Multiple-beam antenna with photonic bandgap material

The invention relates to a multi-beam antenna comprising:<ul id="ul0001" list-style="dash" compact="compact"><li>a BIP material (Photonic Prohibition Tape) capable of filtering spatially and frequency electromagnetic waves, this BIP material having at least one non-conducting band and forming an outer radiating surface in emission and / or reception,</li><li>at least one periodicity defect of the BIP material so as to create at least a narrow bandwidth within said at least one non-pass band of this BIP material, and</li><li>an excitation device adapted to emit and / or receive electromagnetic waves within said at least one narrow bandwidth created by said at least one defect.</li></ul>

Multi-beam antennas are widely used in space applications and especially in geostationary satellites to transmit to the earth's surface and / or receive information from the Earth's surface. They comprise for this purpose several radiating elements each generating a beam of electromagnetic waves spaced from the other beams. These radiating elements are, for example, placed near the focus of a parabola forming reflector of electromagnetic wave beams, the parabola and the multi-beam antenna being housed in a geostationary satellite. The parabola is intended to direct each beam on a corresponding area of ​​the earth's surface. Each area of ​​the Earth's surface illuminated by a beam of the multi-beam antenna is commonly referred to as a coverage area. Thus, each coverage area corresponds to a radiating element.

Currently, the radiating elements used are known as "horns" and the multi-beam antenna equipped with such horns is referred to as a horn antenna. Each horn produces a substantially circular radiating spot forming the base of a conical beam radiated emission or reception. These horns are arranged next to each other so as to bring as close as possible the radiant spots of each other.

FIG. 1A diagrammatically represents a multi-beam antenna with cornets in front view in which seven squares F1 to F7 indicate the bulk of seven cones arranged contiguously to one another. Seven circles S1 to S7, each inscribed in one of the squares F1 to F7, represent the radiating spots produced by the corresponding horns. The antenna of FIG. 1A is placed at the focus of a parabola of a geostationary satellite intended to transmit information on the French territory.

Figure 1B shows areas C1 to C7 of coverage at -3 dB, each corresponding to a radiating spot of the antenna of Figure 1A. The center of each circle corresponds to a point on the earth's surface where the power received is maximum. The perimeter of each circle delimits an area within which the power received on the earth's surface is greater than half the maximum power received at the center of the circle. Although the radiating spots S1 to S7 are substantially contiguous, they produce cover areas at -3 dB disjoined from each other. The regions between the -3 dB coverage areas are referred to here as receiving holes. Each receiving hole therefore corresponds to a region of the earth's surface where the received power is less than half of the maximum power received. In these receiving holes, the received power may be insufficient for a receiver floor to function properly.

To solve this reception hole problem, it has been proposed to overlap the radiating spots of the multi-beam antenna with each other. A partial front view of such a multi-beam antenna having a plurality of overlapping radiating spots is illustrated in Figure 2A. In this figure, only two radiating spots SR1 and SR2 have been represented. Each radiant spot is produced from seven independent and distinct radiation sources. The radiating spot SR1 is formed from the SdR1 to SdR7 radiation sources arranged contiguously next to one another. A radiating spot SR2 is produced from SdR1, SdR2, SdR3 and SdR7 radiation sources and from SdR8 to SdR10 radiation sources. The SdR1 to SdR7 radiation sources are adapted to work at a first working frequency to create a first substantially uniform electromagnetic wave beam at this first frequency. The sources of radiation SdR1 to SdR3 and SdR7 to SdR10 are adapted to work at a second working frequency so as to create a second beam of electromagnetic waves substantially uniform at this second working frequency. Thus, the sources of radiation SdR1 to SdR3 and SdR7 are able to work simultaneously at the first and second working frequencies. The first and second working frequencies are different from each other so as to limit interference between the first and second beams produced.

Thus, in such a multi-beam antenna, radiation sources, such as SdR1-3 radiation sources, are used both to create the SR1 radiating spot and the SR2 radiating spot, thereby producing an overlap of these two. radiating spots SR1 and SR2. An illustration of the arrangement of the -3 dB coverage areas created by a multi-beam antenna with overlapping radiating spots is shown in Figure 2B. Such an antenna can significantly reduce the receiving holes, or even make them disappear. However, in part because of the fact that a radiating spot is formed from several independent and distinct radiation sources, at least some of which are also used for other radiating spots, this multi-beam antenna is more complex to order than conventional horn antennas.

