BROADBAND ANTENNA REFLECTOR FOR A CIRCULAR-POLARIZED
PLANAR WIRE ANTENNA AND METHOD FOR PRODUCING SAID
ANTENNA REFLECTOR
The invention is applicable to the field of circular-polarized planar
wire antennas for broadband transmitter or receiver devices. It relates to an
antenna reflector for such an antenna, an antenna device comprising the
reflector and the antenna, and a method for implementing the antenna
5 reflector.
In the framework of certain applications, the antennas must have a
wide band of operating frequencies, for example of the order of a decade, in
other words a frequency band whose maximum frequency is equal to at least
10 ten times the minimum frequency. Circular-polarized planar wire antennas,
such as spiral antennas, belong to these wide frequency band antennas. A
spiral antenna is generally composed of a dielectric substrate into which a
radiating element is etched. The radiating element comprises at least two
strands wound into a spiral whose inner ends are supplied with current. The
15 electromagnetic radiation from the spiral antenna varies depending on the
number of strands and the phase of the current in each strand. The width of
the frequency band depends on the inner and outer diameters of the spiral.
From a theoretical point of view, a planar wire antenna possesses
20 a plane of symmetry and therefore radiates into the whole of space, in
particular in the two directions orthogonal to the plane of the antenna. For
reasons of electromagnetic compatibility, the antennas must not interfere with
the other systems situated nearby. Consequently, they are very often
specified so as to radiate into a half-space. For this reason, the antenna is
25 associated with a reflector which transforms the bidirectional radiation into a
unidirectional radiation. From a practical point of view, this reflector also
serves as a support allowing the antenna to be made more rigid and to be
supplied with current.
30 According to a first solution, the reflector comprises an electrically
conducting plane disposed at a distance from the antenna equal to a quarter
of the mean wavelength of the radiation that it emits or that it receives. At
2
such a distance, the electric field of the reflected backward radiation is then
in phase with the electric field of the forward radiation. The main drawback of
this solution is that the distance can only be adjusted in an optimal manner
for a single wavelength. The electric field of the radiation emitted or received
5 at wavelengths far from this mean wavelength therefore risk being affected,
thus limiting the bandwidth of the antenna. Another drawback of this solution
is that a quarter of a wavelength quickly corresponds to a large distance for
low frequencies, which quickly leads to an overall relatively large thickness
for the antenna. Furthermore, the electrically conducting plane allows the
10 propagation of surface currents and reflection and scattering phenomena
occur at the edge of the antenna, thus generating spurious radiation.
According to a second solution, the antenna reflector comprises a
structure of the Artificial Magnetic Conductor (AMC) type disposed under the
15 plane of the antenna on the side of the backward radiation. A conventional
AMC structure comprises a dielectric substrate, electrically-conducting
patterns disposed periodically on a first surface of the dielectric substrate and
a uniform electrically-conducting plane forming a ground plane on a second
surface of the dielectric substrate. Each conducting pattern can be connected
20 to the ground plane via interconnection holes, generally referred to as "vias"
in the literature. An AMC structure possesses the property of reflecting the
electric field of the backward radiation in phase with the electric field of the
forward radiation. It can therefore be positioned very close to the antenna
and allows a reduction in the thickness of the antenna device comprising the
25 antenna and the AMC structure. An AMC structure can also possess the
property of prohibiting the propagation of electromagnetic waves in certain
directions of the plane in which the conducting patterns are disposed, which
prevents any spurious radiation from being generated. This is referred to as
an electromagnetic band gap (EBG) structure. However, the properties of a
30 structure of the EBG or AMC type are only manifest within a certain band of
frequencies, referred to either as EBG band or as AMC band depending on
the case in question. This band of frequencies, notably its central frequency
and its low and high cutoff frequencies, depends on the shape and on the
dimensions of the conducting patterns, and also on the thickness and on the
35 relative permittivity of the dielectric substrate of the structure. In particular, for
3
a relatively limited thickness of the dielectric substrate, in other words very
small compared to the wavelength, whether either the EBG band or the AMC
band are considered, the bandwidth is very narrow, in other words much less
than an octave. Thus, the constraints relating to the thickness mean that the
5 current antennas comprising a reflector with an EBG or AMC structure do not
allow operation over a wide band of frequencies, greater than a decade.
One aim of the invention is notably to overcome the
aforementioned drawbacks by providing an antenna reflector with a wide
10 frequency band and having a reduced thickness based on a hybrid structure.
