Orientable antenna with conservation of polarization axes

An antenna is orientable, directional and capable of use as a transmit and/or receive antenna. It includes at least one reflector, at least one source of electromagnetic radiation including means for exciting the source with two orthogonal linear polarizations and a mechanical system for positioning and holding the source and the reflector. The orientation of the antenna is made up of depointing and rotation about a preferred direction of propagation of the radiation and the mechanical system enables such rotation while keeping the source fixed, so conserving the orientation of the orthogonal linear polarization. A preferred embodiment of the antenna includes a parabolic main reflector and a hyperbolic auxiliary reflector in a Cassegrain geometry, and the mechanical system enables rotation of both reflectors about the preferred direction of radiation and holds the source fixed to conserve the orthogonal linear polarization axes of the beam. Applications include radar, direct broadcast satellites and telecommunications employing frequency re-use by polarization diversity, especially advantageous in space and airborne applications.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of the invention is that of antennas for transmitting and/or 
receiving electromagnetic radiation and in particular directional and 
orientable antennas adapted to transmit and/or to receive radiation in a 
specific and variable direction. An antenna of this kind can comprise a 
source of radiation and one or more reflectors, the shape of the 
reflector(s) and the disposition of the system of reflector(s) relative to 
the source determining the directional characteristics of the antenna 
obtained and the shape of the beam transmitted or received. 
2. Description of the Prior Art 
The present invention relates to many kinds of directional antenna known to 
the person skilled in the art, including parabolic antennas, Cassegrain 
antennas, Gregorian antennas, etc using either axial or "offset" 
illumination. An offset system has a main reflector whose aperture is 
eccentric to the axis of the surface in question. In the single-reflector 
situation the primary source disposed on this axis is inclined so that it 
points to the center of the reflector. 
The invention is more particularly concerned with antennas adapted to 
transmit and/or to receive with two orthogonal linear polarizations when 
the success of their mission depends on this capability. This applies to 
some telecommunication antennas, for example, which use polarization 
diversity to enable reuse of the spectrum in a given band of frequencies. 
Another example concerns satellite broadcasting antennas for the DBS 
(Direct Broadcast by Satellite) and DTH (Direct to the Home) systems. Some 
radar systems perform independent measurements with orthogonal 
polarizations to determine the radar signature of a complex target, for 
example, or for meteorological and remote sensing applications. 
Most prior art implementations of this kind have been fixed terrestrial 
systems or systems on board terrestrial or airborne vehicles. 
The present invention is particularly advantageous when used in space, on 
board a satellite, an orbital space station, a probe or any other space 
platform. 
A new problem can arise on attempting to extrapolate from prior art 
terrestrial systems to design a space system using polarization diversity, 
namely: the implicit reference axes available on the terrestrial surface, 
the vertical and the horizontal, do not exist in space. Consequently, 
conservation of these axes as reference axes is problematical. 
This problem is not insurmountable and can even be solved very easily if 
various system constraints are accepted. 
For example, a geostationary telecommunication satellite must usually be 
able to communicate with a relatively small number of fixed ground 
stations. The orientations of the orthogonal polarization axes used in a 
system of this kind can be arbitrary, provided that a few initial 
adjustments are made to the ground equipment before transmission of wanted 
information. The constraint to be accepted in this situation is that no 
temporal variation of the geometrical parameters of the link can be 
tolerated, without carrying out a new adjustment sequence. In the prior 
art this is no problem, or virtually no problem, since the geometrical 
parameters of the link with a geostationary satellite are in principle 
invariant. 
The situation is different for a satellite in low Earth orbit, a polar 
orbit or an inclined orbit (Walker, Molnya, etc orbits); these orbits can 
be elliptical or circular. Satellites in such orbits move across the sky 
from the point of view of an observer at a fixed point on the terrestrial 
surface. Consequently, a link between any such "non-geostationary" 
satellite and a fixed ground station will be in a direction that varies 
continuously due to the movement of the satellite. 
