EP1250729B1 - Anisotropic composite antenna - Google Patents

Abstract

The aerial comprises an element (20) capable of radiating or receiving an electromagnetic field, a conductive plane (22), and an anisotropic composite 24, formed by a stack of alternate ferromagnetic and electrically insulated layers. These layers or film are perpendicular to the conductive plane and to the electrical component (E) of the radiated or received field.

Description

Technical area

The present invention relates to an anisotropic composite antenna. It finds application in telecommunications, particularly in the frequency band from about 50 MHz to about 4 GHz. The antenna of the invention can be used both in transmission and reception.

State of the art

The so-called "skin" antennas generally consist of a metal casing above which is disposed an element capable of radiating or receiving an electromagnetic field. The length of this element is generally close to the half-wavelength of the field to be emitted or received. It may consist of a slot pierced in a metal plate or a metal pattern (strand or ribbon).

The appended FIG. 1 thus shows an antenna with an element 10 capable of radiating or receiving, a conductive plane 12, cylindrical or parallelepipedic conducting walls 13, a dielectric layer 14 placed on the front face of the assembly and serving as protection and finally a conductor 16 connecting the element 10 to transmission or reception means not shown. The radiated or received electromagnetic field is symbolically represented by the arrows R.

This type of antenna imposes severe constraints on the distance D to be arranged between the radiating element and the conductive plane constituting the bottom of the housing. This distance must be large enough so that there is no destructive interference between the incident wave and the wave reflected by the housing, without being excessive which would be detrimental to the gain and the bandwidth of the 'antenna.

In an attempt to reduce these constraints, it has been thought to add a high-index dielectric between the element capable of radiating or receiving and the conductive plane, which makes it possible to reduce the interval D. But this reduction is made to the detriment of the bandwidth of the antenna.

Ferrite magnetic substrates have also been thought to be used to tune the antenna over a certain frequency band. But the particular nature of this material (usually ceramic), as well as its mass and its radioelectric properties limit its use, especially for large surfaces. Another important limitation is related to the demagnetizing field of a ferrite substrate. Indeed, a parallelepipedal ferrite substrate are associated demagnetizing coefficients significantly different from zero. This results in a dynamic demagnetizing field, which is the product of a demagnetizing coefficient by the saturation magnetization of ferrite. This field increases the resonance frequency while decreasing the permeability of the ferrite substrate.

The static demagnetizing field (equal to the product of the demagnetizing coefficient in the field direction applied by the saturation magnetization) reduces the interest of the ferrite substrate in the case where an external magnetic field is applied to match the properties of the substrate. 'antenna. In fact, the field to be applied to the substrate is equal to the sum of the internal field and the demagnetizing field, and increasing the value of the field to be applied amounts to increasing the power of the magnet system, or the consumption of an electromagnet.

US Pat. No. 5,563,616 recommends placing an anisotropic material in layers between the element capable of radiating or receiving, this material having a high relative permittivity.

The present invention is intended to remedy all these disadvantages.

Presentation of the invention

To this end, the invention recommends adding, between the conductive plane and the element capable of radiating or receiving, an anisotropic composite formed of a stack of alternately ferromagnetic and electrically insulating layers. These layers are perpendicular to the conductive plane. If they rest directly on this plan, they rest there by their slice. Moreover, these layers are oriented or configured to be perpendicular (or substantially perpendicular) to the electrical component of the radiated or received field component taken in the plane of the antenna.

The composite used in the invention is in itself known and sometimes called "LIFT" for "Ferromagnetic Insulating Lamella on the Slice". It is described in document FR-A-2 698 479. A method for measuring its electromagnetic characteristics is described in FR-A-2,699,683. Such a composite has a high permeability and a low permittivity in the microwave range, for a plane wave arriving at normal incidence, with a rectilinear polarization (magnetic field parallel to the layers and electric field perpendicular to the layers). It is possible to adjust the frequency response of these materials by combining several ferromagnetic materials.

