Microwave Antenna Structure With Aperture Blocking Elimination

Abstract

An aplanatic multi-beam antenna free from aperture-blocking effect comprises a convex main reflector and a concave subsidiary reflector whose axes orthogonally intersect at a planar network of parallel wires including an angle of 45.degree. with these axes. The wire network discriminates between waves polarized in two mutually orthogonal planes, passing one type of wave and reflecting the other. Each curved reflector is provided with means for rotating the plane of polarization of an incident wave through 90.degree. upon reflection. The reflectors form a focusing system with a focal plane containing one or more transducers for emitting or receiving radiation of a polarization passing the discriminator before striking the first reflector whence they are directed, via the reflecting discriminator, to the second reflector; upon leaving the latter reflector, they have a direction of polarization enabling them to clear the discriminator once more.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a microwave antenna structure and refers more particularly to the production of a multi-beam aplanatic antenna.

The need for such an antenna makes itself felt in the field of space communication, for instance, where it is required that a satellite transmits information simultaneously and independently to a plurality of ground stations spread out over a plurality of areas. These transmissions, carried out on the same frequency, involve a plurality of antennas operating at the same frequency, each antenna having a radiation diagram with a main lobe and side lobes. Since it is necessary that the radiation be simultaneous and independent at the same frequency for each antenna, it is desirable that, for each such antenna, the level of the side lobes is as small as possible so that these lobes do not interfere with neighboring main lobes relating to the other antennas.

In these antennas mounted on satellites it is also advisable that the focusing system use should satisfy conditions of aplanatism, i. e. that a slight displacement of the primary source or sources from the focus of the system does not bring about a deterioration in the parallelism of the rays transmitted by the system. Such a deterioration would result in an enlargement of the main lobe and the appearance of aberration secondary or side lobes.

The requirements for maintenance of the side lobes at a low level and aplanatism are met by giving the antenna a specific configuration.

A multi-beam, aplanatic antenna, the so-called Schwarzschild antenna, is known which is derived from the so-called Cassegrain antenna and is formed by two surface revolution reflectors, namely a main reflector and an auxiliary reflector or subreflector having the same axis and meridians precisely determined so that the assembly is aplanatic.

Just the same, the Schwarzschild antenna is unsuitable for the purpose of the present invention. In fact, this antenna has a masking or aperture blocking effect due to the fact that the sources are generally situated near the main reflector and also to the presence of the subreflector in front of the main reflector. This masking effect tends to increase the level of the side lobes.

The object of the invention is to overcome the drawbacks which have been pointed out and to provide a twin-reflector antenna of the multi-beam type which is aplanatic and causes no aperture blocking.

A microwave antenna embodying our invention comprises two conjugate reflectors whose meridians are such that they fulfill the condition of aplanatism and whose axes of symmetry intersect; between the sources and the focusing system formed by the two reflectors there is interposed a member which discriminates between differently polarized electromagnetic waves, passing waves polarized in one plane and reflecting those polarized in another plane.

BRIEF DESCRIPTION OF THE DRAWING

Other objects and advantages of the invention will become apparent from the following description of an embodiment and from the attached drawing in which:

FIG. 1 shows a so-called Schwarzschild antenna according to the prior art;

FIG. 2 shows an antenna according to the invention;

FIG. 3, is view of the system of FIG. 2, showing the construction of the beams traversing the focusing system, and

FIG. 4, is a radiation diagram obtained with the antenna according to the invention.

DESCRIPTION OF THE PRIOR ART

The so-called Schwarzschild multi-beam, aplanatic antenna is a twin-reflector antenna belonging to the class of twin-reflector antennas of which the best known is the Cassegrain antenna.

Such a Cassegrain or Schwarzschild antenna is shown in FIG. 1. It comprises a main reflector 1 and an auxiliary reflector 2 or subreflector whose axis coincides with the axis of the main reflector. A source 3 is situated at the apex S.sub.1 of the main reflector, this apex generally coinciding with a focus of the subreflector which, in the case of the Cassegrain antenna, is a hyperbolic segment whereas the main reflector is a parabolic segment. These reflectors are so mounted with respect to one another that a beam of parallel rays directed onto the main reflector 1 by an object situated on the axis at infinity is reflected toward the focus F of the main reflector and intercepted by the subreflector 2 which causes it to converge onto the source 3.

Conversely, a beam emitted by primary source 3 toward the subreflector 2 is reflected toward the main reflector 1 which in turn reflects it in the form of a beam of rays parallel to the axis of the focusing system.

DESCRIPTION OF A PREFERRED EMBODIMENT

Certain modifications have already been suggested on to these antennas and particularly on Cassegrain antennas to reduce the aperture-blocking effect which increases with the size of the subsidiary reflector or subreflector. These suggestions include a reduction in the dimensions of the subreflector and; an attempt to make the feed more directive by moving it closer to the subreflector. It is also possible to build a semi-transparent subreflector which reflects the beams issuing from the source with a certain polarization and allows beams to pass whose polarization has been turned by 90.degree. at the main reflector, but, in this case, the aperture blocking due to the primary source persists. Therefore, these measures are inadequate to decrease the level of the secondary lobes as required for certain applications. Moreover, since the main reflector is parabolic it does not fulfill the condition of aplanatism. The Schwarzschild antenna overcomes this defect of the Cassegrain antenna by utilizing a main reflector and a subreflector whose shape in slightly different from that of the reflectors of the conventional Cassegrain antenna, the meridians of these reflectors being calculated in a manner designed to satisfy the conditions of aplanatism.

Thus, we have ascertained that the Schwarzschild antenna is aplanatic but still has secondary lobes of too high a level for the application envisaged for the present invention.

