EP0399524B1 - Structure for the realisation of circuits and components, applied to microwave frequencies - Google Patents

Description

The invention relates to a structure for producing circuits and components applied to microwave frequencies.

The increasing development of the use of electromagnetic waves in fields as diverse as telecommunications, medical applications, radar ... has led to a variety of techniques used in order, on the one hand, to control their propagation, on the other hand , to control their influence. The means used in one case as in the other are defined by the general radio-electric characteristics required: frequency bands, necessary powers, admissible loss levels, complexity levels of the connectors, mission in the broad sense of the term, as well as by a non-specifically radio set of other criteria involving parameters such as the mass, the volume of the circuits or even the admissible temperature range that the technologies used will have to withstand. All of these additional constraints are again governed by the "mission in the broad sense" aspect; the precise choice of a technology having to integrate radio criteria as well as mechanical, structural and thermal criteria.

It is easy to understand that the environment and location data are different when it comes to mounting microwave equipment on a satellite, an airplane, or in a submarine for example and that this has an impact on the definition. and the choice of technology required to make the equipment.

Undoubtedly the best known means of conveying an electromagnetic wave is undoubtedly the hollow tube. This can take simple shapes of rectangular or circular section or even more elaborate shapes, for example hexagonal section. Its field of use in frequency is very wide from a few gigahertz to several hundred gigahertz, that is to say from the centimeter or sub-millimeter. Below a few gigahertz, the use of the waveguide proves difficult because of its size and its mass. Other types of propagation are then used.

• les lignes coaxiales et dérivées,
• les lignes triplaques,
• les lignes "microstrip" et dérivées,

qui sont largement utilisées pour propager des signaux allant du continu jusqu'à quelques dizaines de gigahertz. De façon simple on peut dire que les propriétés radioélectriques (impédance, constante de propagation etc...) résultent du positionnement de deux conducteurs l'un par rapport à l'autre à l'aide d'un matériau support ou espaceur diélectrique. Dans la pratique on emploie couramment des matériaux dont les constantes diélectriques varient de 1 à 10, voire 40 pour certaines applications.Non-exhaustively, we can cite:

• coaxial and derived lines,
• triplate lines,
• "microstrip" and derived lines,

which are widely used to propagate signals ranging from continuous to a few tens of gigahertz. In a simple way, it can be said that the radioelectric properties (impedance, propagation constant, etc.) result from the positioning of two conductors relative to one another using a dielectric support material or spacer. In practice, materials are commonly used whose dielectric constants vary from 1 to 10, or even 40 for certain applications.

With regard to radiation, radiant elements have appeared over the past ten years that are remarkable in terms of their simplicity of construction and their characteristics of lightness and ability to be shaped: These are printed antennas, the principle of which uses a resonant element. etched on a dielectric support, the assembly being located on a ground plane. Again, such concepts make it possible to offer very competitive solutions in terms of volume, compactness and mass.

These two areas of interest (production of circuits and radiating elements) have led manufacturers to offer an increasingly wide range of dielectric materials with increasingly wide fields of application.

American patent US-A-4 623 893 in the name of SABBAN describes an antenna whose signal supply is produced in microstrip technology on a first dielectric substrate, while the radiating elements are of the "patch" type printed on a second substrate dielectric. Thus, this antenna is formed by a plurality of dielectric layers, at least two of which are provided with conductive elements (ground plane, radiating element, resonator, etc.), and the dielectric layers are assembled by means of spacers. In the antenna described, the mechanical rigidity of the assembly comes only from the dielectric substrates, the spacers being preferably punctual and arranged in regions where the fields electrical will be weak (away from resonators and radiating elements). However, the dielectric substrates must also have radioelectric performances whose values depend on the physical dimensions. The region between the dielectric planes is filled either with air, or with a material having a dielectric constant close to that of air (honeycomb, plastic or rubber foam, etc.). As a result, the mechanical parameters on the one hand, and radioelectric parameters on the other hand, cannot be optimized separately and individually, because the rigidity and the dielectric losses depend on the materials used for the different layers, as well as on their physical dimensions and of their relative provision.