The aim of the invention is to remedy this drawback by proposing a simpler overlapping multi-beam antenna with radiating spots.

It therefore relates to an antenna as defined above, characterized:<ul id="ul0002" list-style="dash" compact="compact"><li>in that the excitation device is able to work simultaneously at least around a first and a second different working frequency,</li><li>in that the excitation device comprises a first and a second excitation element that are distinct and independent of one another, each capable of transmitting and / or receiving electromagnetic waves, the first excitation element being capable of working at the first working frequency and the second excitation element being able to work at the second working frequency,</li><li>in that the or each frequency defect of the BIP material forms a leak resonant cavity having a constant height in a direction orthogonal to said radiating outer surface, and determined lateral dimensions parallel to said radiating outer surface,</li><li>in that the first and second working frequencies are able to excite the same resonance mode of a resonant leak cavity, this resonance mode being established identically regardless of the lateral dimensions of the cavity, so as to create on said outer surface respectively a first and a second radiating spots, each of these radiating spots representing the origin of a beam of electromagnetic waves radiated in emission and / or reception by the antenna,</li><li>in that each of the radiating spots has a geometric center whose position is a function of the position of the excitation element which gives rise to it and whose surface is greater than that of the radiating element giving rise to it, and</li><li>in that the first and second excitation elements are placed relative to one another so that the first and second radiating spots are arranged on the outer surface of the BIP material next to each other. the other and overlap partially.</li></ul>

In the multi-beam antenna described above, each excitation element produces a single radiating spot forming the base or cross-section at the origin of an electromagnetic wave beam. Thus, from this point of view, this antenna is comparable with conventional horn antennas where a horn produces a single radiating spot. The control of this antenna is therefore similar to that of a conventional horn antenna. In addition, the excitation elements are placed so as to overlap the radiating spots. This antenna thus has the advantages of a multi-beam antenna with overlapping radiating spots without the complexity of the control of the excitation elements has been increased compared to that of multi-beam horn antennas.

According to other characteristics of a multi-beam antenna according to the invention:<ul id="ul0003" list-style="dash" compact="compact"><li>each radiant spot is substantially circular, the geometric center corresponding to a maximum of power transmitted and / or received and the periphery corresponding to a transmitted and / or received power equal to a fraction of the maximum power transmitted and / or received at its center, and the distance, in a plane parallel to the outer surface, separating the geometric centers of the two excitation elements, is strictly less than the radius of the radiating spot produced by the first excitation element added to the radius of the radiating spot produced by the second excitation element,</li><li>the geometric center of each radiating spot is placed on the line orthogonal to said radiating outer surface and passing through the geometric center of the excitation element giving rise to it,</li><li>the first and the second excitation elements are placed inside the same cavity,</li><li>the first and second working frequencies are located within the same narrow bandwidth created by the same cavity,</li><li>the first and second excitation elements are each placed inside distinct resonant cavities, and the first and second working frequencies are each able to excite a resonance mode independent of the lateral dimensions of their respective cavity,</li><li>a reflective plane of electromagnetic radiation associated with the BIP material, this reflective plane being deformed so as to form said distinct cavities,</li><li>the or each cavity is of parallelepipedal shape,</li><li>the device for focusing the electromagnetic waves comprises a half-cylinder shaped reflector, and the BIP material of the antenna has a convex surface corresponding to the half-cylinder-shaped surface of the reflector.</li></ul>

The invention also relates to a system for transmitting and / or receiving electromagnetic waves comprising:<ul id="ul0004" list-style="dash" compact="compact"><li>a device capable of focusing the electromagnetic waves emitted and / or received by the system on a focal point, and</li><li>an emitter and / or receiver of electromagnetic waves placed substantially at the focal point so as to emit and / or receive said electromagnetic waves, characterized in that it comprises an antenna according to the invention, whose radiating outer surface is substantially placed on the focal point so as to form said transmitter and / or receiver of electromagnetic waves.</li></ul>

