Multiple-beam antenna with photonic bandgap material

Translation of description of equivalent WO 2004040694 A1

MULTI-BEAM ANTENNA A METERIAU BIP

The invention relates to a multi-beam antenna comprising:

- A PBG material (Photonic Band Prohibition) able to spatially filter and frequency-electromagnetic waves, this material

BIP having at least one stopband and forming a radiating outer surface in transmission and / or reception,

- At least one defect of periodicity of the photonic bandgap material so as to create at least a narrow bandwidth within the at least one non bandwidth of this PBG material, and

- An excitation device capable of transmitting and / or receiving electromagnetic waves inside said at least one narrow pass band created by said at least one defect.

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

Currently, the radiating elements used are known under the term "cone" and equipped multi-beam antenna such cones is known as cones antenna name. Each horn produces a roughly circular radiating spot forming the base of a conical beam radiated in transmission or reception. These cones are disposed one beside the other so as to close as possible to the radiating spots each other. Figure 1A schematically shows a multi-antenna horns beam in front view in which seven squares F1 to F7 indicate congestion seven cones arranged contiguously to each other. Seven circles S1 to S7, each included in one of the squares F1 to F7 represent the radiating spots produced by the respective cones. The antenna 1A is placed at the focus of a parabola satellite nary a geostation for transmitting information on French territory.

1B shows the areas C1 to C7 cover -3 dB, each corresponding to a radiating spot of the antenna of Figure 1 A. The center of each circle corresponds to the land surface of the point where the received power is Max. The circumference of each circle defines an area inside which the received power on the earth's surface is greater than half of the maximum received power at the center of the circle. Although the radiating spots S1 to S7 are substantially contiguous, celles- practices produce coverage areas -3 dB disjoint from each other. The areas between the coverage areas -3 dB are called here receiving holes. Each receiving hole corresponds to a land surface area, where the received power is less than the half of the received maximum power. In these receiving holes, the received power may be insufficient for a ground receiver to operate properly.

To solve this reception gap problem, it was proposed to ride together the radiating spots of the multi-beam antenna. A partial front view of such a multi-beam antenna having a plurality of radiating spots overlapping is illustrated in Figure 2A. In this figure, only two radiating spots SR1 and SR2 are shown. Each radiating spot is produced from seven independent radiation sources and distinct from each other. The radiating spot SR1 is formed from the radiation sources SdR1 to SdR7 arranged contiguously sides to each other. A radiating spot SR2 is produced from the radiation sources SdR1, SDR2, SdR3 and SdR7 and SdRδ radiation sources SdR10. The radiation sources are clean SdR7 SdR1 to work at a first working frequency create a first beam of electromagnetic waves substantially uniform in this first frequency. Radiation sources SdR1 to SdR3 and SdR7 to SdR10 are unique to working at a second frequency working to create a second substantially uniform beam of electromagnetic waves to this second working frequency. Thus the radiation sources SdR1 to SdR3 SdR7 and are able to work simultaneously with 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 radiation sources SdR1 to 3, are used both to create the radiating spot SR1 and the radiating spot SR2, which produces an overlap of these two radiating spots SR1 and SR2. An illustration of the arrangement of coverage areas -3 dB created by a multi-beam antenna having overlapping radiating spots is shown in Figure 2B. Such an antenna can significantly reduce the receiving holes, or even make them disappear. However, due in part to the fact that a radiating spot is formed from several independent sources of radiation and distinct from one another, at least some are also used for other radiating spots, this multi-beam antenna more complex to control than conventional antennas cones.