For these non-geostationary satellites there is not necessarily any 
insurmountable problem in using orthogonal linear polarizations provided 
that certain constraints on system design are accepted. For example, a 
linear polarization can be chosen parallel to the path of the satellite, 
known from astronomical tables, with the other polarization chosen 
perpendicular to this path and to the nadir. Each fixed ground station 
knows in advance the orientations of the polarization axes used by the 
satellite and the ground antenna can be adjusted accordingly. 
The extent and the frequency of such adjustments will depend on the freedom 
to be allowed in respect of the geometrical parameters of the link 
established between the non-geostationary satellite and the ground 
station. If the link is used only when these parameters are identical or 
virtually so (small variations in their values can be tolerated within a 
range whose extent is determined by the cross polarization link balance), 
there is no foreseeable problem of interference between two transmission 
channels using the same frequency with orthogonal polarizations (this is 
polarization diversity). 
However, this constraint is a problem in the prior art systems in that the 
possibility of orienting the onboard antenna is limited by the radio 
performance specifications promulgated by national and international 
regulatory bodies (FCC, CCITT, ITU, etc) for radio transmissions. In known 
systems the orientation of the antenna can cause performance to vary 
outside the narrow range allowed by such standards and specifications. 
Frequency re-use through polarization diversity can also have advantages in 
direct satellite broadcasting. A user on the ground will not be obliged to 
re-orient his antenna to point at a second satellite in order to pick up a 
second "bouquet" of transmissions if a first satellite can provide the 
programs of the second bouquet along with those of the first bouquet, from 
the unique orbital position of the first satellite, using cross 
polarizations. 
The invention is directed to remedying the drawbacks of the prior art for 
telecommunication satellites (transmit and/or receive antenna) and direct 
broadcast satellites (transmit antenna). 
In meteorological and remote sensing radar systems, the polarization of the 
wave received by the equipment can be used to probe the target better. For 
example, backscattering and depolarization of the polarized wave 
transmitted by the satellite can reveal the nature of atmospheric 
precipitation, since the depolarization depends on the size, the 
concentration and the phase state (ice, liquid droplets, vapor), of the 
substances probed. To give another example, polarization measurements on 
radar backscattering from the surface of the sea can indicate how rough 
the sea is. 
Sensitivity to polarization varies according to the mission. In these last 
two examples, the polarization of the initial wave can be arbitrary 
without this affecting the result because the targets themselves are not 
fixed but, to the contrary, have an arbitrary orientation. 
The situation is different in observing a fixed target illuminated by a 
polarized wave at different moments in time. Such successive measurements 
can be used to observe the evolution of the target or to improve the 
signal to noise ratio and the resolution of the fixed image by correlating 
successive images (background subtraction). A typical case is the 
observation of the same geographical area or the same object on the ground 
on successive passes of a non-geostationary satellite. The successive 
orbits of any such satellite are usually not closed as seen from the 
terrestrial surface, but rather trace out a spiral advancing in the 
direction of longitude. This applies to heliosynchronous orbits, for 
example. 
One problem with any such prior art system is that although the orthogonal 
polarization vectors can be arbitrary for isolated observations, they must 
be conserved for correlating successive measurements. However, these 
vectors tend to evolve for at least two reasons. Firstly, the precession 
of the orbit introduces variable but predictable geometric factors and, 
secondly, viewing the same location on the ground in successive orbits 
generates other variations of the geometrical parameters which must be 
allowed for in the correlations to be carried out. 
Expressed in the most general terms, the new problem to which the invention 
is addressed is as follows: an antenna is required whose elements can be 
oriented at will to enable arbitrary orientation of the transmitted or 
received beam of radiation, whilst allowing conservation of orthogonal 
linear polarization axes regardless of the orientation of the beam. 
Moreover, the antenna of the invention must allow conservation of 
orthogonal linear polarization axes even in the situation in which the 
beam rotates about its main direction of propagation. 