British patent application GB-A-2 194 101 in the name of MATSUSHITA describes a planar antenna the structure of which is produced by stacking successive planes comprising conductive patterns on dielectric substrate, including a supply network printed on a first dielectric substrate , which supplies by electromagnetic coupling the radiating elements of the "patch" type printed on a second dielectric substrate, these two planes being separated from a ground plane and separated from each other by perforated dielectric spacers. Thus, this antenna is constituted by a plurality of dielectric layers, at least three of which are provided with conductive elements (ground plane, radiating elements, power supplies, etc.), and the dielectric layers are assembled by means of spacers. In the described antenna, the mechanical rigidity of the assembly comes only from the dielectric substrates, the spacers being preferably perforated and made of a foam material in order to obtain a dielectric constant as close to that of air as possible. The dielectric spacers are perforated so as not to present a dielectric constant appreciable only in regions where the electric fields will be weak (far from resonators and radiating elements). However, the dielectric substrates must also have radioelectric performances whose values depend on the physical dimensions. As in the previous document, the region between the dielectric planes is therefore filled either with air, or with a material having a dielectric constant close to that of air (honeycomb, plastic or rubber foam,. ..). As in the previous case, the mechanical parameters on the one hand, and radioelectric parameters on the other hand, cannot be optimized separately and individually.

Finally, American patent US-A-2 919 441 mentions the idea of removing a mechanical structure between two conductive tapes to reduce losses, but the antenna described in this patent has nothing to do with an antenna patch type.

• la masse des équipements,
• les plages de température et les contraintes thermiques,
• les niveaux de vibration,
• la stabilité physique au vide (non dégazage).

The constraints of use in space environment are well known and generally relate to:

• the mass of the equipment,
• temperature ranges and thermal constraints,
• vibration levels,
• physical stability under vacuum (not degassing).

The object of the invention is to propose an embodiment of substrates with variable permittivity.

• une structure mécanique (26) formant une pluralité d'enceintes,
• une pluralité d'éléments conducteurs (30), et
• un plan de masse (31) métallique,

ladite structure mécanique (26) formant une pluralité d'enceintes se trouvant entre lesdits éléments conducteurs (30) et ledit plan de masse (31), caractérisée en ce que : un milieu isolant (27) est disposé dans chaque dite enceinte, ladite structure mécanique 26 étant située en dehors desdites enceintes ; et en ce que chaque élément conducteur (30) est disposé au dessus dudit milieu isolant (27), lesdites fonctions mécanique et électrique étant ainsi localement dissociées.To this end, the invention provides a patch type antenna structure, in which the mechanical and electrical functions are globally integrated, further comprising

• a mechanical structure (26) forming a plurality of enclosures,
• a plurality of conductive elements (30), and
• a metallic ground plane (31),

said mechanical structure (26) forming a plurality of enclosures located between said conductive elements (30) and said ground plane (31), characterized in that: an insulating medium (27) is disposed in each said enclosure, said structure mechanical 26 being located outside said enclosures; and in that each conductive element (30) is disposed above said insulating medium (27), said mechanical and electrical functions being thus locally dissociated.

According to a preferred embodiment of the invention, said insulating medium (27) is a dielectric material, which can for example be a solid or a gas. In a variant, said insulating medium (27) is a vacuum.

The advantage of the invention results from its versatility and its considerable weight gain compared to more conventional solutions. Its simplicity of making dielectrics with any constant and its low mass make this solution very attractive for space uses.

• les figures 1, 2 et 3 illustrent des réalisations de l'art connu ;
• les figures 4 et 5 illustrent une vue en coupe et une vue de dessus, en partie éclatée, d'une structure de circuits et composants appliquée aux hyperfréquences selon l'invention.

The characteristics and advantages of the invention will become apparent from the description which follows, by way of nonlimiting example, with reference to the appended figures in which:

• Figures 1, 2 and 3 illustrate embodiments of the known art;
• Figures 4 and 5 illustrate a sectional view and a top view, partially exploded, of a circuit structure and components applied to microwave according to the invention.

