EP2811574B1 - Rigid radome for a concave reflector antenna - Google Patents

Description

The present invention relates to a telecommunication antenna with concave reflector having for example the shape of at least one parabola portion. These antennas, in particular of the microwave type, are usually used in mobile communication networks. These antennas operate indifferently in transmitter mode or in receiver mode, corresponding to two opposite directions of RF wave propagation.

The reflector is associated with a radome having an impervious protective surface which partitions the space defined by the reflector, with or without a skirt, vis-à-vis the outside. This radome can be flexible or rigid. A rigid radome, the most currently used, has the advantage of good resistance to the external climate environment such as rain, wind or snow.

Nevertheless, the thickness of the material used in a rigid radome is problematic since this thickness is determined as a function of the frequency band used by the antenna. For example, the thickness of a rigid radome implemented in an antenna transmitting with a frequency of 40 GHz is practically half of the thickness of a rigid radome of the same kind used in an antenna transmitting with a frequency of 20 GHz. It is understood that to use the antenna over a wide frequency band, ranging from 5 to 25 GHz, for example, it is necessary to use five radomes of different thickness. These radomes must be dismantled and replaced at each frequency domain change.

• une grande transparence aux ondes radioélectriques sur la plus grande bande passante possible,
• une bonne résistance mécanique à des charges supérieures à 300 Kg/m², ce qui correspond à des vents de 250 Km/h,
• une stabilité suffisante vis-à-vis des rayons ultraviolets (UV), de la pluie, des brouillards salins et des écarts de température dans la plage -45°C à +70°C,
• un coût le plus bas possible, notamment en ce qui concerne les radômes de grand diamètre.

In addition, a radome must have the following qualities:

• a high transparency to the radio waves on the greatest possible bandwidth,
• good mechanical strength at loads greater than 300 Kg / m ² , which corresponds to winds of 250 Km / h,
• sufficient stability against ultraviolet (UV) rays, rain, salt spray and temperature differences in the -45 ° C to + 70 ° C range,
• the lowest possible cost, especially for large diameter radomes.

For example, for antennas transmitting with a frequency of 25 GHz, we know radomes having a sandwich structure consisting of a cellular core structure with a thickness of about 3 mm and two external plates having a thickness of order of 0.35 mm each. Such a structure has excellent radio performance for frequencies around 25 GHz, but has more modest performance for frequencies far from this target.

This phenomenon is due to the difference between the actual thickness of the radome and the reference thickness which is adapted to the value of a half-wavelength at the center frequency of the bandwidth. Such a radome having a thickness equal to the reference thickness is called a "half-wave radome". For large deviations from the central frequency of operation of the antenna, and therefore the ideal thickness, the reflection coefficients of the radome can exceed -20 dB, which affects the performance of the antenna . The prior art US2010103072 describes a shielded radome with honeycomb. The present invention aims to eliminate the disadvantages of the prior art, by providing a rigid radome for operation with satisfactory performance of a parabolic antenna in a wider frequency range than the prior art, without which it is necessary to change it.

More particularly, the present invention therefore aims to improve the performance of the antennas for frequencies below the target frequencies of the radome and the antenna, without the need to change it.

The object of the present invention is a radome intended to be mounted on an antenna, said radome comprising a first layer and at least a second layer attached to said first layer so as to increase the effective thickness of said radome on at least a portion of its surface, the area of said second layer being strictly less than 100% of the area of said first layer.

The first layer may be made of a rigid or flexible material, and adopt a flat, curved or conical shape.

This has the advantage of achieving a lightweight and low cost radome allowing applications for millimeter waves by easy modification of high frequency radomes, which are themselves lightweight and inexpensive.

Indeed, for low frequencies a half-wave radome is usually preferred, which is very thick and therefore expensive material. The present invention makes it possible to adapt a radome with a sandwich-type structure to use at low frequencies while avoiding the use of a radome with a very thick "sandwich" type structure or a radome with a conical shape. or spherical, which is very difficult to achieve with multilayer materials.