This hybrid structure comprises both an electrically conducting plane of the
type of the first solution and a structure of the AMC type based on the second
solution. For this purpose, one subject of the invention is an antenna reflector
locally exhibiting either electromagnetic properties of an electrical conductor,
15 or electromagnetic properties akin to a magnetic conductor, depending on
the radiation emitted or received locally by the antenna. More particularly,
one subject of the invention is an antenna reflector onto which a circularpolarized
planar wire antenna can be mounted that is capable of emitting
electromagnetic radiation in two directions orthogonal to the plane of the
20 antenna over a predetermined frequency band, the antenna reflector being
characterized in that it comprises:
• a first reflection region designed to reflect, with a phase-shift close to
180 degrees, an electric field of the electromagnetic radiation referred to as
backward radiation whose frequency is included within a first sub-band of
25 frequencies, the first reflection region being designed to face a region of the
antenna able to emit electromagnetic radiation in the first sub-band of
frequencies, at a distance allowing the electric field of the backward
electromagnetic radiation to be reflected substantially in phase with the
electric field of the electromagnetic radiation referred to as forward radiation,
30 and
• a second reflection region designed to reflect, with a phase-shift
included between two values of angle on either side of the value of zero
degrees, the electric field of the backward electromagnetic radiation whose
frequency is included within a second sub-band of frequencies, the second
35 reflection region being designed to face a region of the antenna able to emit
4
electromagnetic radiation in the second sub-band of frequencies, at a
distance allowing the electric field of the backward electromagnetic radiation
to be reflected substantially in phase with the electric field of the forward
electromagnetic radiation.
5
The reflector may comprise several reflection regions each
designed to reflect, with a phase-shift included between two values on either
side of the value of zero degrees, the electric field of the backward
electromagnetic radiation whose frequency is included within a sub-band of
10 frequencies. Each reflection region is then designed to face a region of the
antenna able to emit electromagnetic radiation in the sub-band of frequencies
in question, at a distance allowing the electric field of the backward
electromagnetic radiation to be reflected substantially in phase with the
electric field of the forward electromagnetic radiation.
15
Similarly, the reflector may comprise several reflection regions
each designed to reflect, with a phase-shift close to 180 degrees, the electric
field of the backward electromagnetic radiation whose frequency is included
within a sub-band of frequencies. Each reflection region is then designed to
20 face a region of the antenna able to emit electromagnetic radiation in the
sub-band of frequencies in question, at a distance allowing the electric field
of the backward electromagnetic radiation to be reflected substantially in
phase with the electric field of the forward electromagnetic radiation.
25 According to one particular embodiment, the first sub-band of
frequencies corresponds to the highest frequencies of the predetermined
frequency band. The reflector can thus be placed at a distance from the
antenna substantially equal to a quarter of the wavelength of the central
frequency of this sub-band of frequencies, this being relatively close to the
30 antenna.
Advantageously, the sub-bands of frequencies, taken as a whole,
cover substantially the whole of the predetermined frequency band. The
electric field of the backward electromagnetic radiation can thus be in phase
4
5
with the electric field of the forward electromagnetic radiation over the whole
frequency band of the antenna.
The reflector may comprise a substrate made of dielectric material
5 and a ground plane formed on a first surface of the substrate, the first
reflection region being formed on a second surface of the substrate by an
electrically conducting pattern, the other reflection region or regions each
being formed on the second surface of the substrate by a set of electricallyconducting
patterns disposed in a non-conjoined manner.
10
According to a first embodiment, the first and second surfaces of
the substrate are substantially plane and parallel to each other. According to
a second embodiment, the second surface of the substrate has a conical
shape.
15
The electrically-conducting patterns of the sets forming reflection
regions designed to reflect the electric field of the backward electromagnetic
radiation with a phase-shift included between two values on either side of the
value of zero degrees can be electrically connected to the ground plane.
20
According to one particular embodiment, the two values of angle
on either side of the value of zero degrees are substantially equal to -120
degrees and +120 degrees.
25 Another subject of the invention is an antenna device comprising a
circular-polarized planar wire antenna capable of emitting electromagnetic
radiation over a predetermined frequency band and an antenna reflector
according to the invention.
30 Another subject of the invention is a method for implementing the
antenna reflector according to the invention. The method comprises the
following steps:
• a step for determining, in a near-field region, an amplitude distribution
of an electromagnetic radiation able to be emitted by the antenna in the
6
absence of the antenna reflector for at least a first and a second sub-band of
frequencies belonging to the predetermined frequency band,
• a step for determining the shape and dimensions of a first reflection
region of the antenna reflector designed to reflect, with a phase-shift close to
5 180 degrees, an electric field of the electromagnetic radiation referred to as
backward radiation whose frequency is included in the first sub-band of
frequencies, in such a manner that this reflection region can be situated
facing the region of the antenna where the electromagnetic radiation able to
be emitted by the antenna in the first sub-band of frequencies has the highest
10 amplitude, at a distance allowing the electric field of the backward
electromagnetic radiation to be reflected substantially in phase with the
electric field of the electromagnetic radiation referred to as forward radiation,
and
• a step for determining the shape and dimensions of a second
15 reflection region of the antenna reflector designed to reflect, with a phaseshift
included between two values of angle on either side of the value of zero
degrees, the electric field of the backward electromagnetic radiation whose
frequency is included in the second sub-band of frequencies, in such a
manner that this reflection region can be situated facing the region of the
20 antenna where the electromagnetic radiation able to be emitted by the
antenna in the second sub-band of frequencies has the highest amplitude, at
a distance allowing the electric field of the backward electromagnetic
radiation to be reflected substantially in phase with the electric field of the
forward electromagnetic radiation.