The invention aims to remedy this drawback by proposing a multi-beam antenna for radiating spots easier rides. It therefore relates to an antenna as defined above, characterized:

- In that the excitation device is capable of working simultaneously at least around a first and a second distinct working frequency, - in that the excitation device comprises a first and second excitation elements 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 in the second working frequency,

- In that the or each periodicity defect of the PBG material forms a resonant cavity leaks having a constant height in a direction orthogonal to said radiating outer surface and determined lateral dimensions parallel to said radiating outer surface,

- In that the first and second working frequencies are capable of exciting the same resonance mode of a resonant cavity to leak, this mode of resonance being established identically regardless of the lateral dimensions of the cavity, of so as to create on said outer surface, respectively a first and a second radiating spots, each of these radiant spots representing the origin of a beam of electromagnetic waves radiated in transmission and / or reception by the antenna, - in that each of the radiant spots has a geometric center the position of which depends on the position of the excitation element which gives rise to the surface of which is greater than that of the radiating element giving birth, and

- In that the first and second excitation elements are placed relative to each other so that the first and second radiating patches are disposed on the outer surface of the PBG material to one side from each other and partially overlap.

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 a beam of electromagnetic waves. 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 thus similar to that of a conventional horn antenna. In addition, the excitation elements are placed so as to overlap the radiating spots. This antenna therefore has the advantages of a multi-beam antenna with overlapping radiating spots without the complexity of the control of excitation elements has been increased relative to that of the multi-beam in horn antennas. According to other characteristics of a multibeam antenna according to the invention:

- Each radiating spot is substantially circular, the geometric center corresponding to a maximum power transmitted and / or received and the periphery corresponding to a transmit power and / or received 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 radiant spot produced by the first excitation element added to the radius of the radiant spot produced by the second excitation element,

- The geometric center of each radiant spot is placed on the line orthogonal to said radiating outer surface and passing through the geometric center of the excitation element giving birth,

- The first and second excitation elements are placed inside the same cavity,

- The first and second working frequencies are located within the same narrow pass band created by this same cavity,

- The first and second excitation elements are each positioned inside separate resonant cavities, and the first and second working frequencies are each capable of exciting a resonance mode independent of the lateral dimensions of their respective cavity,

- An electromagnetic radiation reflector plane associated with the PBG material, this reflector plane being deformed so as to form said distinct cavities, - the or each cavity is parallelepipedal in shape,

- The device capable of focussing the electromagnetic waves comprises a semi-cylindrical reflector, and the PBG material of the antenna has a convex surface corresponding to the shaped surface of half cylinder reflector. The invention also relates to a transmitting and / or receiving electromagnetic waves comprising:

- A device capable of focussing the electromagnetic waves emitted and / or received by the system on a focal point, and - A transmitter and / or receiver of electromagnetic waves placed substantially at the focal point so as to transmit and / or receive said electromagnetic waves, characterized in that it comprises an antenna according to the invention, the radiating outer surface is substantially placed on the focal point so as to form said transmitter and / or receiver of electromagnetic waves.

According to other features of the system according to the invention:

- The device capable of focussing the electromagnetic waves is a parabolic reflector, - the device capable of focussing the electromagnetic waves is an electromagnetic lens.

The invention will be better understood from reading the following description given purely by way of example and with reference to the drawings, in which: - Figures 1A, 1B, 2A and 2B antennas multi- known beams and the resulting coverage areas;

- Figure 3 is a perspective view of a multibeam antenna in accordance with the invention;

- Figure 4 is a graph showing the antenna transmission coefficient of Figure 3;

- Figure 5 is a graph showing the antenna radiation pattern of Figure 3;

- Figure 6 is a schematic illustration in section of a system for transmitting / receiving electromagnetic waves with the antenna of Figure 3;

- Figure 7 shows a second embodiment of a multibeam antenna according to the invention;

- Figure 8 shows the transmission coefficient of the antenna of Figure 7; - Figure 9 shows a third embodiment of a multibeam antenna according to the invention; and

- Figure 10 is an illustration of a semi-cylindrical antenna according to the invention. Figure 3 shows a multi-beam antenna 4. The antenna 4 is formed of a material 20 to ban photonic band or PBG material associated with a metal plane 22 Electromagnetic wave reflector NetIQ ues. The PBG materials are known and the design of a PBG material such as 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 technique will be described in detail here.