For the production of a structure (respectively of propagation circuits) as shown in FIG. 1, the main design problem is to maintain a conductive element 10 at a precise distance from a ground plane 11 (from two planes respectively massive).

The medium 12, thus delimited by the conductive element 10, the ground plane (s) 11 and a characteristic distance d chosen during the design as a function of its influence on the phenomena of interaction between the electromagnetic field and the matter contained in this medium, must present the electrical characteristics ε _r (dielectric constant) and tg δ (loss factor) chosen by the designer.

• légèreté,
• rigidité,
• tenue en température (typiquement 130°C),
• faible dégazage,
• stabilité dimensionnelle (faible coefficient de dilatation thermique, faible coefficient de dilatation par désorption d'humidité, conductivité thermique élevée).

On the other hand, the entire device must present performance compatible with its use. For example, for a spatial application, the main performances will be:

• lightness,
• rigidity,
• temperature resistance (typically 130 ° C),
• low degassing,
• dimensional stability (low coefficient of expansion thermal, low coefficient of expansion by moisture desorption, high thermal conductivity).

Several solutions from a radioelectric point of view are usually adopted.

Thus, in the field of a propagation circuit, it is possible to confer, as shown in FIG. 2, significant rigidity on the ground planes 17 and it is thus possible to maintain between them the conductor 15 and the dielectric material 16. There is then the central conductor 15 disposed between two layers 16 of dielectric material, two structures 17 forming a ground plane being located on either side of this assembly. Each of these structures is formed for example of a "carbon skin sandwich 18-" honeycomb "made of aluminum 19-carbon skin 20, the carbon skin 20 located inwards being metallized 21. The material dielectric 15 can be produced in "honeycomb", in organic foam or by dielectric spacers for example.

The dielectric material 15 is chosen for its radioelectric performance, which allows a great latitude of choice. We can finally obtain a powerful solution from a radioelectric point of view. On the other hand, the addition of mechanical elements (stiffening of the ground planes, maintenance of the central conductor and of the dielectric medium) leads to poor mechanical performance. This type of solution is therefore well suited for small devices (surfaces typically less than 0.5 m²) and / or for devices where the ground planes are used to provide additional mechanical functions (maintenance of radiating elements of type horns or propellers for example).

In the case where high mechanical performance is required (in the case of large antennas for example), radically opposite solutions are generally adopted. These consist in fact of a total integration of the mechanical and electrical functions. This is obtained, as shown in Figure 3, by involving the dielectric material 22 in the mechanical rigidity of the assembly by gluing in particular. There is then the central metal conductor 25 disposed between two layers of dielectric 22, and two metal planes 23 forming ground planes, bonding layers 24 being located between each of the planes in contact. The advantage is then to use materials with high specific rigidity (composite materials for example) as far as possible from the neutral fiber of the "sandwich" (lower and upper surfaces of the panel) and to bond between these faces a material having good shear properties and low density ("honeycomb", for example). This principle is well suited for the production of large devices where a very low surface mass is sought (antenna, distributor, typically 5 kg / m²). The constraints to be taken into account when choosing the dielectric material are very strong, since it must satisfy the radioelectric, mechanical and environmental resistance requirements. We generally arrive at a good compromise, but the electrical performance is not always sufficient (loss factor too high due to the presence of adhesive films) or even the mechanical performance can be deteriorated (if for example we want to use a dielectric with a constant greater than 2 with a thickness greater than a millimeter).

The invention relates to a structure in which the electrical and mechanical functions are globally integrated, but locally dissociated.

As shown in Figures 4 and 5, the structure according to the invention comprises a mechanical structure 26 forming an enclosure 33 in which can be disposed a block 27 of dielectric material. On either side of the assembly thus formed is disposed a layer of dielectric material 28, (29), the first 28 supporting the conductive element 30 disposed above the dielectric pad 27, the other 29 supporting the plane of mass 31 metallic. A bonding layer 32 is disposed between the mechanical structure and each of the two dielectric layers.