25
The method may comprise the following additional steps:
• a step for determining a minimum distance dgmin that can separate the
antenna from the first reflection region of the antenna reflector without
significantly altering the amplitude distribution of the electromagnetic
30 radiation emitted by the antenna in the first sub-band of frequencies,
• a step for determining a minimum distance dBmin that can separate the
antenna from the second reflection region of the antenna reflector without
significantly altering the amplitude distribution of the electromagnetic
radiation emitted by the antenna in the second sub-band of frequencies.
35
*
7
The invention notably offers the advantage of allowing a reflection
coefficient to be maintained close to unity over a wide frequency band,
nominally over the whole operating frequency band of the antenna.
5 The invention will be better understood and other advantages will
become apparent upon reading the description that follows, presented with
reference to the appended drawings in which:
- figure 1 shows one example of an antenna device comprising a
spiral antenna and an antenna reflector according to the invention;
10 - figure 2 shows possible steps in the method for implementing
an antenna reflector according to the invention;
- figures 3a and 3b show examples of distributions of amplitude
of the electromagnetic radiation emitted by a spiral antenna at a given
frequency according to whether the electromagnetic radiation is altered or not
15 by the presence of the antenna reflector;
- figure 4 shows one example of a phase diagram obtained in a
step of the method for implementing an antenna reflector according to the
invention.
20 A perfect electrical conductor, or PEC, is a structure with a surface
having an infinite electrical conductivity. The electric field tangent to this
surface is therefore always zero. An incident electric field encountering the
surface is reflected in phase opposition, irrespective of its frequency. In the
following part of the description, the electrical conductors will be considered
25 as perfect electrical conductors. A perfect magnetic conductor, or PMC, is a
structure comprising a surface on which the tangential magnetic field is
always zero. A magnetic field incident on this surface is cancelled, whereas
the incident electric field is reflected in phase. Structures exhibiting properties
of perfect magnetic conductors cannot be implemented in practice. It is
30 nevertheless possible to form structures exhibiting electromagnetic properties
close to perfect within a certain frequency band and for a given polarization.
It is considered that a surface exhibiting electromagnetic properties close to a
perfect magnetic conductor within a given frequency band is a surface for
which the phase of the reflection coefficient at the frequencies in question is
35 included between two values around 0°. The phase of the reflection
8
coefficient is for example included between -120 and 120 degrees. A surface
exhibiting electromagnetic properties close to a perfect magnetic conductor
within a given frequency band is generally designated as being a highimpedance
surface for this frequency band.
5
Figure 1 shows one example of an antenna device 1 comprising a
spiral antenna 2 and an antenna reflector 3 according to the invention. The
spiral antenna 2 is capable of emitting over a predetermined frequency band,
known as operating frequency band AF. It can emit electromagnetic radiation
10 in two directions orthogonal to its plane. The electromagnetic radiation
propagating in the direction opposite to the antenna reflector 3 is called
forward radiation, and the electromagnetic radiation propagating in the
opposite direction is called backward radiation. The spiral antenna 2
comprises a dielectric substrate 21 and two electrically conducting strands
15 22a and 22b forming the radiating element of the spiral antenna 2. The
dielectric substrate 21 is for example an epoxide board of the printed circuit
type. It comprises an upper surface 24 and a lower surface 25 substantially
plane and parallel. The conducting strands 22a and 22b have an identical
length and are mutually wound around a central point O so as to form a spiral
20 26 on the upper surface 24. The first strand 22a extends between an inner
end A and an outer end B of the spiral 26. The second strand 22b extends
between an inner end C and an outer end D of the spiral 26. The spiral
antenna 2 also comprises means for powering the radiating element, not
shown. Normally, the two strands 22a and 22b are powered on their inner
25 ends A and C by microwave signals in phase opposition. The strands 22a
and 22b may be printed or etched onto the upper surface 24. They may also
be formed in an electrically-conducting material and fixed onto the upper
surface 24.
30 In figure 1, a planar wire antenna of the Archimedes spiral type is
formed. In such an antenna, each conducting strand has a constant
thickness and a constant spacing with respect to the other strand.