The composite in question is anisotropic, that is to say that its electromagnetic properties are very different depending on the orientation of the magnetic and electrical fields with respect to the layers. If the electric field is perpendicular to the ferromagnetic layers, the material allows the electromagnetic wave to penetrate. If, on the contrary, the electric field is parallel to the conductive lamellae, it is totally reflected by the material, which then behaves like a metal.

où e est l'épaisseur du composite, Z₀ une impédance caractéristique, N² = ε_⊥.µ _∥ et Z² = (µ _∥ ε_⊥), où ε_⊥ et µ _∥ sont respectivement la permittivité comptée perpendiculairement aux couches et µ _∥ la perméabilité comptée parallèlement aux couches.When such an anisotropic composite is disposed in an antenna directly on the conductive plane, the surface impedance it presents corresponds to a short circuit seen through the line formed by the composite and that for the favorable polarization (magnetic field parallel to the lamellae and electric field perpendicular to the layers). This impedance Z is defined by: $$\mathrm{Z}={\mathrm{Z}}_0\mathrm{t}\mathrm{h}\left(\mathrm{j}.2\mathrm{\pi }.\mathrm{NOT}.\mathrm{e}/\mathrm{\lambda }\right),$$

where e is the thickness of the composite, Z _{0 is} a characteristic impedance, N ² = ε _⊥ .μ _∥ and Z ² = (μ _∥ ε _⊥ ), where ε _⊥ and μ _∥ are respectively the permittivity counted perpendicular to the layers and μ _∥ the permeability counted parallel to the layers.

For the other polarizations, the impedance of the composite is close to that of a metal, that is to say close to zero.

The materials constituting an anisotropic composite are light and easy to form. In addition, one can easily obtain particular frequency responses by varying the permeability of the materials. On the other hand, the conductive nature of the composite for a particular direction of the field may be an advantage.

In addition, the application to the anisotropic component of a magnetic field does not have the disadvantages encountered with the ferrites. Indeed, high permeabilities can be obtained with low volume fractions of magnetic material. The demagnetizing field is then proportional to the saturation magnetization divided by its volume fraction. Static and dynamic demagnetizing field values are thus significantly lower than in the case of ferrites. An anisotropic composite antenna according to the invention can therefore be used to use an external magnetic field, either to modify the frequency tuning or to adjust the permeability level (by means of permanent magnets) to the desired frequency. In particular, an external magnetic field may be useful for decreasing magnetic losses at the working frequency.

Precisely, the present invention therefore relates to an antenna as defined in claim 1. It comprises an element capable of radiating or receiving an electromagnetic field, this element being disposed in front of a conductive plane, this antenna further comprises, between the element capable of radiating or receiving and the conductive plane, an anisotropic composite formed of a stack of alternately ferromagnetic and electrically insulating layers, these layers being perpendicular to the conductive plane and to the electrical component of the radiated or sensed field. 'antenna.

The composite can be placed directly on the driver's plane; but not necessarily.

Regarding the element capable of radiating or receiving, it can be of any known shape: straight or spiral slot, straight or spiral strands or ribbons. The layers of the composite must be oriented accordingly to always be perpendicular (or substantially perpendicular) to the electrical component of the radiated or received field. This component is the component in the plane of the antenna (it does not take into account the component of the electric field oriented perpendicularly to the plane of the antenna).

Brief description of the drawings

• Figure 1, already described, shows in section a skin antenna according to the state of the art;
• Figure 2 shows an antenna with a straight slot;
• Figure 3 shows the electromagnetic characteristics of a LIFT composite placed under a straight slot antenna;
• FIG. 4 shows the variation of the standing wave rate as a function of the frequency for an antenna according to FIGS. 2A and 2B with the composite of FIG. 3;
• FIG. 5 shows the gain of the antenna with and without anisotropic composite as a function of the height of the antenna;
• Figure 6 shows the adapter band of the antenna;
• Figure 7 gives the electromagnetic characteristics for a composite based on CoNbZr;
• Figures 8A, 8B show in plan view and in section a spiral slot antenna;
• Figure 9 shows a top view of the shape of the composite in the case of Figures 8A and 8B;
• Fig. 10 shows a slot antenna with central excitation;
• FIGS. 11A and 11B show, in plan view and in section, an antenna with two straight conductive strands;
• Figure 12 shows an antenna with two conductive ribbons;
• FIGS. 13A and 13B show, in plan view and in section, an antenna with spiral conductive strips.