According to other features of the system according to the invention:<ul id="ul0005" list-style="dash" compact="compact"><li>the device capable of focusing the electromagnetic waves is a parabolic reflector,</li><li>the device capable of focusing the electromagnetic waves is an electromagnetic lens.</li></ul>

The invention will be better understood on reading the description which follows, given solely by way of example, and with reference to the drawings, in which:<ul id="ul0006" list-style="dash" compact="compact"><li>FIGS. 1A, 1B, 2A and 2B show known multi-beam antennas as well as the resulting coverage areas;</li><li>Figure 3 is a perspective view of a multi-beam antenna according to the invention;</li><li>Fig. 4 is a graph showing the transmission coefficient of the antenna of Fig. 3;</li><li>Fig. 5 is a graph showing the radiation pattern of the antenna of Fig. 3;</li><li>Figure 6 is a schematic and sectional illustration of an electromagnetic wave transmission / reception system equipped with the antenna of Figure 3;</li><li>FIG. 7 represents a second embodiment of a multi-beam antenna according to the invention;</li><li>FIG. 8 represents the transmission coefficient of the antenna of FIG. 7;</li><li>FIG. 9 represents a third embodiment of a multi-beam antenna according to the invention; and</li><li>Figure 10 is an illustration of a semicylindrical antenna according to the invention.</li></ul>

FIG. 3 represents a multi-beam antenna 4. This antenna 4 is formed of a photonic ban band material or BIP material associated with a metal plane 22 reflecting electromagnetic waves.

The BIP materials are known and the design of a BIP material such as the material 20 is, for example, described in the patent application FR 99 14521. Thus, only the specific characteristics of the antenna 4 with respect to this state of the art. the technique will be described here in detail.

It is recalled that a BIP material is a material which has the property of absorbing certain frequency ranges, that is to say of prohibiting any transmission in said aforementioned frequency ranges. These frequency ranges form what is called here a non-conducting band.

A non-conducting band B of the material 20 is illustrated in FIG. This FIG. 4 represents a curve representing the variations of the transmission coefficient expressed in decibels as a function of the frequency of the electromagnetic wave emitted or received. This transmission coefficient is representative of the energy transmitted on one side of the BIP material with respect to the energy received on the other side. In the case of the material 20, the non-conducting band B or absorption band B extends substantially from 7 GHz to 17 GHz.

The position and width of this non-conducting band B depends solely on the properties and characteristics of the BIP material.

The BIP material generally consists of a periodic arrangement of dielectric permittivity and / or variable permeability. Here, the material 20 is formed from two blades 30, 32 made of a first magnetic material such as alumina and two blades 34 and 36 formed in a second magnetic material such as air. The blade 34 is interposed between the blades 30 and 32, while the blade 36 is interposed between the blade 32 and the reflective plane 22. The blade 30 is disposed at one end of this stack of blades. It has an outer surface 38 opposite its surface in contact with the blade 34. This surface 38 forms a radiating surface in emission and / or reception.

In a known manner, the introduction of a break in this geometric and / or radio frequency periodicity, a break that is also called a defect, makes it possible to generate an absorption defect and therefore the creation of a narrow bandwidth within the non-magnetic band. passing through the BIP material. Under these conditions, the material is referred to as defective BIP material.

Here, a break in geometric periodicity is created by choosing the height or thickness H of the blade 36 greater than that of the blade 34. In known manner, and so as to create a narrow band E (Figure 4) substantially in the middle of the bandwidth B, this height H is defined by the following relation: <maths id="math0001" num=""><math display="block"><mrow><mi mathvariant="normal">H</mi><mo>=</mo><mn>0</mn><mo>,</mo><mn>5</mn><mo>×</mo><mrow><mi mathvariant="normal">λ</mi><mo>/</mo><mrow><msqrt><mrow><msub><mi>ε</mi><mi>r</mi></msub><mo>×</mo><msub><mi>μ</mi><mi>r</mi></msub></mrow></msqrt></mrow></mrow></mrow></math><img file="EP1568104B1_D0001.tif" /></maths>or :<ul id="ul0007" list-style="dash" compact="compact"><li>λ is the wavelength corresponding to the median frequency f_m bandwidth E,</li><li>ε_r is the relative permittivity of the air, and</li><li>μ_r is the relative permeability of the air.</li></ul>

Here the median frequency f_m is substantially equal to 1.2 GHz.