Nevertheless, the invention is also applicable to any type of circular-polarized
planar wire antenna. It is notably applicable to equiangular spiral antennas,
35 also called logarithmic spiral antennas, in which the width of the strands and
9
the spacing between the strands increase with the distance from the center
of the spiral. Similarly, the spiral antenna in figure 1 comprises two
electrically conducting strands. However, the invention is also applicable to
antennas comprising a different number of strands.
5
The antenna reflector, being the subject of the invention, uses the
operating properties of planar wire antennas. The radiating element of such
an antenna, when it is excited, emits electromagnetic radiation from a
localized region of operation, associated with the relative arrangement of the
10 strands and with the phase offset of the current flowing in the various
strands. This region of operation exhibits the particularity of varying as a
function of the frequency according to a law specific to each type of planar
wire antenna. In particular, for an Archimedes spiral antenna whose strands
are powered in phase opposition, the region of operation from which
15 electromagnetic radiation is emitted at a given frequency forms a ring whose
mean diameter is substantially equal to the wavelength of the
electromagnetic radiation divided by the number Pi (D=A/TT). The antenna
reflector according to the invention, on which an antenna is designed to be
mounted, thus comprises at least two reflection regions whose
20 electromagnetic properties are adapted to the electromagnetic radiation
emitted locally by the antenna. A first reflection region exhibits
electromagnetic properties of an electrical conductor, notably in a first subband
of frequencies AF1. This sub-band of frequencies AF1 corresponds for
example to high frequencies of the operating frequency band AF within which
25 the planar wire antenna emits. A second reflection region exhibits
electromagnetic properties close to a perfect magnetic conductor in a second
sub-band of frequencies AF2. This second sub-band of frequencies AF2
corresponds for example to lower frequencies than those of the first subband
of frequencies AF1. The antenna reflector thus comprises reflection
30 regions of two different types, namely at least one reflection region exhibiting
electromagnetic properties of an electrical conductor, and at least one
reflection region exhibiting electromagnetic properties close to a perfect
magnetic conductor. The antenna reflector can also comprise additional
regions exhibiting either electromagnetic properties of an electrical conductor
35 (reflection regions of the first type), or electromagnetic properties close to a
10
perfect magnetic conductor (reflection regions of the second type) in other
sub-bands of frequencies. Advantageously, these various sub-bands of
frequencies are determined in such a manner as to cover, with the first subband
of frequencies AF1, the whole of the operating frequency band AF.
5 According to one particular embodiment, the regions exhibiting
electromagnetic properties of an electrical conductor are alternated with
regions exhibiting electromagnetic properties close to a perfect magnetic
conductor.
10 In the exemplary embodiment shown in figure 1, the antenna
reflector 3 comprises a dielectric substrate 31, a ground plane 32 carried by a
lower surface 33 of the dielectric substrate 31, and three sets 341, 342, 343
of electrically-conducting patterns 34 carried by an upper surface 35 of the
dielectric substrate 31. The dielectric substrate 31 can be an epoxide board
15 of the printed circuit type whose upper surface 35 and lower surface 33 are
substantially plane and parallel. The conducting patterns 34 can then be
printed or etched onto the upper surface 35 of the dielectric substrate 31.
More generally, they can be formed by any conventional technique for
construction of printed circuits. They may also be formed in an electrically
20 conducting material and fixed onto the upper surface 35. The lower surface
25 of the dielectric substrate 21 of the spiral antenna 2 is situated opposite
the upper surface 35 of the dielectric substrate 31 of the antenna reflector 3.
The dielectric substrate 21 may come directly into contact with the
conducting patterns 34. The dielectric substrate 21 then fulfills a function of
25 electromagnetic isolation between the spiral antenna 2 and the antenna
reflector 3. This isolation may nevertheless be provided by any other means.
Each set 341, 342, 343 of conducting patterns 34 is configured in such a
manner as to form a reflection region whose electromagnetic properties may
differ from those of the other regions in order to be adapted to the
30 electromagnetic radiation to be reflected locally. The first set 341 of
conducting patterns 34 only comprises a single conducting pattern in the
shape of a disk. The conducting disk 36 thus forms a first reflection region
341A whose electromagnetic properties match those of an electrical
conductor. This zone 341A therefore belongs to the first type of reflection
35 region. In particular, the conducting disk 36 exhibits electromagnetic
11
properties of an electrical conductor at least in the first sub-band of
frequencies AF1. The antenna reflector 3 can thus be placed at a distance
relatively close to the spiral antenna 2. The distance in question between the
antenna reflector 3 and the spiral antenna 2 can be the distance between the
5 upper surface 35 of the dielectric substrate 31 of the antenna reflector 3 and
the upper surface 24 of the dielectric substrate 21 of the spiral antenna 2,
called height h. Theoretically, the height h may be substantially equal to an
odd integer multiple of a quarter of a wavelength of the central frequency of
the first sub-band of frequencies AF1 ((2.N+1).A/4, where N is a natural
10 integer), the backward reflected electromagnetic radiation then being in
phase with the incident radiation on the upper surface 24 of the dielectric
substrate 21 of the spiral antenna 2. The height h is for example substantially
equal to a quarter of the wavelength of the central frequency of the first subband
of frequencies AF1. The second set 342 of conducting patterns 34
15 comprises several non-conjoined electrically-conducting patterns 34
disposed on the upper surface 35 in such a manner as to form overall an
annular reflection region 342A surrounding the conducting disk 36 and whose
center substantially coincides with the center of the conducting disk 36.