Detailed description of particular embodiments

où Z est l'impédance de surface.As already indicated, the composite used according to the invention plays, in particular, the role of impedance transformer. It must be designed so that the efficiency of the antenna is as great as possible. An order of magnitude of the radiation efficiency of the antenna with respect to a similar antenna without short circuit can be given by the formula: $$\mathrm{E}=-10\log \left(\left|\mathrm{Z}/\left(\mathrm{Z}+1\right)\right|\right).$$

where Z is the surface impedance.

où e est la hauteur du composite et λ la longueur d'onde dans le vide.For a composite whose charge rate of ferromagnetic material is not too low (typically greater than 2%), and for thicknesses much less than a quarter of the wavelength, the surface impedance is given as a first approximation by : $$\mathrm{Z}=\mathrm{j}\mathrm{two}\mathrm{\pi }{\mathrm{\mu }}_{\parallel }\mathrm{e}/\mathrm{\lambda }.$$

where e is the height of the composite and λ is the wavelength in vacuum.

The composite placed on a conductive plane must have a sufficiently large normalized surface impedance (greater than 0.5) at the frequency considered so that the efficiency E is not too low. The typical thickness of the composite will be less than λ / 20. The composite may optionally be surmounted by a layer of dielectric or air, located between it and the radiating element. The thickness of this layer does not exceed, in general, λ / 10.

A favorable case is one where the level of loss remains low (μ '' / μ '<0.15 where μ' 'is the imaginary part of the permeability and μ' the real part) so that the stationary waves penetrating in the material and participating in the radiation of the antenna are not too quickly attenuated.

To produce the composite, it is possible to use a ferromagnetic material having a gyromagnetic resonance frequency greater than half the operating frequency of the antenna and for example less than 1.2 times this frequency. The volume fraction of ferromagnetic material may be at least 5%.

The permeability of an anisotropic composite depends on the properties of the ferromagnetic material. Dependence laws can be found in the article titled "Demonstration of anisotropic composites with tuneable microwave permeability manufactured from ferromagnetic thin films" by O. ACHER, P. LE GOURRIEREC, G. PERRIN, P. BACLET and O. ROBLIN, published in "IEEE Trans., Microwave Theory and Techniques", vol. 44, 674, 1996.

The microwave properties of a number of ferromagnetic materials are described in particular in the article entitled "Investigation of the gyromagnetic permeability of amorphous CoFeNiMoSiB manufactured by different techniques" by O. ACHER, C. BOSCHER, P. LE GUELLEC, P. BACLET , G. PERRIN, published in "IEEE Trans by Magn.", Vol. 32, 4833 (1996) and in the article entitled "Microwave permeability of ferromagnetic thin films with stripe domain structure" by. O. ACHER, C. BOSCHER, B. BRULE, G. PERRIN, N. VUKADINOVIC, G. SURAN and H. JOISTEN, published in "Journal of Appl. Phys." 81, 4057 (1997).

The frequency range of use of the antenna of the invention is the band from about 50 MHz to about 4 GHz. Above 4 GHz, the permeability levels obtained with the thin layers make them less attractive and the thicknesses necessary for the production of the antennas become less than one centimeter, so that to reduce this thickness again is of little use.

Figure 2 shows an example of an antenna emitting around 1.9 GHz. The element capable of radiating or receiving is a slot 20 pierced in a conductive plate 21. The conductive plane 22 supports the anisotropic composite 24. The electrical connection is referenced 26. The electrical component of the field is denoted E.

The slot 20 may have a length of 79 mm and a width of 2 mm. The metal plate 21 may be a square plate of 300x300 mm ² . Several heights D were tested, namely 40 mm, 20 mm, 10 mm and 5 mm, which correspond respectively to λ / 4, λ / 8, λ / 16, λ / 32.

The composite 24 is formed of flat slats and is arranged in such a way that these slats are all parallel to the longitudinal edges of the slit 20.

The composite can be made from a ferromagnetic layer of composition Co ₈₂ Zr ₈ Nb ₁₀ deposited on a film of mylar (trademark). In an exemplary embodiment, the thickness of the ferromagnetic was 1.3 microns and that of the mylar of 10 microns. The layers rest by their edge on the metal plane. The electric field at the slot is perpendicular to it and is therefore perpendicular to the lamellae.

