METHOD FOR ORIENTING THE BEAM OF AN ELECTRONIC SCANNING
ANTENNA, AND SENDlNGlRECElVlNG SYSTEM IMPLEMENTING SUCH
A METHOD
The present invention relates to a method for orienting the beam radiated by
an electronic scanning antenna. It also relates to an electromagnetic sending
and receiving system implementing such a method. In particular, it is
applicable to any type of electronic scanning antenna, used, for example, in
5 radars, telecommunication systems or multifunctional arrays.
Electronic scanning antennas are formed from modules-positioned in an
array. Each module comprises at least one radiating element contributing to
the formation of the sending and/or receiving beam. It is known that the
10 direction of the radiated beam is determined by the phase applied to the
signal sent or received at each radiating element. Stated otherwise, the
direction of the radiated beam is controlled by the phases applied to the
radiating elements according to a known law. The modules may or may not
be active, the active modules moreover integrating an amplifier of the sent
15 signal.
Thus, an electronic scanning antenna has, for a radar for example, a
microwave-frequency architecture consisting of channels comprising, in
particular, amplifier modules that may be used for sending and for receiving,
which are associated with multifunctional circuits comprising phase-shift
20 elements for aiming the beam in directions other than the normal to the array,
each module being equipped with a radiating element.
A drawback of electronic scanning antennas is that they are subject to a
misalignment of the radiated beam depending on the temperature. A
misalignment such as this is not acceptable with the angular precisions
25 demanded for most radar applications in particular. This misalignment is due
to the mechanical deformation of the antenna. More particularly, when the
temperature increases, the array structure expands. Conversely, when the
temperature decreases, the structure contracts. In any case, the phase
controls used for angularly aiming the radiated beam are no longer valid and
30 lead to an aiming error which may be crippling.
A known solution for solving this problem is to carry out a calibration of the
electronic scanning array. For this, the operating temperature range of the
antenna is sampled, hence between the minimum operating temperature and
the maximum operating temperature, and defects of illumination are recorded
for amplitudes and phases of the various microwave-frequency channels of
the array, a channel being associated with each module of the array. The
5 defects measured during the calibration phase are stored in a table, a socalled
calibration table. In the operational phase, the temperature-dependent
defects are thus ascertained by reading off the calibration table. At a given
temperature, the defect read off the table may thus be corrected by modifying
the phase values in order to offset this defect.
10 A drawback of this solution is that it is tricky and lengthy to implement.
Indeed, the measurements must be made for each temperature and copied
into the calibration table. The number of measurements is important, as the
range of operating temperatures must be sampled sufficiently and the
measurements themselves must be made with care on account of the small
15 misalignments involved. Although small, these misalignments may
nonetheless impair the precision of detection of a radar.
An aim of the invention is to overcome the aforementioned drawbacks. To
this end, a subject of the invention is a method for orienting the beam of an
20 electronic scanning antenna, said antenna being composed of an array of
radiating elements positioned in an initial geometric configuration at a
reference temperature To, geometric configuration models of said array as a
function of the temperature having been set up beforehand, the orientation of
said beam is carried out by:
25 - a first phase of measuring the temperature of said array in order to
select a model corresponding to the measured temperature;
- a second phase of calculating the phases, depending on the
direction of aiming (9, (p), to be applied to the signals of the radiating
elements for the selected model.
30 The geometric configuration models are, for example, calculated in a
preliminary step with respect to said initial configuration depending on the
temperature and on a thermal expansion coefficient TEC specific to said
array.
A model indicates, for example, the geometric position of said radiating
35 elements with respect to an axis system.
In the case in which the array is planar, the position of the radiating elements
is, for example, defined by their coordinates (xi, yj) in an X, Y axis system,
said phases depending on said coordinates.
In the case in which the array is linear, the position of the radiating elements
5 is, for example, defined by their abscissae (xi) along an X axis, said phases
depending on said abscissae.
In a possible implementation, said antenna operating in a given temperature
range, the models are calculated for the temperatures sampled between the
minimum value and the maximum value of the range according to a given
10 increment.
Another subject of the invention is an electromagnetic sending and receiving
system comprising an electronic scanning antenna composed of an array of
radiating elements implementing the preceding method.
15 In particular, the system comprises, for example, means for storing said
geometric configuration models as well as means for calculating said phases
to be applied. ---
Advantageously, this system is notably capable of equipping a radar.