Similarly, the third set 343 of conducting patterns 34 comprises several non-
20 conjoined conducting patterns 34 forming overall an annular reflection region
343A with a diameter greater than the diameter of the annular region 342A
formed by the second set 342 of conducting patterns 34. The conducting
patterns 34 of the second and third sets 342 and 343 may be electrically
connected to the ground plane 32, for example by means of metallized holes
25 formed in the dielectric substrate 31 of the antenna reflector 3. Each set 342
and 343 of conducting patterns 34 thus forms a reflection region exhibiting
electromagnetic properties close to a perfect magnetic conductor. The
geometric shape and the dimensions of the conducting patterns 34 are
determined in such a manner that each annular reflection region 342A and
30 343A, designed to locally form a reflector for the region of operation of the
spiral antenna 2 in a sub-band of frequencies AF2 or AF3, exhibits
electromagnetic properties close to a perfect magnetic conductor at least in
this sub-band of frequencies AF2 or AF3. The reflection regions 342A and
343A thus belong to the second type of reflection region. The antenna
35 reflector 3 may also comprise other reflection regions whose electromagnetic
"
12
properties match those of an electrical conductor (reflection regions of the
first type). These reflection regions are designed to come to a distance from
the antenna 2 so as to be able to reflect the electric field of the backward
electromagnetic radiation substantially in phase with the electric field of the
5 forward electromagnetic radiation on the upper surface 24 of the antenna 2.
In theory, the height, or distance between these reflection regions and the
antenna 2, must be substantially equal to an even integer multiple of a
quarter of a wavelength of the central frequency of the respective sub-band
of frequencies (2.N.A/4, where N is a natural integer). In practice, the height
10 may differ depending on the near field emitted by the antenna 2, as explained
hereinbelow.
Figure 2 illustrates possible steps of the method for implementing
an antenna reflector according to the invention for a planar wire antenna. For
15 the following part of the description, the particular case of a spiral antenna
such as that shown in figure 1 continues to be considered. The method is
nevertheless applicable to any type of circular-polarized planar wire antenna.
In a first step 101, the electromagnetic radiation emitted by the spiral antenna
2 alone, in other words without the antenna reflector 3, is characterized for at
20 least two frequencies belonging to the operating frequency band AF of the
spiral antenna 2. It is of course possible to characterize the electromagnetic
radiation over two sub-bands of frequencies belonging to the operating
frequency band AF. For the rest of the description, it is considered that the
electromagnetic radiation is characterized for the frequency sub-bands AF1,
25 AF2 and AF3. More particularly, distributions are determined in amplitude and
in phase of electromagnetic fields emitted by the spiral antenna 2 in the near
field region in a plane substantially parallel to the plane of the spiral antenna
2, in this case, the upper surface 24. For this purpose, the conducting strands
22a and 22b of the spiral antenna 2 are powered on their inner ends A and C
30 by electrical currents with the same amplitudes and, in general, having a
phase difference of 180 degrees. As indicated hereinabove, the
electromagnetic radiation emitted by the spiral antenna 2 has a maximum
amplitude when the currents flowing in the strands 22a and 22b are locally in
phase. In practice, the electromagnetic radiation emitted by the spiral
35 antenna 2 at a given frequency has a maximum amplitude in a region forming
13
a circular ring whose mean diameter is substantially equal to the wavelength
of the electromagnetic radiation divided by the number Pi. In a second step
102, the minimum distance dEmin that can separate the spiral antenna 2 from
an electrical conductor without altering the amplitude distribution of the
5 electromagnetic radiation emitted by the spiral antenna 2 in the sub-band of
frequencies AF1 is determined. The amplitude distribution is for example
considered in the near field region. The distance in question is for example
the height h between the upper surface 35 of the dielectric substrate 31 of
the antenna reflector 3 and the upper surface 24 of the dielectric substrate 21
10 of the spiral antenna 2. The step 102 can be carried out over a wide
frequency band, for example over the whole operating frequency band AF. In
practice, the idea is essentially to determine the minimum distance that must
separate the spiral antenna 2 from the reflection region 341A exhibiting
electromagnetic properties of an electrical conductor. The step 102 is
15 therefore carried out at least for the sub-band of frequencies AF1.
