the contrary, the diagram for the longitudinal polarisation in the plane perpendicular to the colinear dipoles has a greater radiation width.

An antenna of this kind, however, is not suitable for example for the investigation of the outer ionosphere. More especially it is thus not possible to use a linear cross-dipole array as a primary radiator for a mirror antenna to achieve a stronger directional characteristic. In order to achieve a high aperture efficiency for circular polarisation, it is, on the contrary, necessary for the radiation diagrams to have the same shape in the plane perpendicular to the focal line for both polarisation directions.

The object of the invention is, therefore, to provide a linear cross-dipole array with a reflector for radiation of a circularly-polarised waved composed of two linearly-polarised waves which, with the aid of simple technical means, for the two linear polarisations has radiation diagrams having the same shape in the plane perpendicular to the co-linear dipoles and which if used in connection with a mirror results in a very high aperture efficiency with favourable mirror dimensions as well as a good circular polarisation of the overall antenna; moreover, the difference in impedance between the dipoles radiating the longitudinal and the transverse polarisation is to be matched.

Starting from a linear crossed-dipole assembly of the kind mentioned at the introduction of this specification, this problem is solved in that the reflector, for transverse polarisation, is designed as a plane or flat reflector and at the same time, by means of mounting thereon of conductor elements is designed, for longitudinal polarisation, as a corner reflector.

Advantageous developments of the invention are defined in the sub-claims appended hereto.

The advantages achieved with the invention consist more especially in that, as a result of the adaptation of the shape of the two radiation diagrams, produced by the longitudinal and transverse dipoles, in the plane perpendi cular to the collinear dipoles, a high efficiency circular polarisation is achieved. If the linear cross-dipole array is used as primary radiator of a cylindrical parabolic antenna with offset (or decentered) feed, then a considerably higher aperture efficiency is achieved. Accordingly by using the linear crossed-dipole as sembly in accordance with the invention as a primary radiator mirror antennae of smaller dimensions than has hitherto been possible can be constructed. More especially in the case of VHF antennae having large mirror diameters, the invention enables a considerable saving in material and costs to be achieved.As a result of the obviation of differences in impedance between the dipoles radiating the longitudinal polarisation and the transverse polarisation, moreover the overall efficiency of such antennae is further increased.

Two exemplified embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figure la is a side or sectional view of a preferred embodiment of the linear crosseddipole assembly in accordance with the invention; Figure ib is a plan view corresponding to Fig. 1 a; and Figure 2 is a fragmentary side eievation illustrating a cylindrical parabolic antenna having a linear crossed-dipole assembly in accordance with the invention as its primary radiator.

In the embodiment shown in Figs. 1 a and 1 b, crossed dipoles 1, 2. . ., i, . . nare arranged in a row on a plane or flat reflector 11. Disposed symmetrically and parallel to the collinear dipoles of the crossed dipoles 1, 2 . . ., i, . . ., n, are rod-shaped conductor ele- ments, 21, 22 carried on dielectric supports 23, 24 so as to extend at a height h above the reflector 11. The height h, as well as the spacing a from the collinear dipoles, of the rod-shaped conductor elements 21, 22, amounts preferably to 0.45 of the operating wave length A. The spacing of the crossed dipoles 1, 2 . . ., i, . . ., n from one another amounts to 0.7 of the wave length A.

Mounted parallel to the collinear dipoles of the crossed dipoles 1, 2 . . ., i, . . ., n are, additionally, further rod-shaped conductor elements 31, 32, which are connected in electrically-conductive conductive manner to the screening outer holders or mounting supports or the symmetrisation of the crossed dipoles 1, 2 . . ., i,. . ., n, and are at a small spacing from the reflector 11. An electrical connection is, however, not absolutely necessary. The spacing from reflector 11 preferably amounts to approximately 0.1 of the operating wave length A. The diameters of the rod-shaped conductor elements 21, 22; 31, 32 amounts to approximately 0.025 of the operating wave length A. Furthermore, the rod-shaped conductor elements 21, 22; 31, 32 are approximately of the same length as the reflector 11.