The electromagnetic characteristics of the composite, for the favorable polarization (ie the permittivity perpendicular to the plane of the layers (ε ' _⊥ , ε'' _⊥ ) and the permeability in the plane of the layers (μ' _∥ , μ '' _∥ ) are given in figure 3 for the material defined above The thickness of the composite plate is 1.9 mm, which gives it an impedance with a modulus close to 1.5 to 1.9 GHz It is recalled that the permittivity of the compositions parallel to the plane of these layers is very large and can be considered infinite.

The experimental characteristics of the antenna thus produced are given in FIGS. 4 and 5 as a function of the distance D, which is expressed in fractions of the wavelength. Figure 4 gives the stationary wave ratio (TOS) and Figure 5 the gain, G expressed in dB. As soon as the height of the cavity D is less than 10 mm, that is to say λ / 16, the TOS at the input of the antenna increases considerably in the metal configuration of the prior art (curve 25 ), while it remains very low (of the order of 1.5) in the configuration of the invention (curve 26). For lower thicknesses, the absence of composite becomes prohibitive (TOS of 7 for D = 5 mm in conventional metallic configuration), whereas with the composite (for a thickness of 1.9 mm), a TOS of 3 is obtained , which remains quite acceptable for many applications.

As for the gain (FIG. 5), for a height D = 10 mm, this gain is the same with (curve 83) or without composite (curve 82). For even lower thicknesses, a rapid degradation is observed in the metal case (prior art), while only 3 dB is lost in the composite case.

Figures 4 and 5 show that for a thickness D less than 10 mm, all the performance of the antenna of the invention are greater than those of a conventional antenna.

Other measurements were made, with a similar structure but with a slot length of 14 cm, suitable for operation around 1.1 GHz. The lateral dimensions were identical. The height D being chosen between λ / 4 and λ / 64. Figure 6 thus shows the adaptation band with a TOS less than 3. It is remarkable to note that this bandwidth is very wide even when approaching the plated configuration. In the case of metal alone, (prior art), the TOS degrades and the associated bandwidth is reduced.

One can seek to improve the microwave behavior of the antenna, in particular its gain, placing under the slot a composite which totally absorbs the radiated wave towards it, that is to say a impedance equal to 1. It is also advantageous to increase the quantity | Z / (Z + 1) | by increasing the thickness or charge rate of the composite. It is thus possible to prefer a material having a certain permeability μ 'but a low permeability μ''at the working frequency rather than a high permeability μ''. This last path is interesting to the extent that the less magnetic losses are introduced into the antenna environment, the less energy in the vicinity of the metal plane is likely to be absorbed, especially in modes or for are generally not taken into account. On the other hand, it is reflected in phase with the radiated wave and therefore increases the efficiency of the antenna.

Thus, for an antenna operating around 200 MHz, it will be possible to retain a material such as the one whose electromagnetic characteristics are given in FIG. 7. It is a LIFT made from CoNbZr of thickness 0.9 μm deposited. on a kapton (trade mark) film 12.7 μm thick; the average glue thickness is 2.5 μm; the density of the material is 1.8. With a permeability equal to 21-3j at 200 MHz, this material has limited losses. With a thickness of 11 mm, an impedance of which the module is close to the unit is achieved, which makes it possible either to press the slot on the composite or to place it at a distance of less than λ / 16 (ie 93 mm ).

Figures 8A and 8B further illustrate a slot antenna but in the case of a spiral slot. On the 8A, which is a view from above, a spiral slot 30 is pierced in a conductive plate 31. Figure 8B, which is a section along AA, better shows the conductive plane 32, the composite 34 and the connection 36. This composite is shown in plan view in Figure 9 (the radiating element having been removed). The circles of the composite are thus seen in the spiral slot 30 (FIG. 8A). The electrical component of the radiated or received field is marked E.

In the illustrated embodiment, the ferromagnetic and insulating layers are cylindrical. The spiral of the radiating gap and the layers of the composite are therefore not strictly parallel, but the deviation from the parallelism is small (less than 10 °) and does not affect the performance of the antenna.