It is recalled that PBG material is a material which has the property of absorbing certain frequency ranges, that is to say to prohibit transmission in said abovementioned frequency ranges. These frequency ranges form what is called here a non bandwidth.

A stopband B of material 20 is illustrated in Figure 4.

FIG 4 shows a curve representing the variations of the transmission coefficient expressed in decibels as a function of the frequency of the emitted or received electromagnetic wave. This transmission coefficient is representative of the energy transmitted from one side of the PBG material with respect to the energy received from the other side. In the case of material 20, the stopband B or absorption band B extends substantially from 7 GHz to 17 GHz. The position and the width of the non bandwidth B is a function of the properties and characteristics of the PBG material.

The PBG material generally consists of a periodic arrangement of dielectric permittivity and / or permeability variable. Here, the material 20 is formed from two plates 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 reflector plane 22. The blade 30 is disposed at one end of said stack of blades. It has an outer surface 38 opposite to its surface in contact with the blade 34. This surface 38 forms a radiating surface in transmission and / or reception.

Known manner, the introduction of a break in the geometric periodicity and / or radio, also called fault rupture, allows generate an absorption defect and therefore the creation of a narrow bandwidth within the stopband of the PBG material. The material is, under these conditions, designated by BIP defect material.

Here, a break of geometric periodicity is created by choosing the height or thickness H of the blade 36 than that of the blade 34. In a known manner, and so as to create a narrow bandwidth E (Figure 4) substantially in the middle of bandwidth B, this height H is defined by the following relationship:

H = 0.5 x λl ^_Er x μ_r or :

- Λ is the wavelength corresponding to the center frequency f_m bandwidth E,

- ε_r is the relative permittivity of the air, and

- μ_r is the relative permeability of air. Here, the median frequency f_m is substantially equal to 1.2 GHz.

The blade 36 forms a resonant cavity parallelepiped leak whose height H is constant and whose lateral dimensions are defined by the lateral dimensions of the PBG material 20 and the reflector 22. These blades 30 and 32 and the reflector plane 22, are rectangular and lateral dimensions. Here, these lateral dimensions are selected to be several times larger than the radius R defined by the following empirical formula:

G_dB > 20Iog ^ -2.5. (1) where: - GOB is the decibel gain desired for the antenna,

- Φ 2 = R

- Λ is the wavelength corresponding to the center frequency fm

For example, for a gain of 20 dB, the radius R is substantially equal to 2.15 λ. Known manner, such a rectangular resonant cavity has a plurality of resonance frequencies of families. Each family resonance frequency 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 cavity resonance mode. These resonance mode are known under the terms of TM modes of resonance₀, TMi, ..., TMj .... These resonance modes are described in more detail in the document by F. Cardiol, "Electromagnetism, Treaty of Electricity, Electronics and Electrical", Ed. Dunod 1987.

It is recalled here that the resonance mode TMo is likely to be excited by a range of frequencies of neighboring excitation of a fundamental frequency f_m0. Similarly, each TMj mode is likely to be excited by a range of frequencies of neighboring excitement of a fundamental frequency f_mid. Each resonance mode corresponds to a radiation pattern of the antenna and particularly to a radiating spot in transmission and / or reception formed on the outer surface 38. The radiating spot is here the outer surface area 38 containing all points where the power radiated in transmission and / or reception is greater than or equal to half of the maximum power radiated from the outer surface by the antenna 4. Each radiating spot admits a geometric center corresponding to the point where the power radiated is substantially equal to the maximum radiated power.

In the case of the resonance mode TM₀This radiating spot is part of a circle whose diameter φ is given by equation (1). For the resonant mode TMo, the radiation pattern is here 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 TMo mode of resonance is illustrated in Figure 5.

The frequencies f_mid are placed within the narrow bandwidth E. Finally, four excitation elements 40 to 43 are placed one beside the other in the cavity 36 of the reflector plane 22. In the example described here, the centers geometric these excitation elements are placed the four corners of a rhombus whose side dimensions are strictly less than 2R.