Fig. 2 shows, as a further exemplified embodiment, the arrangement of the linear crossed-dipole assembly, in accordance with the invention, as primary radiator of a cylindrical parabolic antenna. In this respect, the dipoles, radiating the longitudinally-polarised wave, of the crossed dipoles 1, 2,.

i, . . ., n are arranged in the focal line of cylindrical parabolic reflector 12 and the dipoles, radiating the transversely-polarised wave, of the crossed dipoles 1, 2 . . ., i, . . ., n are arranged perpendicularly thereto. The radiation of the circularly-polarised electromagnetic wave is effected in the direction towards the cylindrical parabolic reflector 1 2. The dimensions of the cylindrical parabolic reflector 12 have been so selected, in the exemplified embodiment, that the quotient of the length of the reflector in the focal line and its diameter perpendicular thereto amounts to approximately 0.45.In this way, in addition to a high mechanical stability, an optimum aperture efficiency of the antenna of aproximately 90% is achieved with pre-supposed uniform excitation of the linear crosseddipole assembly.

The mode of operation of the linear crossdipole assembly, in accordance with the invention, for the radiation of a circularly-polarised wave consists in using a kind of polarisation-dependent reflector which acts for the transverse polarisation as a plane reflector and acts, at the same time, for the longitudinal polarisation as a corner reflector. This is achieved by the mounting of the rod-shaped conductor elements 21, 22 above a plane reflector 11. The rod-shaped conductor elements 31, 32 serve for the impedance matching of the longitudinal and transverse dipoles.

They act, for the longitudinal polarisation, as a shifting or displacement of the reflector 11 without, however, affecting the impedance and the radiation diagram of the transverse polarisation. The influence of these conductor elements 31, 32 on the radiation diagram of the longitudinal polarisation is negligibly small.

The linear cross-dipole assembly in accordance with the invention is particularly suitable as primary radiator of mirror antennae with a focal line or focal plane.

Claims

1. A dipole/reflector assembly comprising a row of crossed-dipoles and a reflector for radiating a circularly-polarised wave composed of two linearly-polarised waves, characterised in that the reflector is designed as a flat reflector for transverse polarisation, and, at the same time, by means of the mounting thereon of conductor elements is designed as a corner or angular reflector for longitudinal polarisation.

2. An assembly as claimed in claim 1, characterised in that the conductor elements are each of rod shape, are arranged symmetrically and parallel relative to the coliinear dipoles of the crossed dipoles and in that they extend, at a specific spacing from the reflec tor, parallel to the latter.

3. An assembly as claimed in claim 2, characterised in that the conductor elements are arranged at a height above the reflector which is smaller than half of the operating wave length (A) of the crossed dipoles.

4. An assembly as claimed in claim 3, characterised in that the spacing from the reflector of the crossed dipoles is equal to i of the operating wave length, in that at each side of the collinear dipoles is a respective rod-shaped conductor element which is arranged at a distance (a) from the row of dipoles of 0.45 of the operating wave length (A), and in that the two conductor elements are at a spacing (h) from the reflector of 0.45 of the operating wave length (A).

5. An assembly as claimed in any preceding claim characterised in that there are mounted, at both sides of the collinear dipoles and parallel to the row additional rod-shaped conductor elements which are at a small spacing from the reflector.

6. An assembly as claimed in claim 5, characterised in that the additional rod-shaped conductor elements are arranged at a distance equal to approximately 0.1 of the operating wave length (A) from the reflector.

7. An assembly as claimed in any of claims 2 to 6, characterised in that the rodshaped conductor elements each have a diameter of the order of magnitude of 0.025 of the operating wave length (A).

8. An assembly as claimed in any preceding claim characterised in that the conductor elements are of the same length as the reflector.

9. An assembly as claimed in any preceding claim characterised in that the dipoles, for radiating the longitudinally polarised wave, of the crossed dipoles are arranged parallel in the focal line of a cylindrical parabolic reflector and the dipoles, radiating the transverselypolarised wave, of the crossed dipoles are arranged perpendicularly thereto, the radiation being directed towards the cylindrical parabolic reflector.

10. A dipole reflector assembly substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.