To obtain a broadband antenna emitting around 500 MHz (which corresponds to a wavelength of 600 mm), it will be possible to adopt a slit length of the order of λ / 2, ie 300 mm. The composite can be made from CoFeNiSiB with a thickness of 1.3 μm and a glue thickness of 2.5 μm. The density of the material is then 2.3. Thicknesses as low as 1 mm making it possible to obtain impedances greater than 1.5, hence good properties for cavity depths of the order of λ / 10 or less.

The realization of a spiral layer composite substantially parallel to the slot can be done by winding strips on preforms, or by any other means.

The area of radiation of the spiral slit is a function of the radius thereof, this value being related to the frequency. The optimization of the thickness of the composite material must be a function of the radius of the cavity.

Another variant, which is easier to produce, consists in producing a composite toroid by winding and placing the spiral slit in a concentric manner. This solution is less respectful of the geometry of the fields, but is acceptable if the opening of the spiral is less than 30 °.

Figure 10 further illustrates a slot antenna but in a variant where the slot is wide and excited at its center. The slot is referenced 40, the conductive plane 42, the composite 44 and the power connection 46. The lamellae of the composite are still oriented parallel to the longitudinal edges of the slot, that is to say perpendicular to the component E.

FIGS. 11A and 11B illustrate, respectively in plan view and in section along AA, an embodiment in which the antenna is of the dipole type. The element able to radiate or receive is constituted by two conductive strands 50. The conductive plane 52 supports the composite 54 and a dielectric layer 55 can support the two strands. The connection 56 is double. The lamellae of the composite 54 are oriented perpendicularly to the strands. For operation at 2 GHz, the length of each strand may be close to 75 mm for operation in λ / 2. For the composite, the material whose characteristics have been illustrated in FIG. 3 with a thickness of 1.5 to 3 mm can be used. The thickness of the dielectric layer 56 does not exceed λ / 16.

The strands may be replaced by conductive ribbons as shown in FIG. 12. These ribbons bear the reference 60, the conductive plane the reference 62 and the composite the reference 64. The layers of the composite are still lamellae perpendicular to the large dimension of the ribbons 60.

Finally, in FIGS. 13A and 13B, which are respectively views from above and in section along AA, the conductive strands 70 are no longer rectilinear but have a spiral shape. The composite 74 is then formed of radial strips substantially perpendicular to the conductive strands. The connection 76 is double and feeds the spiral strands.

Claims (8)

1. Aerial comprising an element (10) able to radiate or receive an electromagnetic field, a conductive plane (12) and, between the element able to radiate or receive (20, 30, 40, 50, 70) and the conductive plane (22, 32, 42, 52, 62, 72), an anisotropic substrate, characterized in that said substrate is an anisotropic composite (24, 34, 44, 54, 64, 74) formed by a stack of alternatively ferromagnetic and electrically insulating layers, said layers being perpendicular to the conductive plane and to the electrical component (E) of the radiated or received field, said composite having a permittivity perpendicular to the plane of the layers with a real part less than 5 and an imaginary part of substantially 0.
2. Aerial according to claim 1, in which the anisotropic composite (24, 34, 44, 54, 64, 74) rests directly on the conductive plane (22, 32, 42, 52, 62, 72).
3. Aerial according to claim 1, in which the element capable of radiating or receiving is a straight slot (20, 40) drilled in a conductive plate (21), the layers of the anisotropic composite (24, 34, 44, 54, 64, 74) being in this case flat lamina wafers parallel to the said slot.
4. Aerial according to claim 1, in which the element capable of radiating or receiving comprises at least one spiralled slot (30) drilled in a conductive plate (31), the layers of the composite being in this case wound approximately parallel to the said slot.
5. Aerial according to claim 1, in which the element capable of radiating or receiving is formed of two straight conductor wires (50) or conductive strips (60), the layers of the composite in this case being flat lamina wafers perpendicular to the two wires (50) or strips (60).
6. Aerial according to claim 1, in which the element capable of radiating or receiving comprises at least one conductor wire or strip spirally-wound (70), the layers of the composite being in this case radial and approximately perpendicular to the wire or strip (70).
7. Aerial according to claim 1, in which the ferromagnetic layers or film have a gyromagnetic resonance frequency lower than 1.2 times the working frequency of the aerial.
8. Aerial according to claim 1, in which the volume fraction of ferromagnetic material is at least equal to 5%.

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