Crossed Polarization Same-Zone Two-Frequency Antenna for Telecommunications Satellites

The present invention relates to a crossed polarization same-zone two-frequency antenna for telecommunications satellites enabling identical zones on the surface of the globe to be covered by two electromagnetic waves which are orthogonally polarized to each other.

In order to cover identical zones on the surface of the globe using two radiated waves at different frequencies, a known antenna is constituted by a reflector of paraboloid shape situated opposite a primary source of electromagnetic waves placed at the focus of the reflector, the primary source being horn-shaped, for example, and being placed at the end of an electromagnetic waveguide.

Since the radiation pattern of the primary source has an aperture which varies as a function of the frequency of the radiated electromagnetic wave, this type of implementation provides an antenna whose efficiency is not the same for both of the waves reflected by the reflector, and in order to obtain signals of the same energy at the surface of the globe, the primary source must be adapted to compensate for the energy loss to which one of the waves is subjected relative to the other, with such compensation requiring the transmitter power supply devices in the satellite to be over-dimensioned.

Further, known antennas comprising a single reflector do not conserve completely orthogonal electric fields in each of their planes of polarization after reflection, so the isolation between the transmission channels constituted by the waves of different frequency cannot be totally effective.

Preferred embodiments of the present invention remedy these drawbacks.

The present invention provides a crossed polarization same-zone two-frequency antenna for telecommunications satellites, the antenna being of the type comprising a parabolic reflector having an apex S, an elliptical periphery, with a major axis Dx.sub.1 and a minor axis Dy.sub.1, and a primary source of spherical electromagnetic waves placed at the focus of the parabolic reflector, wherein said reflector comprises a first grating of conductive wires fixed to the concave face of the reflector which extend parallel to one another and to a plane passing through the axis of revolution of the paraboloid and along the major axis of the paraboloid, and a second grating of conductor wires orthogonal to the conductor wires of the first grating and placed inside the the first grating of conductors to form a reflecting surface having an elliptical periphery whose major axis Dx.sub.2 is less than Dx.sub.1 and whose minor axis Dy.sub.2 is less than Dy.sub.1, the centers of the ellipses formed by the peripheries of each of the reflecting surfaces being common, and the sizes of the major and minor axes of the ellipses of the two reflecting surfaces formed by the two gratings of conductors being so determined as to obtain the same coverage zone for the high frequency wave as for the low frequency wave.

An embodiment of the invention is described by way of example with reference to the accompanying drawing, in which:

FIG. 1 is a perspective view of an antenna provided with a polarized parabolic reflector in accordance with the invention;

FIG. 2 is a front view of the reflector in accordance with the invention,

FIG. 3 is a front view of the reflector in accordance with the invention;

FIG. 4 is a section taken the line IV--IV of FIG. 3; and

FIG. 5 is a diagrammatic perspective view showing the beam aperture angles for high and low frequency waves.

The parabolic antenna shown in FIG. 1 comprises a parabolic reflector 1 having an apex S and a primary source 2. The primary source 2 is constituted, for example, by means of a rectangular section horn and is located at the focus of the reflector by means of support arms 3, 4 and 5 which bear against the edge 6 delimiting the concave and convex surfaces of the reflector. The reflector 1 comprises a rigid parabolic structure made of synthetic material, e.g. Kevlar, aramid fiber, or any other equivalent dielectric material. KEVLAR is a registered trademark of Dupont de Nemours Corporation. A first electrically conducting polarizing grating 7 is disposed directly on the concave parabolic face of the reflector directly opposite the primary source 2, and a second polarizing grating 8, whose conductor wires are orthogonal to those of the first grating 7 is situated in the middle portion of the reflector. The first grating 7 is constituted by conductor wires extending over the entire area of the reflector facing the primary source along lines which mark the intersection of planes which are parallel to one another and to the direction of the main axis AA' of the paraboloid, said axis AA' passing through the apex S and the focus F of the paraboloid (see FIG. 1). The second grating 8 is likewise constituted by conductors which are likewise situated along the lines of intersection of mutually parallel planes which are also parallel to the direction of the axis AA' and which are orthogonal to the preceding planes defining the first conductor grating 7. The reflector is disposed relative to the primary source 2 in such a manner that the parallel wires of the gratings 7 and 8 are also parallel to the electric fields of respective ones of the two orthogonally polarized electromagnetic waves to ensure that each grating reflects only the corresponding one of the waves.

The conductors constituting the gratings 7 and 8 may be obtained by burying metal wires in the dielectric material or else by overall etching using a mask in contact with the surface of the reflector, or else by local etching as shown in FIGS. 3 and 4 using a laser, or else by etching the surface of the developed plane of the reflector as described in published French patent application No. 2 302 603, for example.

A reflector in accordance with the invention as described above has the advantage of reflecting two electromagnetic waves which are orthogonally polarized relative to each other and which are at different frequencies in such a manner as to obtain the same geographical coverage on the surface of the globe. The central portion of the reflector constituted by the area common to both orthogonal gratings 7 and 8 reflects both orthogonally polarized waves, whereas the peripheral portion outside the central grating 8 only reflects the low frequency polarized wave. The same zone coverage is obtained by determining the area and shape of the central grating in such a manner as to obtain the same zone coverage with the high frequency wave as is obtained by the grating 7 for the low frequency wave. The relevant calculations are explained below with reference to the front view of the reflector as shown in FIG. 2.

In FIG. 2, the reflector 1 extends over an elliptical area having a major axis Dx.sub.1 and a minor axis Dy.sub.1 as does grating 7, with the elliptical ratio being close to that required for the desired ground coverage. The grating 8 disposed in the middle of the reflector likewise extends over an interior elliptical zone of the reflector having a major axis Dx.sub.2 and a minor axis Dy.sub.2. The ellipses delimiting the areas of the gratings 7 and 8 have the same center. The low frequency spherical wave from the primary source 2 is transformed into a plane wave by the entire area of the reflector 1. In FIG. 5, the three decibel width of the resulting secondary radiation pattern has the following values in the main planes:

and

where

.theta.x.sub.1 and .theta.y.sub.1 designate the aperture angles of the beam in the corresponding main planes;

K.sub.11 is a weighting coefficient for a section orthogonal to the electric field;

K.sub.12 is a weighting coefficient for a section parallel to the electric field; and

.lambda..sub.1 is the wavelength of the low frequency wave.

The high frequency spherical wave is likewise transformed by the grating 8 in the middle of the reflector into a plane wave whose radiation pattern has a 3 dB width in the main planes as follows:

and

where

.theta.x.sub.2 and .theta.y.sub.2 designate the aperture angles of the beam in the corresponding main planes in FIG. 5;

K.sub.21 is a weighting coefficient for a section orthogonal to the electric field;

K.sub.22 is a weighting coefficient for a section parallel to the electric field; and

.lambda..sub.2 is the wavelength of the low frequency wave.

The two waves of wavelengths .lambda..sub.1 and .lambda..sub.2 have the same coverage zone when:

and

i.e. when

and

When these conditions are satisfied, the beam aperture for the high frequency wave is very close to the aperture obtained for the low frequency, wave and zone coverage is provided at the same gain for both frequencies.

The invention is not limited to the embodiment described above, and naturally other embodiments are possible, in particular as a function of various kinds of primary source for use with such a reflector. In particular, it will be understood that the elliptical shape of the reflector and of the inner grating could readily be reduced to circles for use with some kinds of primary source in such antennas.

Further, in some particular applications, the centers of the ellipses delimiting the areas of the gratings 7 and 8 need not necessarily be located at the same point as the apex S of the reflector, e.g. when providing an offset type reflector.