Thus, in the structure according to the invention, the medium in the vicinity of the conductive element consists of a dielectric material whose selection criteria are mainly electrical (ε _r , tg δ) and which does not participate in mechanical rigidity from the whole. Beyond this vicinity, a mechanical structure makes it possible to contain the preceding dielectric material and to guarantee the overall mechanical performance of the device. Selection criteria materials constituting this structure being mainly mechanical (E / ρ, E = Young's modulus, ρ density), this one can be very effective.

• performances radioélectriques élevées et ajustables (ε_r) : un matériau diélectrique quelconque pouvant être utilisé, pourvu qu'il soit léger et résistant à l'environnement, de plus il n'est pas fait appel à un film de colle,
• performances mécaniques élevées : la structure étant réalisées à l'aide du matériau le mieux adapté, voire même à l'aide d'un matériau conducteur (composite à renfort graphite par exemple) si cela est admissible du point de vue radioélectrique.

The advantages of the invention are as follows:

• high and adjustable radio performance (ε _r ): any dielectric material that can be used, provided that it is light and resistant to the environment, moreover, no adhesive film is used,
• high mechanical performance: the structure being made using the most suitable material, or even using a conductive material (composite with graphite reinforcement for example) if this is admissible from the radioelectric point of view.

• ε_r = 2,5
• tg δ   aussi faible que possible
  E/ρ (rigidité spécifique) aussi élevée que possible.

In a first exemplary embodiment, it is possible to produce, with a height h for example of 3 mm, an antenna printed on a dielectric having the following desired performances:

• ε _r = 2.5
• tg δ as low as possible
  E / ρ (specific stiffness) as high as possible.

With the devices of the known art, where mechanical and electrical functions are integrated, the most suitable materials are PTFE (polytetrafluoroethylene) matrices with glass reinforcement. In fact, the epoxide and polyimide matrices, although they make it possible to achieve superior mechanical properties, bring up the values of ε _r and tg δ.

d'où les performances suivantes :

• Radiofréquence (RF)
     . tg δ = 9.10⁻⁴
• Mécanique
  • . γ   = 6,99 kg/m² (masse surfacique brute : sans connecteur, contrôle thermique,...)
  • . f   = 13 Hz (première fréquence de résonance pour une plaque carrée de 0,5 m de côté, dont les bords sont simplement supportés).

We have the following table:

hence the following performances:

• Radiofrequency (RF)
  . tg δ = 9.10⁻⁴
• Mechanical
  • . γ = 6.99 kg / m² (gross areal mass: without connector, thermal control, ...)
  • . f = 13 Hz (first resonance frequency for a square plate of 0.5 m side, whose edges are simply supported).

Whereas in the case of the device of the invention, the dielectric material is chosen for its radioelectric properties only. For example, with Alumina felt we get: ρ = 750 kg / m³ ε _r = 2.5 tg δ = 2.10⁻⁴ (assuming a linear variation of ε _r and tg δ depending on the density).

The material constituting the structure is mainly chosen for its mechanical characteristics.

• radiofréquence : tg   δ = 2.10⁻⁴
• mécanique (avec une structure en Kevlar/epoxy, de largeur 2 mm) :
  • . f = 19,8 Hz
  • . γ = 2,83 kg/m²

The performances obtained in this example are:

• radio frequency: tg δ = 2.10⁻⁴
• mechanical (with Kevlar / epoxy structure, width 2 mm):
  • . f = 19.8 Hz
  • . γ = 2.83 kg / m²

With a device according to the invention, the gain can therefore be a factor of 4 on the R.F. losses and a factor of around 2.5 on the mass.

In a second embodiment, an antenna printed on a dielectric can be produced having a constant as close as possible to 1, with a patch / ground plane distance = 6 mm, the performances sought being those of the first embodiment with ε _r ≃ 1.

With the devices of known art, where mechanical and electrical functions are integrated, the most suitable architectures are obtained by bonding a very aerated organic material (foam, honeycomb) between the substrates supporting the elements. radiant and the ground plane by means of glue films or layers of composite materials.