The blade 36 forms a parallelepipedal resonant cavity with leaks whose height H is constant and whose lateral dimensions are defined by the lateral dimensions of the BIP material 20 and the reflector 22. These blades 30 and 32, as well as the reflective plane 22, are rectangular and of identical lateral dimensions. Here, these lateral dimensions are chosen to be several times larger than the radius R defined by the following empirical formula:<maths id="math0002" num="(1)"><math display="block"><mrow><msub><mi mathvariant="normal">G</mi><mrow><mi mathvariant="normal">dB</mi></mrow></msub><mo>≥</mo><mn>20</mn><mi mathvariant="normal">log</mi><mfrac><mrow><mi>π</mi><mi mathvariant="normal">Φ</mi></mrow><mi>λ</mi></mfrac><mo>-</mo><mn>2</mn><mo>,</mo><mn>5.</mn></mrow></math><img file="EP1568104B1_D0002.tif" /></maths>or:<ul id="ul0008" list-style="dash" compact="compact"><li>G_dB is the gain in decibels desired for the antenna,</li><li>Φ = 2 R,</li><li>λ is the wavelength corresponding to the median frequency f_m</li></ul>

For example, for a gain of 20 dB, the radius R is substantially equal to 2.15 λ.

In known manner, such a parallelepiped resonant cavity has several families of resonant frequencies. Each family of resonance frequencies is formed by a fundamental frequency and its harmonics or integer multiples of the fundamental frequency. Each resonance frequency of the same family excites the same mode of resonance of the cavity. These resonance modes are known as TM resonance modes₀, TM₁, ..., TM_iThese modes of resonance are described in more detail in the document by F. Cardiol, "Electromagnetism, Electricity, Electronics and Electrical Engineering", Ed. Dunod, 1987.

It is recalled here that the resonance mode TM₀ is likely to be excited by a range of excitation frequencies close to a fundamental frequency f_m0. Similarly, each TM mode_i is likely to be excited by a range of excitation frequencies close to a fundamental frequency f_mid. Each resonance mode corresponds to a radiation pattern of the particular antenna and to a transmitting and / or receiving radiating spot formed on the outer surface 38. The radiating spot is here the zone of the outer surface 38 containing all the points where the radiated power in emission and / or in reception is greater than or equal to half of the maximum power radiated from this external surface by the antenna 4. Each radiating spot has a geometric center corresponding to the point where the radiated power is substantially equal to the maximum radiated power.

In the case of TM resonance mode₀, this radiant spot is part of a circle whose diameter φ is given by the formula (1). For TM resonance mode₀the radiation pattern here is highly directional along a direction perpendicular to the outer surface 38 and passing through the geometric center of the radiating spot. The radiation pattern corresponding to the TM resonance mode₀ is illustrated in Figure 5.

Frequencies f_mid are placed inside the narrow bandwidth E.

Finally, four excitation elements 40 to 43 are placed next to one another in the cavity 36 on the reflector plane 22. In the example described here, the geometric centers of these excitation elements are placed at the four angles of a rhombus whose dimensions of the sides are strictly smaller than 2R.

Each of these excitation elements is able to emit and / or receive an electromagnetic wave at a working frequency._Ti different from that of the other excitation elements. Here, the frequency f_Ti of each excitation element is close to f_m0 in order to excite the TM resonance mode₀ of the cavity 36. These excitation elements 40 to 43 are connected to a conventional generator / receiver 45 of electrical signals intended to be transformed by each excitation element into an electromagnetic wave and vice versa.

These excitation elements are, for example, constituted by a radiating dipole, a radiating slot, a plate probe or a radiating patch. The lateral bulk of each radiating element, that is to say in a plane parallel to the outer surface 38, is strictly smaller than the surface of the radiating spot to which it gives rise.

FIG. 6 illustrates an example of application of the antenna 4. FIG. 6 represents a system 60 for transmitting and / or receiving electromagnetic waves suitable for equipping a geostationary satellite. This system 60 comprises a parabola 62 forming an electromagnetic wave beam reflector and the antenna 4 placed at the focus of this parabola 62. The electromagnetic wave beams emitted or received by the outer surface 38 of the antenna 4 are represented in this figure by lines 64.