SUMMARY OF THE INVENTION 
To solve this problem, the invention consists in an antenna including at 
least one reflector and at least one source of electromagnetic radiation, 
each source being capable of transmitting and/or receiving radiation in a 
primary direction which links said source to at least one reflector; said 
source including at least one radiating element and means for exciting 
said element, said antenna being adapted to transmit or to receive a beam 
of electromagnetic radiation of arbitrary cross-section and in a preferred 
radiation direction determined by the disposition and the orientation of 
said reflector and said source, said reflector having any shape and said 
beam having polarization axes conferred on it by the excitation applied to 
said source, said beam being orientable by movement of said antenna or its 
component parts, said antenna further including mechanical means which 
determine the relative disposition of said reflector and said source and 
enable said reflector to rotate about an electromagnetic radiation 
propagation axis while holding said source in a position such that the 
polarization axes remain fixed during said rotation. 
The designer will determine the nature of the source to suit the mission to 
be accomplished. For example, the source can be a basic horn, a microstrip 
("patch") radiator, a slot, etc or a complex or extensive source, for 
example an array of patches of slots, possibly associated with cavities. 
The complex source can be made up of a plurality of separate sources with 
a polarization-selective reflector or with a plurality of 
frequency-selective reflectors. The source can be a direct source or 
periscopic source. In brief, the invention can be implemented using any 
source known to the person skilled in the art for such applications. 
In accordance with one feature of the invention, the movement of at least 
one reflector includes rotation of the reflector about the preferred 
direction of radiation. In accordance with another feature of the 
invention this movement includes angular displacement (depointing) of the 
preferred direction about a point which represents the position of the 
source. In one embodiment of the invention this movement includes rotation 
of the reflector about the radiation propagation direction linking the 
source to the reflector. 
According to one specific feature of the invention the direction of 
propagation between the source and the reflector coincides with the 
preferred direction of radiation. 
In one specific embodiment of the invention the at least one reflector is a 
single reflector having parabolic generatrices, the reflector being 
illuminated by the source disposed substantially at its focus, and the 
reflector can be turned about the radiation direction with the source 
fixed. The geometry of the system is centered. 
In one embodiment of the invention the single parabolic reflector is 
illuminated by a source with an "offset" geometry and the reflector can be 
turned about the radiation direction with the source fixed. 
In another specific embodiment of the invention the antenna includes at 
least two reflectors disposed in an offset or centered "Gregorian" 
geometry. The two reflectors are disposed with their concave surfaces 
facing each other and the illumination of each is either offset or 
centered. 
In another and particularly advantageous embodiment of the invention the 
antenna includes at least two reflectors disposed in a Cassegrain 
geometry, namely a main reflector which reflects the beam and an auxiliary 
reflector which is illuminated by the source, and at least the main 
reflector can be turned about the preferred direction of radiation with 
the source fixed. In one embodiment of the invention the system of 
reflectors can be turned about the preferred direction of radiation with 
the source fixed. In accordance with an additional feature of the 
invention the antenna further includes mechanical means for depointing all 
of its component parts without modifying their relative disposition, in 
addition to the mechanical means previously described. 
In all embodiments of the invention the focusing reflectors have an 
arbitrary shape; however, the invention will be particularly advantageous 
if at least one reflector has no axial symmetry (of rotation about an 
axis). 
The reflector can be simple or complex. 
For example, a complex reflector can be a dual gridded reflector made up of 
two reflectors disposed one in front of the other in a direction of 
propagation of the beam, the first reflector being reflective for a first 
linear polarization and transparent for an orthogonal second linear 
polarization which is reflected by the second reflector disposed behind 
the first reflector. This dual gridded type of reflector is well known to 
the person skilled in the art. In an embodiment of the invention using 
this type of reflector the mechanical means rotate the source, which is of 
any shape, and hold the reflector(s) fixed. 
Other features and advantages of the invention will emerge from the 
following detailed description with reference to the appended drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The drawings show embodiments of the invention by way of non-limiting 
example. The same reference numbers in the various figures always denote 
the same items. Some of the figures are not to scale, to make them 
clearer. 