20 Other features and advantages of the invention will become apparent with
the aid of the description which follows, made in relation to the appended
drawings which show:
- figure I, an illustration of the misalignment of the beam of a linear
array antenna;
25 - figure 2, an illustration of the misalignment of the beam of a planar
array antenna;
- figure 3, an illustration of the spherical coordinates of the beam;
- figure 4: possible steps for the implementation of the method
according to the invention.
Figure 1 illustrates, in one dimension, the misalignment of a radiated beam 1
of an electronic scanning array antenna 10 due to a variation in ambient
temperature. More specifically, figure 1 shows a linear array of radiating
elements 2 positioned along an X axis.
In the example of figure 1, the variation in temperature is manifested as an
increase in temperature. In the nominal state, the radiating elements 2,
shown by dotted lines, are positioned regularly along the X axis. After the
increase in temperature, the array of modules expands and the radiating
5 elements are located in position 2', the distance between two modules
growing.
In order to ensure that an array antenna operates correctly, the mesh of the
array must be such that no grating periodicity lobe appears in the radiating
space. As a general rule, this mesh is regular as illustrated in figure 1, in one
10 dimension. It is defined by the spacing period between the radiating elements
2, defining the sampling of the radiating aperture by these radiating elements.
In a first approach, this condition is obtained for a spacing between two
radiating elements that is smaller than A,, A, being the wavelength
corresponding to the maximum operating frequency of the antenna 10. For
15 an electronic scanning antenna the beam of which is misaligned up to an
angle OM, counted from the normal 3 to the array 10, this condition translates
into a spacing smaller than h,/(l + cos OM). -
For an operating frequency F = c I A, the phases to be applied to the radiating
elements to angularly aim the radiated beam 1 in a direction 8 are known. A
20 radiating element of order i is positioned at an abscissa xi on the X axis. The
phase Qi to be applied to the channel of order i, in degrees, is given by the
following relationship:
Qi = 360" xi sine/ A (1)
25
by choosing, for example, the origin of the axis of the abscissae X at the
center of the array.
For a regular array'the radiating elements of which are spaced a distance d
apart, the phase increment between two adjacent channels is therefore:
30
A@ = 360" d sin81 A (2)
The invention will subsequently be described for a regular array, but it may
be applied to any type of array.
Relationship (2) may thus be defined as a phase slope to be applied to the
aperture of the array in order to misalign the beam. This slope p is defined by
the following relationship:
p in fact defining a phase slope by unit length, A@ being expressed as a
function of the distance d by A@ = p x d.
10 By inverting relationship (2), it is apparent that at a given frequency F, hence
at a given wavelength A, the angular direction of radiated aiming 8 is given by
the following relationship:
8 = arcsin [ (A. A@) 1(360°. d) ] (4)
15 This relationship shows that, at a given frequency, if the gap d between
radiating elements increases, then the angular aiming 8 of the beam 1
decreases, hence the beam deviates 5 toward the normal 3 to the array as
shown in figure 1.
20 Figure 2 illustrates a beam misalignment in a case of application to a planar
array antenna 20. The module array is shown in an X, Y axis system. The
modules 2 are positioned, in this example, in a rectangular mesh.
As in the preceding, one-dimensional, case, it is known how to calculate the
phases to be applied to the channels in order to aim the beam 1 in a direction
25 (8, cp) at a frequency F = c 1 A, 8 and cp being the angles conventionally
defined in a spherical coordinate system, as depicted in figure 3 showing the
spherical coordinates (8, cp) of the direction 11 of the beam.
A radiating element of order i along the X axis and of order j on the Y axis is
positioned at the abscissa xi and at the ordinate yj, hence having the
30 coordinates (xi, yj) in the plane X, Y, by selecting, for example, the center of
the array as the origin of the axes.
The phase @ij to be applied to the channel (i, j), in degrees, is given by the
following relationship:
For a regular array, a rectangular mesh for example, the radiating elements
of which are spaced apart by a distance dx along the X axis and by a
5 distance dy along the Y axis, the phase increment behveen adjacent
channels is given by the following relationships:
AQl = 360" dx sin9coscp 1 A along the X axis (6)
A@2 = 360" dy sinesincp I A along the Y axis (7)
Similar expressions may be used for a non-rectangular regular mesh, in
particular for a triangular mesh.