Figures 3a and 3b show two examples of distributions of amplitude
of the electromagnetic radiation emitted by a spiral antenna 2 at a given
frequency in a plane belonging to the near field region parallel to the plane of
20 the spiral antenna 2. The first distribution, shown in figure 3a, is related to a
distance between the spiral antenna 2 and the antenna reflector 3 for which
the electromagnetic radiation is not altered; the second distribution, shown in
figure 3b, relates to a distance for which the electromagnetic radiation is
altered. In figure 3a, several different circular rings 301 to 305 corresponding
25 to various amplitudes of the electrical energy density can be seen. The rings
301 and 305, 302 and 304, and 303 exhibit for example mean amplitudes
respectively equal to 2.10"7 J/m3, 6.10"7 J/m3, and 1.5.10"6 J/m3. The ring 303
thus corresponds to the region of operation of the spiral antenna 2 at the
given frequency. The annular shape of the amplitude distribution allows it to
30 be deduced that the electromagnetic radiation is not altered. In figure 3b,
several different regions with an amplitude of irregular shape are seen. A first
zone 306 exhibits a mean amplitude substantially equal to 2.10"7 J/m3. Two
regions 307a and 307b exhibit a mean amplitude substantially equal to
2.5.10'6 J/m3, and two regions 308a and 308b exhibit a mean amplitude
35 substantially equal to 5.5.106 J/m3. The fact that the regions exhibiting a
14
maximum amplitude do not form a continuous annular region allows it to be
deduced that the electromagnetic radiation is altered. Of course, the altered
or unaltered nature of the electromagnetic radiation must be examined
according to the geometry of the antenna in question. In the case of a spiral
5 antenna, the discriminating shape is a circular ring.
In a third step 103 of the method for implementing an antenna
reflector 3 according to the invention, the minimum distance dBmin that can
separate the spiral antenna 2 from a perfect magnetic conductor without
10 altering the amplitude distribution of the electromagnetic radiation emitted by
the spiral antenna 2, at least in one of the sub-bands of frequencies AF2 and
AF3, is determined. The amplitude distribution is for example considered in
the near field region. The distance in question may also be the height h. The
step 103 can be carried out over a wide frequency band, for example over
15 the whole of the operating frequency band AF. In practice, the idea is
essentially to determine the minimum distance dBmin that needs to separate
the spiral antenna 2 from the reflection regions 342A and 343A whose
electromagnetic properties match those of a perfect magnetic conductor. The
step 103 is therefore preferably carried out for the sub-bands of frequencies
20 AF2 and AF3. Where appropriate, it is carried out for each of the sub-bands
of frequencies in question outside of the frequency sub-band AF1. In a fourth
step 104, the shape and the dimensions of the first reflection region 341 A,
exhibiting electromagnetic properties of an electrical conductor in the subband
of frequencies AF1 (reflection region of the first type), are determined in ?
25 such a manner that this reflection region 341A comes into the vicinity of the
region of operation of the spiral antenna 2 in this sub-band of frequencies
AF1. The step 104 essentially consists in determining the diameter of the
conducting disk 36. In a fifth step 105, the shape and the dimensions of the
reflection regions 342A and 343A, exhibiting electromagnetic properties close
30 to a perfect magnetic conductor in the respective sub-bands of frequencies
AF2 and AF3 (reflection regions of the second type), are also determined in
such a manner that each reflection region 342A and 343A comes into the
vicinity of the region of operation of the spiral antenna 2 in the respective
sub-band of frequencies AF2 or AF3. The step 105 essentially consists in
35 determining the inner and outer diameters of the reflection regions 342A and
':
:
|
I
i.
15
343A together with the lengths of the arcs of circles radially bounding the
conducting patterns 34. More generally, the step 105 consists in determining
the location and the surface area of the conducting patterns 34 in such a
manner that each set of conducting patterns forms a surface exhibiting
5 electromagnetic properties close to a perfect magnetic conductor in a subband
of frequencies. In the steps 104 and 105, it is considered that a
reflection region comes into the vicinity of a region of operation of the spiral
antenna 2 when it allows the electromagnetic radiation emitted by this region
of operation to be reflected in the desired direction of radiation. It is to be
10 noted that the steps of the method for implementing the antenna reflector 3
may be carried out in a different order, as long as the first step 101 is carried
out prior to the steps 104 and 105.