Each of these excitation elements is capable of emitting and / or receiving an electromagnetic wave to a different operating frequency π from the other excitation elements. Here, the fτι frequency of each excitation element is close to f_m0 so as to excite the resonance mode TM₀ of the cavity 36. These excitation elements 40 to 43 are connected to a generator / receiver 45 vector of electrical signals for processing by each excitation element in an electromagnetic wave and vice versa.

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

6 illustrates an application example of the antenna 4. Figure 6 shows a system 60 for transmitting and / or receiving electromagnetic waves own to equip a geostationary satellite. This system 60 includes a parabola reflector 62 forming electromagnetic wave beams and the antenna 4 placed at the focus of the parabola 62. The electromagnetic wave beams transmitted or received by the outer surface 38 of the antenna 4 are shown in this figure by 64 strokes.

The operation of the antenna of Figure 3 will now be described in the particular case of the system of Figure 6. In transmission, the excitation element 40, activated by the generator / receiver 45 transmits an electromagnetic wave at a fτo working frequency and excites the resonant mode TM₀ of the cavity 36. The other radiating elements 41 to 43 are, for example, simultaneously activated by the generator / receiver 45 and do the same respectively to the working frequencies f_Tι, fτ2 and f_T3-

It has been found that for the resonance mode TM₀, The radiating spot and the corresponding radiation pattern are independent of the lateral dimensions of the cavity 36. Indeed, the resonance mode TM₀ is only a function of the thickness and the nature of the materials of each of the blades 30-36 and settled irrespective of the lateral dimensions of the cavity 36 when they are a multiple of the radius R defined above. Thus, several resonance modes TM₀ can simultaneously set one next to the other and therefore simultaneously generate several radiating spots arranged one beside the other. This is what occurs when the excitation elements 40-43 excite, each at different points of space, the same resonance mode. 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 the geometric center is positioned vertically above the geometric center of the element 40. Similarly, the excitation by the elements 41 to 43 of the resonance mode TMo is reflected by the appearance of the vertical geometric center each of these elements, respectively of radiating spots 47 to 49. the geometric center of the element 40 being at a distance strictly less than 2R from the geometric center of the elements 41 and 43, the radiating spot 46 partly overlaps the radiating spots 47 and 49 respectively corresponding to the radiating elements 41 and 43. For the same reasons, the radiating spot 49 partly overlaps the radiating spots 46 and 48, the radiating spot 48 overlaps partly radiating spots 49 and 47 and the radiating spot 47 partly overlaps the radiating spots 46 and 48.

Each radiating spot corresponds to the base or cross section to the origin of a beam of electromagnetic waves radiated to the dish 62 and reflected by this parable 62 to the earth's surface. Thus, similar to the multi-beams overlapping radiating spots known antennas, coverage areas on the earth's surface corresponding to each of the transmitted beams are close to each other or overlap, so as to eliminate or reduce the holes reception.

In reception, similar to what was described in issue, each radiating spot of the outer surface 38 corresponds to a coverage area on the Earth's surface. For example, if an electromagnetic wave is emitted from the coverage area corresponding to the radiating patch 46, it is received in the surface corresponding to the 46 spot after being reflected by the dish 62. If the received wave is a frequency within the bandwidth E, it is not absorbed by the photonic bandgap material 20 and is received by the excitation element 40. Each electromagnetic wave received by an excitation element is transmitted as an electrical signal generator / receiver 45.

7 shows an antenna 70 made of a PBG material 72 and a reflector of electromagnetic waves 74 and Figure 8 the changes in the coefficient of transmission of the antenna depending on the frequency.

BIP 72 material is, for example, identical to the PBG material 20 and has the same non bandwidth B (Figure 8). The blades forming the photonic bandgap material already described with reference to Figure 3 bear the same reference numbers.