The operation of the antenna of FIG. 3 will now be described in the particular case of the system of FIG. 6.

In transmission, the excitation element 40, activated by the generator / receiver 45, emits an electromagnetic wave at a working frequency f_T0 and excites the TM resonance mode₀ of the cavity 36. The other radiating elements 41 to 43 are, for example, simultaneously activated by the generator / receiver 45 and likewise respectively at the working frequencies f_T1, f_T2 and F_T3.

It has been discovered that for TM resonance mode₀, the radiating spot and the corresponding radiation pattern are independent of the lateral dimensions of the cavity 36. Indeed, the TM resonance mode₀ depends only on the thickness and nature of the materials of each of the blades 30 to 36 and is established independently of the lateral dimensions of the cavity 36 when they are several times greater than the radius R defined above. Thus, several TM resonance modes₀ can be established simultaneously next to each other and thus simultaneously generate several radiating spots arranged next to each other. This is what happens when the excitation elements 40 to 43 excite, each at different points of space, the same mode of resonance. Therefore, the excitation by the excitation element 40 of the resonance mode TM₀ results in the appearance of a substantially circular radiating spot 46 and whose geometric center is placed vertically to the geometric center of the element 40. Similarly, the excitation by the elements 41 to 43 of the resonance mode TM₀ This results in the appearance, vertically of the geometric center of each of these elements, respectively of radiating spots 47 to 49. Since the geometric center of the element 40 is at a distance strictly smaller than 2R from the geometric center of the elements 41 and 43, the radiating spot 46 partially overlaps the radiating spots 47 and 49 respectively corresponding to the radiating elements 41 and 43. For the same reasons, the radiating spot 49 partially overlaps the radiating spots 46 and 48, the radiating spot 48 partially overlaps the radiating spots 49 and 47 and the radiating spot 47 partially overlaps the radiating spots 46 and 48.

Each radiating spot corresponds to the base or cross-section at the origin of an electromagnetic wave beam radiated towards the dish 62 and reflected by this parabola 62 towards the terrestrial surface. Thus, similar to the known overlapping multi-beam multi-beam antennas, the coverage areas on the terrestrial surface corresponding to each of the emitted beams are close to each other, or even overlap, so as to eliminate or reduce the holes reception.

In reception, similar to what has been described in emission, each radiating spot of the outer surface 38 corresponds to a coverage area on the earth's surface. Thus, for example, if an electromagnetic wave is emitted from the coverage area corresponding to the radiating spot 46, it is received in the surface corresponding to the spot 46 after being reflected by the dish 62. If the received wave is at a frequency in the narrow bandwidth E, it is not absorbed by the BIP material 20 and is received by the excitation element 40. Each electromagnetic wave received by an excitation element is transmitted in the form of an electrical signal to the generator / receiver 45.

FIG. 7 represents an antenna 70 made from a BIP material 72 and a reflector 74 of electromagnetic waves, and FIG. 8 shows the evolution of the transmission coefficient of this antenna as a function of frequency.

The BIP material 72 is, for example, identical to the BIP material 20 and has the same non-conducting band B (FIG. 8). The blades forming this BIP material already described with reference to FIG. 3 bear the same numerical references.

The reflector 74 is formed, for example, from the reflective plane 22 deformed so as to divide the cavity 36 into two resonant cavities 76 and 78 of different heights. The constant height H₁ of the cavity 76 is determined so as to place, within the non-conducting band B, a narrow bandwidth E₁ (Figure 8), for example, around the frequency of 10 GHz. Similarly, the height H₂ of the resonant cavity 78 is determined to place, within the same non-conducting band B, a narrow bandwidth E₂ (Figure 8), for example centered around 14 GHz. The reflector 74 is composed here of two reflective half-planes 80 and 82 arranged in steps and electrically connected to one another. The reflective half plane 80 is parallel to the blade 32 and spaced therefrom from the height H₁. The half-plane 82 is parallel to the blade 32 and spaced therefrom from the constant height H₂.

Finally, an excitation element 84 is disposed in the cavity 76 and an excitation element 86 is disposed in the cavity 78. These excitation elements 84, 86 are, for example, identical to the excitation elements 40 to 43. except that the excitation element 84 is able to excite the TM resonance mode₀ of the cavity 76, while the excitation element 86 is able to excite the resonance mode TM₀ of the cavity 78.