Analysis of relationships (6) and (7), in a manner analogous to the case of
15 the linear array of figure 1, shows that at a given frequency, if the distance
between radiating elements increases along one axis or along both axes,
-then the angular aiming of the beam decreases along one axis or along both
axes, the beam deviating toward the normal 3 to the array 20.
20 An electronic scanning antenna comprises active channels executed in the
form of modules mechanically mounted using a reference plane in order to
guarantee correct mechanical alignment of the modules.
When the ambient temperature varies, the antenna deforms
thermomechanically. If the temperature increases, expansion occurs. The
25 radiating elements move away from one another. As shown previously, for a
control of the phase law effecting a given angular aiming of the beam at a
given frequency, mechanical expansion of the array leads to a change in the
aiming angle of the beam that in this case moves toward the axis 3 of the
antenna. The effect is inverted in the case of a decrease in temperature, the
30 radiating elements moving toward one another.
Aiming precision is of course an essential characteristic for a radar.
Specifically, levels of precision of the order of, for example, a milliradian
(about 0.06") are desired for a radar operating in the X band.
The temperature-dependent behavior of a material is characterized by a
thermal expansion coefficient, denoted by TEC hereinafter. For example, for
a material such as light alloy 5086, this TEC coefficient is of the order of
24.10.~ per degree Kelvin (K) and per unit.length. Stated otherwise, if Lo is a
5 reference dimension at the ambient temperature To corresponding to the
nominal dimensions of the mesh, then the length distortion at a temperature
T is expressed by the following relationship:
AL = CTE. AT.Lo (8)
10
AT = T -To being the temperature gradient and AL the variation in reference
length.
For example, to misalign a beam oriented at go = 60" at 10 GHz, a phase
15 slope po = 360" sin6O0 I A must be applied, where A = 30 mm, the slope being
in " I mm (degrees per millimeter).
A temperature difference of AT leads to mechanical expansion, hence a
change in the phase slope p that becomes:
p = 360" sin60" I A.Lo I (Lo - AL) (9)
For a small variation, this slope may be given by an approximate value,
namely:
Thereby leading to a change in the angular aiming of the beam 1, this aiming
being given by its angle 8, in degrees:
8 = arcsin [A.po l(360 . ( I + ALILo)) (11)
For a temperature variation AT = 25" on a material for which the expansion
coefficient TEC = 24.10-~K1 per unit length,
ALILo = CTE . AT = 6.10.~
Taking po = 360" sin6O0 I A from the preceding example, according to
relationships (1 1) and (12), the aiming angle of the beam then becomes:
It follows that the variation in angular aiming is of the same order as the
desired angular precision.
10 Figure 4 illustrates possible steps of the method according to the invention.
The invention advantageously makes use of the knowledge of how thermal
expansion changes the geometry of an electronic scanning array antenna 10,
20 in order to correct the angular aiming controls of the radiated beam 1.
Again advantageously, the contribution of the error linked to the temperature-
15 dependent expansion of the antenna may be taken into account by modeling
in order to compensate, using a simple calculation, for the defect in angular
aiming of the beam resulting therefrom. It is indeed possible to calculate a
model of the array as a function of the temperature, phase shift values per
radiating element being associated with each temperature. The operating
20 temperature range is sampled in such a way that a model is calculated per
temperature increment. For example, a temperature increment equal to 1
degree Celsius may be taken.
Thus, according to the invention, by knowing the mechanical expansion, or
contraction, coefficient of the antenna as a function of the temperature, it is
25 possible to set up new phase controls that take into account the deformation
of the antenna array in order to aim the radiated beam in the correct angular
direction.
In a preliminary step 30, an associated geometric model is calculated for
each temperature. More specifically, the position of the radiating elements is
30 calculated. The positions are calculated with respect to nominal positions
corresponding to the reference temperature To, 20°C for example. In
particular, for each radiating element 2, it is known how to calculate, based
on the thermal expansion coefficient TEC, its position with respect to its
nominal position, as a function of the temperature. The geometry of the
35 antenna is modeled over its range of operating temperatures, for temperature
values sampled between the minimum temperature and the maximum
temperature.