The reflector 74 is formed, for example, from the plane reflector 22 deformed so as to divide the cavity 36 into two resonant cavities 76 and 78 of different heights. The constant height Hi of the cavity 76 is determined so as to place, in the stopband B, a narrow bandwidth Ei (Figure 8), for example, around the frequency of 10 GHz. Similarly, the height H₂ the resonant cavity 78 is determined to place, even within the non bandwidth B, a narrow bandwidth E₂ (Figure 8), for example centered around 14 GHz. The reflector 74 here consists of two half-planes reflectors 80 and 82 arranged in tiers and electrically connected to each other. The half-plane reflector 80 is parallel to the blade 32 and spaced therefrom from the Hi height. The half-plane 82 is parallel to the blade 32 and spaced therefrom to 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 40-43 except that the excitation element 84 is adapted to excite the resonance mode TMo to the cavity 76, while the excitation element 86 is adapted 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, between the geometric center of the elements excitation 84 and 86 is strictly less than the sum of the radii of two radiating spots produced respectively by the 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 low bandwidth Ei, E₂ respectively. Thus, unlike the antenna 4 of Figure 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 band positions passentes Ei, E₂ are chosen so as to be able to use imposed working frequencies.

9 shows a multi-beam antenna 100. This antenna 100 is similar to the antenna 4 with the exception that the mono-defect photonic bandgap material 20 of the radiating device 4 is replaced by a PBG material 102 to several defects. In Figure 7, the elements described above with reference to FIG 4 bear the same reference numbers.

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

BIP material 102 comprises two successive groups 104 and 106 of blades made of a first dielectric material. Groups 104 and 106 are superimposed in the direction perpendicular to the reflector plane 22. Each group 104, 106 is formed, by way of non limiting example, respectively by two plates 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 the same group. In the case of group 106, each blade has a thickness e₂ = Λ / 2 where λ denotes the wavelength of the center frequency of the narrow band created by the defects of the PBG material.

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

The calculation of these thicknesses and e ei₂ stems from the teaching disclosed in French patent 99 14521 (2801428). Between each blade of the defective PFB material 102 is interposed a blade in a second dielectric material such as air. The thickness of these plates separating the blades 110, 112, 114 and 116 is equal to λ / 4.

The first blade 116 is arranged vis-a-vis the reflector plane 22 and separated from it by a plane blade in second dielectric material of thickness λ / 2 so as to form a parallelepipedal resonant cavity leaks. Preferably, the thickness e_\ blades dielectric material row of each group of sheets of dielectric material is in geometric progression of ratio q in the direction of the groups 104, 106 successive.

Moreover, in the embodiment described here by way of example, the number of stacked groupings is 2 so as not to overload the drawing, and the reason for geometric progression is taken equal to 2. These values ​​are not restrictive. This superposition of PBG material groups having magnetic permeability characteristics, dielectric permittivity and thickness e, different increases the width of the narrow bandwidth created within the same stopband of the PBG material. Thus, the working frequencies of the radiating elements 40 to 43 are chosen more spaced from each other than in the embodiment of Figure 3.

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

Alternatively, the dish 62 is replaced by an electromagnetic lens. The radiating devices described up to the present are made from flat structures. However, alternatively, the surface of these elements is adapted to the shape of the parabola or device capable of focusing the beams of electromagnetic waves. For example, Figure 10 shows an antenna 200 equipped with a device 202 adapted to focus the electromagnetic wave beams on an antenna 204. The device 202 is, for example, a metal reflector in the shape of half cylinder. The antenna 204 is placed at the focus of the device 202. The antenna 204 is similar to the antenna Figure 3, except that the reflector plane, and the blades of the defective PFB material, each have a convex surface corresponding to the concave surface of semi-cylinder.

Alternatively, the radiation emitted or received by each excitation element is polarized in a direction different from that used by neighboring excitation elements. Advantageously, the polarization of each excitation element is orthogonal to that used by neighboring excitation elements. Thus, interference and couplings between adjacent excitation elements are limited. Alternatively, a single excitation element is adapted to operate sequentially or simultaneously at several frequencies of different work. Such an element creates a coverage area in which, for example, the transmission and reception take place at different wavelengths. Such excitation element is also adapted to make the switching frequency.