In this embodiment, the horizontal distance, that is to say parallel to the blade 32, separating the geometric center of the excitation elements 84 and 86, is strictly less than the sum of the radii of two radiating spots produced respectively. by elements 84 and 86.

The operation of this antenna 70 is identical to that of the antenna of FIG. 3. However, in this embodiment, the working frequencies of the excitation elements 84 and 86 are located in narrow bandwidths.₁, E₂ respectively. Thus, unlike the antenna 4 of FIG. 3, the working frequencies of each of these excitation elements are separated from each other by a large frequency interval, for example here 4 GHz. In this embodiment, the positions of the E passbands₁, E₂ are chosen so that they can use imposed working frequencies.

FIG. 9 represents a multi-beam antenna 100. This antenna 100 is similar to the antenna 4 except that the single-defective BIP material 20 of the radiating device 4 is replaced by a multi-fault BIP material 102. In FIG. 7, the elements already described with reference to FIG. 4 bear the same numerical references.

The antenna 100 is shown in section along a section plane perpendicular to the reflector plane 22 and passing through the excitation elements 41 and 43.

The BIP material 102 comprises two successive groups 104 and 106 of blades made of a first dielectric material. The groups 104 and 106 are superimposed in the direction perpendicular to the reflective plane 22. Each group 104, 106 is formed, by way of nonlimiting example, respectively by two blades 110, 112 and 114, 116 parallel to the reflector plane 22. Each blade of a group has the same thickness as the other blades of this same group. In the case of the group 106, each blade has a thickness e₂ = λ / 2 where λ is the wavelength of the median frequency of the narrow band created by the defects of the BIP material.

Each blade of the group 104 has a thickness e₁ = λ / 4.

The calculation of these thicknesses e₁ summer₂ follows from the teaching disclosed in the French patent 99 14521 (2 801 428).

Between each blade of the BIP 102 material is interposed a blade of a second dielectric material, such as air. The thickness of these blades separating the blades 110, 112, 114 and 116 is equal to λ / 4.

The first blade 116 is disposed vis-à-vis the reflector plane 22 and separated from this plane by a blade of second dielectric material thickness λ / 2 so as to form a parallelepiped cavity resonant leak. Preferably, the thickness e_i blades of dielectric material, consecutive to each group of blades of dielectric material, is in geometric progression of reason q in the direction of successive groups 104, 106.

In addition, in the embodiment described here, by way of non-limiting example, the number of superimposed groups is equal to 2 so as not to overload the drawing, and the geometric progression reason is also taken equal to 2. These values ​​are not limiting.

This superposition of groups of BIP material having characteristics of magnetic permeability, dielectric permittivity and thickness e_i different increases the width of the narrow bandwidth created within the same non-pass band of the BIP material. Thus, the working frequencies of the radiating elements 40 to 43 are chosen more spaced apart from one another than in the embodiment of FIG. 3.

The operation of this radiating device 100 derives directly from that of the antenna 4.

Alternatively, the parabola 62 is replaced by an electromagnetic lens.

The radiating devices described until now are made from flat structures. However, alternatively, the surface of these different elements is adapted to the shape of the parabola or device capable of focusing the electromagnetic wave beams. For example, FIG. 10 shows an antenna 200 equipped with a device 202 able to focus the electromagnetic wave beams on an antenna 204. The device 202 is, for example, a metal reflector shaped half-cylinder. The antenna 204 is placed at the focus of this device 202. The antenna 204 is similar to the antenna of FIG. 3, with the exception that the reflector plane, and the blades of the BIP material, each have a convex surface corresponding to the concave surface of the half-cylinder.

In a variant, the radiation emitted or received by each excitation element is polarized in a direction different from that used by the neighboring excitation elements. Advantageously, the polarization of each excitation element is orthogonal to that used by neighboring excitation elements. Thus, interference and coupling between neighboring excitation elements are limited.

Alternatively, the same excitation element is adapted to operate successively or simultaneously at several different working frequencies. Such an element makes it possible to create a coverage area in which, for example, transmission and reception take place at different wavelengths. Such an excitation element is also able to make frequency switching.