• radiofréquence :
  • . ε_r ≃ 1,04
  • . tg δ ≃ 6.10⁻⁴
• mécanique :
  • . γ ≃ 0,928 kg/m2
  • . f ≃ 107 Hz

The following performances are obtained:

• radio frequency:
  • . ε _r ≃ 1.04
  • . tg δ ≃ 6.10⁻⁴
• mechanical :
  • . γ ≃ 0.928 kg / m2
  • . f ≃ 107 Hz

• Radiofréquence. :
     ε_r = 1
     tg δ ≃ 0
• mécanique (avec une structure en fibres de carbone) :

. γ = 1,126 kg/m² (même fréquence de résonance f = 107 Hz)On the other hand, by using the device according to the invention, the volume under the radiating element remaining empty, the following performances are obtained:

• Radio frequency. :
  ε _r = 1
  tg δ ≃ 0
• mechanical (with a carbon fiber structure):

. γ = 1.126 kg / m² (same resonant frequency f = 107 Hz)

For a mass increase of approximately 20%, a radiating element is produced for which the losses are practically zero.

• . de Kevlar ;
• . de carbone ;
• . de verre ;
• . ou de tout autre renfort :

The components of the radiating element according to the invention can be produced using numerous materials, thus:
the mechanical structure 26 can be made of composite materials based, for example:

• . Kevlar;
• . of carbon ;
• . of glass ;
• . or any other reinforcement:

• . de la céramique (ε_r > 1) ; (céramique aérée, ou fibre de céramique ou feutre de céramique)
• . un matériau organique ou composite (ε_r > 1)

   - le volume peut être rempli :

• . de gaz ;
• . d'air ;
• . de vide.

The dielectric material used can be:

• . ceramic (ε _r >1); (aerated ceramic, or ceramic fiber or ceramic felt)
• . an organic or composite material (ε _r > 1)

- the volume can be filled:

• . gas ;
• . air;
• . empty.

Claims (16)

1. A patch type antenna structure in which the mechanical and electrical functions are integrated overall, and further comprises:
      a mechanical structure (26) forming a plurality of enclosures;
      a plurality of conductive elements (30); and
      a metal ground plane (31);
      said mechanical structure (26) forming a plurality of enclosures between said conductive elements (30) and said ground plane (31), the antenna structure being characterized in that: an insulating medium (27) is placed in each of said enclosures, said mechanical structure (26) being situated outside of said enclosures; and in that each conductive element (30) is disposed on the top of said insulating medium (27), said mechanical and electrical functions thus being disassociated locally.
2. An antenna structure according to claim 1, characterized in that a first layer of dielectric material (28) supports each conductive element (30) disposed on the top of said insulating medium (27).
3. An antenna structure according to claim 1 or 2, characterized in that a second dielectric layer (29) supports the metal ground plane (31).
4. An antenna structure according to claim 3, characterized in that a layer of glue (32) is disposed between the mechanical structure (26) and each of the two dielectric layers (28 and 29).
5. An antenna structure according to claim 1, characterized in that the volume available beneath the conductive elements (30) has characteristics desired from the electrical point of view.
6. An antenna structure according to claim 1, characterized in that the mechanical structure is made of composite material.
7. An antenna structure according to claim 6, characterized in that the composite material used is based on Kevlar fiber.
8. An antenna structure according to claim 6, characterized in that the composite material used is based on carbon.
9. An antenna structure according to claim 6, characterized in that the composite material used is based on glass.
10. An antenna structure according to any preceding claim, characterized in that said insulating medium (27) is a dielectric material.
11. An antenna structure according to claim 1, characterized in that in order to obtain a desired dielectric constant, the enclosure (33) is filled with a gas.
12. An antenna structure according to claim 11, characterized in that the gas is at low pressure.
13. An antenna structure according to claim 1, characterized in that said insulating material (27) is a vacuum.
14. An antenna structure according to claim 10, characterized in that said dielectric material used includes ceramic.
15. An antenna structure according to claim 14, characterized in that the ceramic is aerated.
16. An antenna structure according to any one of claims 10 to 12, characterized in that said dielectric material used includes an organic or composite material.

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