FIG. 1 is a diagram showing a satellite Q in Earth orbit. 
The satellite has an orientable antenna; depending on the position of the 
reflector 11, the beam can be directed in various directions to illuminate 
different places on the Earth E. In the FIG. 1 example, the beam F 
directed towards the nadir illuminates the "spot" 1 and the beams F', F" 
respectively illuminate the spots 1', 1" ("spot" is the term of art 
denoting the trace on the ground of a narrow beam directed towards the 
Earth E). 
The beam can be oriented either mechanically by positioning a main 
reflector 11 as shown diagrammatically in this figure or electronically in 
the case of an array antenna by altering the phases of the signal supplied 
to the individual sources of the array. 
All of the remaining description refers exclusively to a transmit antenna. 
However, the person skilled in the art knows the reciprocal nature of the 
theory of passive antennas whereby an antenna operates in the same manner 
in transmission and in reception subject to inverting the sign of the time 
(t) in the equations describing electromagnetic propagation (Maxwell's 
equations). 
Although the antenna of the invention is described in relation to 
transmission it is to be understood that the invention is equally 
concerned with a receive antenna having the same features and with a 
transmit/receive antenna such as a radar or telecommunication antenna. In 
these various embodiments, the amplification electronics associated with 
the antenna must be power amplification electronics in the case of a 
transmit antenna or low-noise amplification electronics in the case of a 
receive antenna or a combination of the two in the case of a 
transmit/receive antenna. 
FIG. 2 shows the traces on the ground of an orientable antenna of the 
invention with conservation of the linear polarization vectors along the 
x, y axes. In this example, spot 1 is an ellipse with axes a, b; the major 
axis of the ellipse is the a axis. The x, y polarization axes coincide 
with the axes a, b of an elliptical spot 1. 
The elliptical spots 1', 1" are illuminated by the beams F', F" from FIG. 
1, for example, obtained by orienting the orientable antenna 11. The 
relative orientation between the spots (1, 1', 1") can be obtained by a 
combination of depointing the antenna to move the spot in translation and 
rotation of the antenna about the main axis of the transmitted beam to 
rotate the axes of the ellipse. 
In a prior art orientable antenna, the antenna is rotated about the main 
axis of the beam by mechanical means which physically turn the antenna 
about this main axis. If the antenna is fed by one or more sources with 
two orthogonal linear polarization axes, the polarization axes are subject 
to the same rotation as the axes of the spot on the ground. For the 
intended applications of the invention rotation of the polarization axes 
cannot be tolerated, as it would inevitably cause interference between 
signals conveyed by channels distinguished only by their polarization. 
The antenna of the invention solves this problem to achieve the result 
shown in FIG. 2. Note that the spots 1', 1" can be illuminated by 
translation and rotation of the elliptical spot 1, but that the 
polarization axes (x, y) are retained regardless of the orientation of the 
axes (a', b'; a", b") of the elliptical spot (1', 1" respectively). In 
this example the elliptical spots are oriented for better coverage of the 
geographical areas indicated on the geopolitical map of Europe. 
To explain more clearly how the invention can solve the problem as stated, 
FIG. 3 is a diagrammatic representation in lateral cross-section of a 
prior art parabolic antenna. The essential components of this antenna are 
the focusing reflector 11 whose shape is a paraboloid of revolution about 
the axis of symmetry z and the source 10 at the focus of the reflector 11. 
In this example the source is a horn 10 fed by a waveguide 12. Mechanical 
means 13 are provided to hold the source 10 at the focus of the reflector 
11 in a fixed and optimal geometrical arrangement. The electromagnetic 
radiation emitted by the source 10 at the focus is reflected by the 
reflector 11 as parallel rays which form a beam F of radiation along the 
main axis z. 
In the case of a main reflector 10 having symmetry of revolution, there is 
no need to rotate the antenna about the main axis z because the spot at 
the nadir will be circular. 
