Parabolic Antennas

Abstract

An ultra high frequency antenna comprising a reflector having a focus and a focal plane, one radiating source at said focus and at least one radiating source in said focal plane, offset with respect to said focus, for illuminating said reflector, said sources producing at least two wave fronts of different directions and phase correcting means in the path of the wave front(s) originating from said offset source(s), said correcting means lying outside the path of said wave front originating from said source located at said focus.

Description

The present invention relates to antennas with radiating beams stepped in elevation.

Antennas of this kind generally comprise several horns situated in the focal plane of a reflector, which is generally parabolic.

Waves radiated by each horn are transformed, after reflection on the reflector, into waves which are plane, as a first approximation. This is strictly true in the case of a horn placed at the focus of the paraboloid. But the farther the horn is placed from the focus, the less plane are the wave surfaces.

It is an object of the invention to provide an antenna of this type in which this drawback is avoided.

According to the invention there is provided an antenna comprising a correcting arrangement for rendering plane the equiphase surfaces of the waves issued from the horns which are not positioned at the reflector focus, this arrangement being placed on the path of these waves and away from the waves issuing from the horn positioned at the reflector focus.

The invention will be better understood from the following description and the appended drawing, in which:

FIGS. 1 and 2 show, very diagrammatically, two embodiments of the antenna according to the invention; and

FIGS. 3 and 4 show in plane view two details of FIG. 1.

The antenna shown in FIG. 1 comprises two horns: a horn S.sub.1 is placed at the focus of a parabolic reflector A and a further horn S.sub.2 is placed at a distance from horn S.sub.1 in the focal plane of reflector A. Both horns transmit vertically polarized waves. Reflector A is built up, as known in the art, by vertical wires, as shown in FIG. 3, which are spaced apart by a distance of the order .lambda./10, .lambda.being the operating wavelength. The arrow in FIG. 3, indicates the direction of the electric field of the wave radiated by horns S.sub.1 and S.sub.2.

A further reflector M, formed by wires inclined by 45.degree. to the horizontal, is positioned for receiving the waves reflected by reflector A. It is followed by reflector E, having a full reflecting surface and spaced by .lambda./4 from reflector M. Reflectors M and E are positioned normally to the axis of reflector A. Reflector M is shown in FIG. 4.

The waves transmitted by horn S.sub.1 are vertically polarized. These waves are reflected as plane waves R by reflector A. The vertically polarized wave R may be considered as the resultant of two component plane waves respectively polarized normally and parallel to the wires of reflector M. Vectors e and e.sub.2 represent respectively the electric fields of wave R and of said two components.

Upon striking reflector M, the plane wave parallel to the wires is reflected by the latter with phase inversion, as indicated by vector e'.sub.2 in opposition to vector e.sub.2, while the wave normal to the wires propagates through reflector M and is reflected by reflector E with a phase inversion. Due to the fact that the distance B between reflectors M and E is .lambda./4 the electric field vector e'.sub.1, at the wires, of the wave reflected by reflector E, is again parallel to e.sub.1. The two reflected component waves combine to form a wave whose electric field is the resultant of vectors e'.sub.1, i.e., e.sub.1 and e'.sub.2, i.e. along the horizontal vector T. Thus, this wave is horizontally polarized and propagates through reflector A.

The same is true for the waves radiated by horn S.sub.2, except for the fact that they do not possess, after reflection upon mirror A, plane equiphase surfaces, horn S.sub.2 being offset with respect to the forms of reflector A.

The shape of this equiphase surface P.sub.1 is shown in FIG. 1. In its upper part, assuming the antenna axis to be horizontal, it is advanced in phase as compared to the plane surface wave shown in dotted line. This alters the shape of the radiation diagram. In order to avoid this defect, according to the invention, reflector M is curved in its upper part C turning its concavity in the same sense as surface P.sub.1 so compensating phase advances. After reflection on this upper part, the shape of the equiphase surface is as shown at P.sub.2.

It will be noted that the only equiphase surfaces to be corrected are those of the waves from horns which are substantially offset with respect to the focus of reflector A.

The advantages of the invention are the following:

a. Sources which are more offset than in conventional systems may be used in combination with a source at the focus, which results in respective plane waves having direction of propagation at large angles with respect to each other; thus the average space angle covered by the whole radiation system, or radiation aperture, may be 10 times, for example, the space angle covered by the radiation originating from a single source.

b. The curvature of mirror M is calculated for the horn which is most offset with respect to the focus. The more offset the intermediate sources, the more they are subject to correction.

c. The shape of the curved part can be easily improved by trail and error.

FIG. 2 shows another embodiment of the invention.

The primary sources S are placed in the focal plane of a parabolic reflector A. The whole assembly is placed in a radome Ra. The correction device C is a lens placed at the top of the radome, and through which propagate the waves radiated by the sources which are offset with respect to the focus of reflector A.

Since the phase shift is a maximum of 360.degree., this lens may be constituted by a zone, at the top of the radome, of a thickness which is variable and greater than the normal thickness of the radome, this thickness not exceeding one wavelength of the radiated wave.

Of course the invention is not limited to the embodiments described and shown which were given solely by way of example.

Claims

1. An ultra high frequency antenna comprising a reflector having a focus and a focal plane, one radiating source at said focus and at least one radiating source in said focal plane, offset with respect to said focus, for illuminating said reflector, said sources producing at least two wave fronts of different directions and phase correcting means in the path of the wave front(s) originating from said offset source(s), said correcting means lying outside the path of said wave front originating from said

2. An antenna as claimed in claim 1, wherein said phase correcting means comprise a lens located in the path of the waves radiated by said offset

3. An antenna as claimed in claim 1, wherein said phase correcting means comprise a second reflector having a planar portion for reflecting wave front originating from said source located at said focus upon reflection thereof on said first mentioned reflector and at least a curved end portion for reflecting wave front(s) originating from said offset

4. An antenna as claimed in claim 1, comprising a plurality of offset sources in said focal plane, said correcting means lying in the path of

5. An antenna as claimed in claim 4, wherein said sources radiate waves polarized in a first direction, said phase correcting means comprise a second reflector having a planar portion for reflecting the wave front originating from said source located at said focus, upon reflection thereof on said first mentioned reflector, and curved end portions for reflecting the wave fronts originating from said plurality of offset sources upon reflection thereof on said first mentioned reflector, said first reflector comprises wires parallel to said first direction, said second reflector comprises parallel wires inclined at 45.degree. to said first direction, a screen being disposed behind said second reflector, parallel thereto, at a distance equal to .lambda./4, .lambda. being the operating wavelength.