FIG. 2 shows a way of producing an aplanatic antenna, which has a multi-beam pattern with low-level secondary lobes, according to our invention.

This antenna comprises two conjugate reflectors, i.e. a main reflector 1 and an auxiliary or subreflector 2 which make up the focusing system of the antenna. The two reflectors have a shape comparable to that of the reflectors in the Schwarzschild antenna.

Under these conditions the focusing system is aplanatic. The axes of these reflectors intersect at a point A Figure are mutually perpendicular. A planar member 4 traverses this point and in the Figure lies at 45.degree. with respect to the axes of the reflectors. This member is formed by a network of parallel metal wires 41 perpendicular to the plane of the two axes. It thus discriminates between differently polarized incident electromagnetic waves, passing those polarized in a plane transverse to its wires and reflecting those polarized in a plane parallel thereto. The feeder 3 comprises three wave transducers 30, 31, 32 located opposite the subreflector in a focal plane of the system 1, 2 so that the waves which they emit or which they receive from an object situated at infinity pass through the polarization discriminator 4.

Since the operation of the antenna according to the invention depends upon the polarization of the waves, the main and subsidiary reflectors are provided on their active surfaces with respective twist reflectors 11 and 21. Such a twist reflector, formed by a network of parallel wires inclined at 45.degree. with respect to the direction of polarization, is placed at a quarter wavelength from the reflecting surface, and is equivalent to a quarter-wave plate rotating the plane of polarization of the incident waves by 90.degree..

The layout of the various components of the antenna system according to FIG. 2, clearly shows that the aperture blocking due to the sources and the subreflector, which generates side lobes, no longer exists. The antenna system according to the invention is thus indeed aplanatic and its radiation diagram has low-level side lobes. FIG. 4 shows such a diagram for two antennas according to the invention.

The operation of such a system is as follows, described with reference to FIG. 3.

A beam of rays B-C is emitted by the source with a polarization P parallel to the plane of the Figure. In this case the parallel wires forming the conductor array 4 which allow such a beam to pass are perpendicular to the plane of polarization P. This beam is reflected by the convex side of subreflector 2 at its point of impingement C in the direction C-E. In passing through the twist reflector 21, the polarization of the reflected beam is rotated by 90.degree. thus becoming perpendicular to the plane of FIG. 3, as indicated at P.sub.1 and therefore parallel to the direction of the wires 41 of the planar network 4. Under these conditions, at point E the beam C-E is reflected from the plane of discriminator 4 along line E-F point F being the point of impingement on the main reflector 1. At this point F the concave side of incident beam is reflected in the direction parallel to the axis S.sub.1 -A of the main reflector. The twistreflector 11 at the surface of the main reflector rotates the polarization of the beam E-F by 90.degree. into a plane P.sub.2 perpendicular to the wires of the network 4 which therefore allows the beam F-G to pass through it. Beam F-G is of telecentric character, i.e. it consists of parallel rays converging at infinity.

Another beam HIJKL emitted from source 32 follows a similar path and is reflected at K in direction K-L.

Conversely, by reason of the reciprocity theorem, beams parallel to the axis S.sub.1 -A emitted by an object situated at infinity will converge after having passed through the system of FIG. 3, at the location of the source 30.

It is to be understood that modifications and variations of the described embodiment of our invention are possible, in conformity with the foregoing teachings, within the scope of the appended claims. What is claimed is:

Claims

1. An antenna structure comprising:

conjugate first and second reflectors with intersecting axes defining a focal plane for a beam of microwave frequency passing between a remote point and said first reflector, the point of intersection of said axes lying between said focal plane and said second reflector;

transducer means for microwave energy at said focal plane; and

discriminating means at said point of intersection for selectively passing and reflecting differently polarized microwaves, each of said reflectors being provided with polarization-changing means for directing a beam with a polarization passed by said discriminating means, incident upon one of said reflectors, back to said discriminating means for reflection onto the other of said reflectors whence the beam is redirected to said discriminating means with a polarization enabling its passage

2. An antenna structure as defined in claim 1 wherein said first reflector has a concave surface and said second reflector has a convex surface

3. An antenna structure as defined in claim 1 wherein said discriminating

4. An antenna structure as defined in claim 3 wherein said conductors

5. An antenna structure as defined in claim 3 wherein said axes intersect orthogonally, said array including an angle of 45.degree. with each of

6. An antenna structure as defined in claim 3 wherein said transducer means comprises an emitter of microwaves with a plane of polarization

7. An antenna structure as defined in claim 3 wherein said polarization-changing means is effective to rotate the plane of

8. An antenna structure comprising:

transducer means for emitting and receiving beams of microwave energy with a predetermined plane of polarization;

a planar array of conductors perpendicular to said plane of polarization disposed in the path of said beams for reflecting microwave energy polarized in a plane parallel to said conductors;

a first and a second reflector for microwave energy having mutually orthogonal axes intersecting at an intermediate point of said array, the latter including an angle of 45.degree. with each of said axes, said second reflector lying in line with said path on the side of said array remote from said transducer means; and

polarization-changing means adjacent the active surface of each of said reflectors for turning the plane of polarization of an incident plane-polarized beam through 90.degree. whereby a beam emitted by said transducer means and passing said array in a direction generally parallel to the axis of said second reflector is successively directed by said second reflector and by said array to said first reflector and is thence redirected to said array with a polarization enabling its passage

9. An antenna structure as defined in claim 8 wherein said reflectors have conjugate curvatures for focusing a beam originating at said transducer means onto an object at infinity upon the second traverse of said array.

10. An antenna structure as defined in claim 9 wherein said first reflector is concave and said second reflector is convex toward said array.