The step 105 may, for example, be carried out by adapting
15 conventional AMC structures. A conventional AMC structure comprises a
dielectric substrate, a ground plane carried by a first surface of the dielectric
substrate, and electrically-conducting patterns with a rectangular shape
arranged according to a regular matrix and carried by a second surface of the
dielectric substrate. The thickness of the dielectric substrate of the
20 conventional AMC structure is preferably chosen to be equal to the thickness
of the dielectric substrate 31 of the antenna reflector 3. An AMC structure
exhibits electromagnetic properties close to a perfect magnetic conductor in a
given sub-band of frequencies. In a first sub-step, for each sub-band of
frequencies outside of the sub-band of frequencies AF1, the dimensions
25 (length and width) of the conducting patterns of a conventional AMC structure
are determined which allow a surface exhibiting properties close to a perfect
magnetic conductor to be formed in the sub-band of frequencies in question.
In the case of a spiral antenna, the surfaces of the conducting patterns
forming the reflector become larger at greater distances from the center of
30 the antenna reflector 3. In a second sub-step, for each of the sub-bands of
frequencies in question, the conducting patterns of the conventional
structures AMC are adapted to the corresponding region of operation of the
spiral antenna 2, each adapted conducting pattern 34 conserving
substantially the same surface area as that in the conventional AMC
35 structure. In a spiral antenna, the conducting patterns 34 therefore take an
16
overall annular shape, as shown in figure 1. In a third sub-step, a phase
diagram is constructed resulting from the association of various phase
diagrams, each being associated with one of the conventional AMC
structures in question. Figure 4 shows one example of such a phase
5 diagram. Phases of the reflection coefficient of the various conventional AMC
structures are plotted on a first graph as a function of the radius of the spiral
antenna 2; the operating frequencies of the spiral antenna 2 are plotted on a
second graph as a function of the radius of the spiral antenna 2. In a fourth
sub-step, based on the phase diagram in figure 4, at least one set 342 of
10 conducting patterns 34 is chosen which allows incident electromagnetic
radiation to be reflected with a phase-shift substantially equal to zero
degrees. Preferably, several sets of conducting patterns 34 are chosen, for
example the two sets 341 and 342, in such a manner as to cover various
regions of operation of the spiral antenna 2 without there being any overlap
15 of conducting patterns 34 between various sets.
The antenna reflector 3 obtained by the method according to the
invention is designed to receive a spiral antenna 2 at a minimum distance for
which neither the first reflection region 341A nor the reflection regions 342A
20 and 343A alter the electromagnetic radiation. The minimum distance
preferably corresponds to the maximum between the distances dEmin and
dsmin determined in the steps 102 and 103. Given that the wavelengths of the
electromagnetic radiation emitted in the first sub-band of frequencies AF1 are
shorter than the wavelengths of the electromagnetic radiation emitted in the
25 second sub-band of frequencies AF2, the electromagnetic radiations emitted
in both the sub-band of frequencies AF1 and in the sub-band of frequencies
AF2 can be in phase with the corresponding reflected electromagnetic
radiations in the near field region. In order to maintain a reflection in phase
over the whole operating frequency band AF of the spiral antenna 2, it is
30 furthermore possible to vary the distance separating the spiral antenna 2
from the antenna reflector 3, or to use magneto-dielectric materials exhibiting
various dielectric permittivities.

17
CLAIMS
1. An antenna reflector onto which a circular-polarized planar
wire antenna (2) can be mounted that is capable of emitting electromagnetic
radiation in two directions orthogonal to the plane of the antenna (2) over a
predetermined frequency band, the antenna reflector (3) being characterized
5 in that it comprises:
• at least one reflection region (341 A) of a first type, each of said
regions being capable of reflecting, with a phase-shift close to 180 degrees,
an electric field of the electromagnetic radiation referred to as backward
radiation whose frequency is included within a sub-band of the frequency
10 band, each of said reflection regions (341A) being designed to face a region
of the antenna (2) able to emit electromagnetic radiation in the corresponding
sub-band of frequencies, at a distance allowing the electric field of the
backward electromagnetic radiation to be reflected substantially in phase with
the electric field of the electromagnetic radiation referred to as forward
15 radiation, and
• at least one reflection region (342A, 343A) of a second type, each of
said regions being capable of reflecting, with a phase-shift included between
two values of angle on either side of the value of zero degrees, the electric
field of the backward electromagnetic radiation whose frequency is included
20 within a sub-band of the frequency band, each of said reflection regions
(342A, 343A) being designed to face a region of the antenna (2) able to emit
electromagnetic radiation in the corresponding sub-band of frequencies, at a
distance allowing the electric field of the backward electromagnetic radiation
to be reflected substantially in phase with the electric field of the forward
25 electromagnetic radiation.
