EP0399524A1 - Structure for the realisation of circuits and components, applied to microwave frequencies - Google Patents

Abstract

The invention relates to a structure for producing circuits and components applied to microwave frequencies, in which the mechanical and electrical functions are globally integrated, but locally dissociated. Application in particular to the space field (space antennas).

Description

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

The growing 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 radioelectric 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 range of admissible temperatures that the technologies used will have to withstand. All of these additional constraints are, here too, 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 for 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.

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 each other using a dielectric support or spacer material. 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.

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.

To this end, the invention proposes a structure for producing circuits and components applied to microwave frequencies, in which the mechanical and electrical functions are globally integrated, but locally dissociated; a mechanical structure forming an enclosure in which is disposed a block of dielectric material. On either side of the mechanical structure-dielectric block assembly is arranged a layer of dielectric material, the first supporting a conductive element disposed above the dielectric pad, the other supporting a metallic ground plane, a bonding layer being disposed between the mechanical structure and each of the two dielectric layers.

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 achievements of the known art;
• - Figures 4 and 5 illustrate a sectional view and a top view, partly 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.

On the other hand, the entire device must have 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 retained.

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 "of aluminum 19-carbon skin 20, the carbon skin 20 located inwards being metallized 21. The material dielectric 15 can be made 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 the 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 (maintaining 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 bonding 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-scale 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 behavior requirements. We generally arrive at a good compromise, but the electrical performance is not always sufficient (too high loss factor due to the presence of glue 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. The selection criteria materials constituting this structure being mainly mechanical (E / ρ, E = Young's modulus, ρ density), this one can be very effective.

The advantages of the invention are as follows:
- high and adjustable radioelectric 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 produced 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.

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 epoxy and polyimide matrices, although they make it possible to achieve superior mechanical properties, raise the values of ε _r and tg δ.

We have the following table: Material ε _r tg δ x 10⁻⁴ E / ρ x 10⁵ (SI) Glass / PTFE 2.5 9 6 Quartz / polyimide 3.6 40 100 Kevlar / epoxy 3.9 130 193 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.

The performances obtained in this example are:
- radio frequency: tg δ = 2.10⁻⁴
- mechanical (with a Kevlar / epoxy structure, 2 mm wide):
. 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 exemplary embodiment, it is possible to produce an antenna printed on a dielectric 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 exemplary 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 via glue films or layers of composite materials.

The following performances are obtained:
- radio frequency:. ε _r ≃ 1.04
. tgδ ≃ 6.10⁻⁴
- mechanical : . γ ≃ 0.928 kg / m2
. 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:
- Radiofrequency. : 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.

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:

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.

It is understood that the present invention has only been described and shown as a preferred example and that its constituent elements can be replaced by equivalent elements without, however, departing from the scope of the invention.

Claims (15)

1 / Structure for producing circuits and components applied to microwave frequencies, in which the mechanical and electrical functions are globally integrated, but locally dissociated, the mechanical function being provided by a mechanical structure (26) forming an enclosure (33), characterized in that that a block of dielectric material (27) is disposed in said enclosure; and in that on either side of the mechanical structure assembly (26) dielectric pad (27) are arranged a conductive element (30) and a metallic ground plane (31), external with respect to said assembly (26 , 27). 2 / Structure according to claim 1, characterized in that a first layer of dielectric material (28) supports the conductive element (30) disposed above the dielectric pad (27). 3 / Structure according to any one of claims 1 or 2, characterized in that a second dielectric layer (29) supports the metallic ground plane (31). 4 / Structure according to claim 3, characterized in that a bonding layer (32) is disposed between the mechanical structure (26) and each of these two dielectric layers (28 and 29). 5 / Structure according to any one of the preceding claims, characterized in that the mechanical function is provided by a mechanical structure (26) located outside the volumes of material under the conductive elements (30). 6 / Structure according to claim 5, characterized in that the volume available under the conductive elements (30) has the desired characteristics from the electrical point of view. 7 / Structure according to claim 1, characterized in that the mechanical structure is made of composite material. 8 / Structure according to claim 7, characterized in that the composite material used is based on Kevlar fiber. 9 / A structure according to claim 7, characterized in that the composite material used is based on carbon. 10 / Structure according to claim 7, characterized in that the composite material used is based on glass. 11 / structure according to claim 1, characterized in that, to obtain the desired dielectric constant, the enclosure (33) is filled of a gas. 12 / Structure according to claim 11, characterized in that the gas has a very low pressure. 13 / Structure according to any one of the preceding claims, characterized in that the dielectric material used comprises ceramic. 14 / structure according to claim 13, characterized in that the ceramic is aerated. 15 / Structure according to any one of claims 1 to 12, characterized in that the dielectric material used comprises an organic or composite material.

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