FIGS. 4A, 4B, 4C are different views of an asymmetric parabolic reflector 
adapted to form an elongate spot on the ground. The shape of the reflector 
11 as seen in plan view in FIG. 4B is virtually rectangular. The 
cross-sections on AA', BB' in FIGS. 4A, 4C respectively, are paraboloid 
arcs of different length. The arcs can have the same focal length despite 
their different lengths, and the reflector 11 will have a single focus. 
The beam resulting from a source at the focus will have a rectangular 
cross-section. 
FIG. 5 shows in cross-section a conventional Cassegrain geometry having a 
source 10 illuminating an auxiliary reflector 21 through a hole 20 in a 
parabolic main reflector 11. The conventional geometry is axisymmetric 
about the axis z which corresponds to the direction of propagation of the 
beam F. The source 10 is either disposed on the z axis (not shown) or 
imaged onto the axis by means of a periscopic third reflector (not shown). 
The shape of the auxiliary reflector 21 is a hyperboloid whose first focus 
C coincides with the focus of the parabolic main reflector 11. The phase 
center of the source 10 is imaged at the second focus C' of the 
hyperboloid. 
In this way, a ray emitted by the source 10 at the point C' at an angle 
.theta. to the z axis will be reflected from the surface of the auxiliary 
reflector 21 towards the main reflector 11 in a direction whose origin is 
the focus C of the parabolic main reflector 11. The rays arriving at the 
focus C are reflected by the parabolic main reflector with a reflection 
angle .theta.' to form a beam F in which all the rays are parallel to the 
z axis. 
The vector N represents the normal to the surface of the auxiliary 
reflector 21 and the vector N' represents the normal to the surface of the 
main reflector 11. 
FIG. 6 is a diagrammatic three-dimensional perspective view of the 
parabolic reflector (11) from FIGS. 4A, 4B, 4C with a system of 
coordinates used to describe movement of the antenna of the invention. The 
apex of the reflector 11 is at the origin O and the z axis represents the 
direction of propagation of reflected waves (not shown). 
The parabolic reflector 11 is approximately rectangular in shape when 
projected onto a plane surface perpendicular to the z axis, for example 
the (x, y) plane. 
D is its width in the x direction and D' is its height in the y direction. 
A section AA' in the (x, z) plane is a parabola and a section B'B in the 
(y, z) plane is a parabola, in conformance with FIGS. 4A, 4B and 4C. 
The system has three degrees of freedom: rotation by an angle .phi. about 
the main axis z and depointing by two angles (.alpha., .beta.) in two 
orthogonal planes intersecting on the main axis z. The depointing can be 
represented by the unit vector u which is oriented in the direction angles 
(.alpha., .beta., .gamma.) to terminate at a point P of the z axis. The 
angle .gamma. can be expressed as a function of the two independent 
variables (.alpha., .beta.). 
The angle .alpha. represents the projection of the vector u onto the (x, z) 
plane and point N' the projection of the point P onto the same (x, z) 
plane. 
The angle .gamma. represents the projection of the vector u onto the (x, y) 
plane and point M the projection of the point P onto this same (x, y) 
plane. The angle .beta. represents the projection of the vector u onto the 
(y, z) plane. The projection of the point P onto this plane is not shown 
in order to simplify the drawing. 
Rotation of the reflector can be expressed either by the angle .phi. about 
the main axis z or by the angle .phi.' about the unit vector u; these 
angles are not independent. 
FIG. 7 is a diagrammatic cross-section of an offset illumination Gregorian 
geometry. The parabolic main reflector 11 is illuminated by the source 10 
via an elliptical auxiliary reflector 13 off the main axis z of the beam F 
which is made up of parallel rays. The source 10 at the first focus of the 
ellipse emits towards the auxiliary reflector 13 along the z" axis and the 
waves are reflected towards the main reflector 11 and focused at a point 
C" (focus of the parabola and second focus of the ellipse), whence they 
diverge to illuminate all of the main reflector 11. This system therefore 
has two axes (z, z") about which rotation can be effected, either rotation 
by an angle .phi. about the z axis or rotation by an angle .phi." about 
the z" axis, respectively. 