2. The reflector as claimed in claim 1 comprising several
reflection regions (342A, 343A) of the second type, each of said reflection
regions (342A, 343A) being designed to face a region of the antenna (2) able
30 to emit electromagnetic radiation in the sub-band of frequencies in question,
at a distance allowing the electric field of the backward electromagnetic
radiation to be reflected substantially in phase with the electric field of the
forward electromagnetic radiation.
18
3. The reflector as claimed in either of claims 1 and 2 comprising
a single reflection region (341A) of the first type at the center of the other
reflection region or regions (342A, 343A) of the second type.
5 4. The reflector as claimed in claim 3, in which the sub-band of
frequencies of the reflection region (341 A) of the first type corresponds to the
highest frequencies of the predetermined frequency band.
5. The reflector as claimed in one of the preceding claims, in
10 which the sub-bands of frequencies of the various reflection regions (341 A,
342A, 343A) are separate from one another and, taken as a whole, cover
substantially the whole of the predetermined frequency band.
6. The reflector as claimed in one of the preceding claims,
15 comprising a substrate (31) made of dielectric material and a ground plane
(32) formed on a first surface (33) of the substrate (31), the reflection region
or regions (341A) of the first type each being formed on a second surface
(35) of the substrate (31) by an electrically conducting pattern (34, 36), the
other reflection region or regions (342A, 343A) of the second type each being
20 formed on the second surface (35) of the substrate (31) by a set (342, 343) of
electrically-conducting patterns (34) disposed in a non-conjoined manner.
7. The reflector as claimed in claim 6, in which the first and
second surfaces (33, 35) of the substrate (31) are substantially plane and
25 parallel to each other.
8. The reflector as claimed in claim 6, in which the second
surface (35) of the substrate (31) has a conical shape.
30 9. The antenna reflector as claimed in one of claims 6 to 8, in
which the electrically-conducting patterns (34) of the sets (342, 343) forming
reflection regions (342A, 343A) of the second type are electrically connected
to the ground plane (32).
19
10. The reflector as claimed in one of the preceding claims, in
which the two values of angle on either side of the value of zero degrees are
substantially equal to -120 degrees and +120 degrees.
5 11. An antenna device comprising a circular-polarized planar wire
antenna (2) capable of emitting electromagnetic radiation over a
predetermined frequency band and an antenna reflector (3) as claimed in
one of the preceding claims.
10 12. A method for implementing an antenna reflector (3) for a
circular-polarized planar wire antenna (2) capable of emitting electromagnetic
radiation in two directions orthogonal to the plane of the antenna (2) over a
predetermined frequency band, characterized in that it comprises the
following steps:
15 "a step (101) for determining, in a near-field region, an amplitude
distribution of electromagnetic radiation able to be emitted by the antenna (2)
in the absence of the antenna reflector (3) for at least a first and a second
sub-band of frequencies belonging to the predetermined frequency band,
• a step (104) for determining the shape and dimensions of a first
20 reflection region (341 A) of the antenna reflector (3) designed to reflect, with a
phase-shift close to 180 degrees, an electric field of the electromagnetic
radiation referred to as backward radiation whose frequency is included
within the first sub-band of frequencies, in such a manner that this reflection
region (341A) can be situated facing the region of the antenna (2) where the
25 electromagnetic radiation able to be emitted by the antenna (2) in the first
sub-band of frequencies has the highest amplitude, at a distance allowing the
electric field of the backward electromagnetic radiation to be reflected
substantially in phase with the electric field of the electromagnetic radiation
referred to as fonA/ard radiation, and
30 "a step (105) for determining the shape and dimensions of a second
reflection region (342A) of the antenna reflector (3) designed to reflect, with a
phase-shift included between two values of angle on either side of the value
of zero degrees, the electric field of the backward electromagnetic radiation
whose frequency is included in the second sub-band of frequencies, in such
35 a manner that this reflection region (342A) can be situated facing the region
20
of the antenna (2) where the electromagnetic radiation able to be emitted by
the antenna (2) in the second sub-band of frequencies has the highest
amplitude, at a distance allowing the electric field of the backward
electromagnetic radiation to be reflected substantially in phase with the
5 electric field of the forward electromagnetic radiation.
13. The method as claimed in claim 12, furthermore comprising
the following steps:
• a step for determining a minimum distance dEmin that can separate the
10 antenna (2) from the first reflection region (341 A) of the antenna reflector (3)
without significantly altering the amplitude distribution of the electromagnetic
radiation emitted by the antenna (2) in the first sub-band of frequencies,
• a step for determining a minimum distance demin that can separate the
antenna (2) from the second reflection region (342A) of the antenna reflector
15 (3) without significantly altering the amplitude distribution of the
electromagnetic radiation emitted by the antenna (2) in the second sub-band
of frequencies.