To angularly aim the beam in a given direction (8, cp) at a temperature T
differing from the reference temperature To by a value AT, it is necessary to
5 apply, to the array of radiating elements (i, j) positioned according to
coordinates (xi, yj), a phase:
Qij = 360" 1 A.[xi sin0coscp + yj sin8sincpl in accordance with relationship (5)
I0 where the coordinates (xi, yj) differ from the initial geometry coordinates (xoi,
yoj) corresponding to the reference temperature To. The difference is a
relative value TEC. AT.
The TEC coefficient is considered to be the same for all of the radiating
elements and to be specific to the array.
15
Returning to figure 4. While operating 300, before the phase 32 of calculating
the aiming of the radiatedheam, a phase 31 of measuring the temperature iscarried
out. The measured temperature indicates the geometric antenna
model to be taken into account for calculating the beam. In particular, this
20 model specifies the coordinates (xi, yj) of the radiating elements to be taken
into account for calculating the beam by applying the phases Qij to the
radiating elements according to relationship (5).
Thus in the first phase 31, the temperature at the array 20 is measured, then
the model corresponding to this temperature is selected.
25 As the models are calculated for the temperatures sampled according to a
given increment, a model corresponds to a measured temperature if that
measured temperature lies in the sampling increment for which the model is
calculated.
In the second phase 32, the phases are calculated that are to be applied to
30 the signals of the radiating elements for the model selected depending on the
desired aiming direction (0, cp).
In the case of application to a linear antenna, in one dimension, phases
defined according to relationship (1) as a function of the abscissa xi will be
applied.
Step 30 of geometrically modeling the antenna array as a function of the
temperature may be carried out just once or periodically according to
mechanical developments of the antenna.
The modeling may advantageously take into account, in addition to the
5 mechanical support, all of the elements constitutive of the array antenna the
behaviour of which varies with temperature, these elements potentially being,
in particular, active elements or transmission lines.
The invention is advantageously applicable to all systems for sending and
10 receiving electromagnetic waves equipped with an electronic scanning
antenna, such as radar systems or telecommunications systems, for
example. Besides the sending and receiving components known elsewhere,
such a sending and receiving system comprises the means for calculating
and controlling the phases of the radiating elements. It also comprises, for
15 example in memory, the models associated with the various temperatures. At
least, a model is stored by storing the coordinates (xi, yj) of the radiating
elements in an axis-system. ~ -

CLAIMS
I. A method for orienting the beam of an electronic scanning antenna, said
antenna (10, 20) being composed of an array of radiating elements
positioned in an initial geometric configuration at a reference temperature
(To), characterized in that, geometric configuration models of said array as a
5 function of the temperature having been set up beforehand, the orientation of
said beam is carried out by:
- a first phase (31) of measuring the temperature of said array in order
to select a model corresponding to the measured temperature;
- a second phase (32) of calculating the phases, depending on the
direction of aiming (9, cp), to be applied to the signals of the radiating
elements (2, 2') for the selected model.
2. The method as claimed in claim 1, characterized in that the geometric
configuration models are calculated in a preliminary step (30) with respect to
15 said initial configuration depending on the temperature and on a thermal -
expansion coefficient TEC specific to said array.
3. The method as claimed in any one of the preceding claims, characterized .
in that a model indicates the geometric position of said radiating elements (2,
20 2') with respect to an axis system.
4. The method as claimed in claim 3, characterized in that said array being
planar, the position of the radiating elements (2, 2') is defined by their
coordinates (xi, yj) in an X, Y axis system, said phases depending on said
25 coordinates.
5. The method as claimed in claim 3, characterized in that said array being
linear, the position of the radiating elements (2) is defined by their abscissae
(xi) along an X axis, said phases depending on said abscissae.
30
6. The method as claimed in any one of the preceding claims, characterized
in that said antenna (10, 20) operating in a given temperature range, the
models are calculated for the temperatures sampled between the minimum
value and the maximum value of the range according to a given increment.
7. A system for sending and receiving electromagnetic waves, comprising an
electronic scanning antenna composed of an array of radiating elements,
characterized in that it implements the method as claimed in any one of the
5 preceding claims.
8. The sending and receiving system as claimed in claim 7, chal-acterized in
that it comprises means for storing said geometric configuration models.
10 9. The sending and receiving system as claimed in either of claims 7 and 8,
characteriied in that it comprises means for calculating said phases to be
applied.
10. The sending and receiving system as claimed in any one of claims 7 to 9,
15 characterized in that it is capable of equipping a radar.