FIG. 8 is a diagrammatic plan view of one embodiment of an orientable 
Cassegrain antenna of the invention with conservation of polarization. As 
in FIG. 5, the parabolic main reflector 11 is illuminated by the source 10 
via the auxiliary hyperbolic reflector 21, one focus of which is at the 
focus of the main parabolic reflector 11. The relative positions of the 
two reflectors (11, 21) are fixed by mechanical supports S.sub.1. 
The combination of the source (10), the reflectors (11, 21) and the 
mechanical positioning means (depointing, rotation) is fixed relative to 
the platform Q (which is a satellite, for example) by supports S.sub.3. 
The positioning means include three stepper motors (R.phi., R.alpha., 
R.beta.) capable of effecting the angular displacement (.phi., .alpha., 
.beta.) explained with reference to FIG. 6. These means are mounted on a 
small platform Q' which rests on the supports S.sub.3. 
The depointing means (R.alpha., R.beta.) are fixed to the small platform Q' 
and drive the support S.sub.2 which supports the axial rotation motor 
R.phi.. This axial rotation motor R.phi. is mechanically fixed to the main 
reflector 11 to rotate the latter (by an angle .phi.) about the main axis 
z. Unlike the prior art antennas, rotation of the main reflector 11 does 
not rotate the source 10, which is not fixed to the reflector 11. 
The source 10 is fed with two orthogonal polarizations which also remain 
fixed relative to the source 10 upon rotation (angle .phi.) of the main 
reflector. 
FIG. 9 is a three-dimensional perspective view from above of the FIG. 8 
embodiment of the invention. Components already described with reference 
to FIG. 8 carry the same reference numbers. The source 10 passes through a 
hole 20 in the main reflector 11 without mechanical contact. This feature, 
already part of the centered Cassegrain geometry, is exploited by the 
invention to isolate the source 10 from rotation about the z axis (angle 
.phi.) of the main reflector and the auxiliary reflector fixed to the main 
reflector 11. 
The orthogonal cross-sections (A, A'; B, B') of the main reflector 11 are 
parabolas as in FIGS. 4A, 4B, 4C and 6. 
The projections of the points A, A'; B, B' onto the x, y plane are 
respectively the points a, a'; b, b' and set the lateral dimensions of the 
main reflector 11 and the auxiliary reflector 21 fixed to the main 
reflector 11 by the supports S.sub.1. In the most general case, and as 
shown in FIG. 6, these lateral dimensions (aa', bb') are not the same and 
the cross-section of the beam F (not shown) can have an arbitrary shape 
dictated by the shape of the perimeter of the main reflector 11, which is 
elliptical in this example. 
As shown in FIG. 9, the source 10 in this example is a horn, but any other 
technology known to the person skilled in the art could be used. For 
example, the source 10 could be an array of individual sources implemented 
in the microstrip ("patch") technology. 
FIG. 10 is a diagrammatic view in axial section of another embodiment of 
the invention which represents a variant of the antenna shown in FIGS. 8 
and 9. 
This is a centered Cassegrain geometry antenna to which has been added a 
periscopic auxiliary reflector 14 which receives radiation from the source 
10 offset on the z' axis parallel to the x axis and perpendicular to the 
main axis z. The auxiliary reflector 14 is disposed so that it reflects 
radiation from the source 10 along the z axis to illuminate the hyperbolic 
auxiliary reflector 21. In every other regard, the description with 
reference to FIGS. 8 and 9 applies here also. 
The source 10 remains fixed relative to the platforms Q and Q', even on 
rotation (angle .phi.) of the main reflector and the auxiliary reflector 
11 by the motor R.phi.. In the event of depointing (angle .alpha.) in the 
x, z plane, the position of the auxiliary reflector 14 is adjusted to 
maintain the reflected radiation from the source 10 on the main axis z to 
illuminate the auxiliary reflector 21. 
FIG. 11 is a diagrammatic view partly in cross-section of another 
embodiment of the invention with an orientable offset Cassegrain antenna 
with conservation of polarization. As in the previous figures, the 
parabolic main reflector 11 is illuminated by the source 10 via an 
auxiliary reflector 15. The main reflector is offset illuminated by the 
auxiliary reflector at an angle .delta. relative to the normal N' to the 
apex of the main reflector 11; the beam F (not shown) is reflected at the 
same angle .delta. to the normal N' along the main axis z. 
In this example depointing of the beam is achieved by positioning of the 
main reflector by the means R.alpha., R.beta.. Different static support 
mechanical means are shown (S.sub.5, S.sub.6, S.sub.7) together with a 
removable support S.sub.4 which supports the platform Q" on the main axis 
z whilst allowing it to move in a plane perpendicular to z. This figure 
also shows various thermal insulation means (I.sub.1, I.sub.2). 
In the FIG. 11 example the main axis z is far from the illumination axis z' 
of the auxiliary reflector 15 and the two axes are parallel. A mobile 
platform Q" on which are mounted the main reflector 11 and its support 
means (S.sub.5, S.sub.6, S.sub.7) and depointing means (R.alpha., R.beta.) 
can be displaced by the means R.phi. through an angle .phi. about the 
primary illumination axis z. Because the source 10 remains fixed relative 
to the platform Q (which is a satellite, for example) on rotation by an 
angle .phi. about the axis z' the polarization axes remain invariant 
relative to the platform Q. 
The support means S.sub.8 for the auxiliary reflector 15 join the latter to 
the mobile platform Q" so that rotation of the latter does not modify the 
relative geometry of the main and auxiliary reflectors 11 and 15. 
These few examples illustrate the principles and a few embodiments of the 
invention on the basis of which the person skilled in the art will know 
how to adapt the invention to the specific needs of a given mission. In 
these examples the depointing means are mechanical in nature and operate 
on the main reflector but the invention can also use electronic depointing 
(by phase shifting the individual sources of an array) or depointing by 
mechanical means operating on an auxiliary reflector, possibly a periscope 
reflector. 
Rotation of the spot formed on the ground without rotation of the 
polarization can be achieved through rotation of an angle .phi. about the 
main axis (z) or by rotation through an angle .phi. of the system of 
reflectors about the primary illumination axis z' or by rotation by an 
angle .phi.' about a depointed main axis u. In all cases decoupling of the 
depointing means and the means for rotation about one of the 
electromagnetic radiation propagation axes (z, z', u) enables orientation 
of the beam and conservation of polarization. Conversely, it is obvious 
that this same decoupling enables the antenna of the invention, subject to 
mechanical adaptations, to rotate the polarization axes whilst maintaining 
the orientation of the beam fixed, although this capability is not needed 
for the intended applications of the examples as described. This invention 
is directed to an alternate embodiment in the form of an antenna including 
at least one reflector and at least one source of electromagnetic 
radiation. Each source is capable of transmitting and/or receiving 
radiation in a primary direction joining the source to at least one 
reflector. Each source may include at least one radiating element and 
means for exciting said element. Such antenna is adapted to transmit or 
receive a beam of electromagnetic radiation of arbitrary cross-section and 
in a preferred direction of radiation. The preferred direction is 
determined by the disposition and orientation of the reflector and of the 
source. The reflector may be a dual gridded reflector of any shape with 
the beam of radiation having orthogonal polarization axes conferred on the 
beam by the orientation of the grids of the reflector. The beam may be 
oriented by movement of the antenna or its component parts. Further, the 
antenna may include mechanical means for determining the relative 
disposition of the reflector and the source and for effecting a rotation 
about an axis of propagation of the electromagnetic radiation while 
keeping the dual gridded reflector in a position so that the polarization 
axes of the beam remain fixed on the rotation of the source.