In one aspect, the second layer of the radome is formed of a material of the same nature as the first layer.

In another aspect, the second layer of the radome is attached to the first layer via a spacer so as to space said second layer of said first layer at least over a portion thereof.

According to another embodiment, a radome has spacers allowing a spacing of the first and second layers between 10 mm and 3 mm.

In addition, the surface of the second layer of the radome may have a shape different from the surface of the first layer.

According to one variant, the surface of the second layer of the radome is curved and the surface of the first layer of the radome is flat.

According to another variant, the surface of the second layer of the radome is flat and the surface of the first layer of the radome is curved.

The second layer may be made of a rigid or flexible material, and adopt a flat or curved shape.

Furthermore, the second layer of the radome may have a polygonal surface, such as for example a triangle or a hexagon or a quadrilateral such as a rectangle or a square, or a shaped surface defined by a closed curve such as a circle, an oval or an oblong.

In one embodiment, the area of the second layer covers at least 70% of the area of said first layer, and preferably at least 50% of the area of said first layer.

In another aspect, the second layer is removably attached to the first layer.

In a radome the space between the first and second layers can be filled with low density foam.

A second object according to the present invention is an antenna having a radome according to any one of the above aspects.

According to one embodiment, the second layer of the radome is arranged so as to face a reflector of said antenna. Preferably, the second layer is disposed on the inner face of the radome facing the waveguide.

A third object according to the invention is a method of manufacturing a radome, comprising the following steps: providing a first layer of a radome and fixing a second layer on the surface of said radome so as to increase the effective thickness of said radome the area of said second layer being strictly less than 100% of the area of said first layer.

In another aspect, the second layer is secured to the first layer of the radome with spacers so as to space said second layer of said first layer of said radome on at least a portion of said second layer.

• la figure 1 illustre une vue en perspective d'une antenne comprenant un radôme selon un mode de réalisation ;
• la figure 2 illustre une vue de la face interne du radôme de la figure 1;
• la figure 3 illustre un moyen de fixation selon un aspect de la présente invention;
• les figures 4 à 6 illustrent diverses variantes d'un radôme en coupe diamétrale « A-A » ;
• les figures 7 et 8 illustrent une comparaison des performances radioélectriques d'un radôme selon l'art antérieur et selon l'invention;
• les figures 9 et 10 illustrent une comparaison des performances radioélectriques d'un radôme selon l'art antérieur et selon l'invention.

Other features and advantages of the present invention will appear on reading the following description of several embodiments, given of course by way of illustration and not limitation, and in the accompanying drawing in which:

• the figure 1 illustrates a perspective view of an antenna comprising a radome according to one embodiment;
• the figure 2 illustrates a view of the inner face of the radome of the figure 1 ;
• the figure 3 illustrates a fastening means according to one aspect of the present invention;
• the Figures 4 to 6 illustrate various variants of a radome in diametrical section "AA";
• the Figures 7 and 8 illustrate a comparison of the radio performance of a radome according to the prior art and according to the invention;
• the Figures 9 and 10 illustrate a comparison of the radio performance of a radome according to the prior art and according to the invention.

In these figures, the identical elements bear the same reference numbers.

In the embodiment illustrated on the figure 1 , there is shown an antenna 10 provided with its fixing means 12, for example to be fixed on Matt. The antenna 10 comprises a parabolic reflector 14 at the center of which is placed a waveguide (not shown). A radome 20 fixed at its periphery on the reflector 14 covers the dish 14. The attachment points of the radome 20 to the dish 14 are illustrated with the reference 22.

The first layer may be made of a rigid or flexible material which allows, as the case may be, to obtain a flat, curved or conical shape. Various materials can be used for the construction of the radome such as a polymer (ABS, PS, PVC, PP) injected or thermoformed. They have in common to attenuate to a minimum the signal sent and received. The radome 20 may for example consist of a sandwich-type multilayer material comprising two dense outer plates surrounding at least one central portion containing a high proportion of air. The outer plates, which are continuous flat thin plates, and the three-dimensional central portion are made of the same polymeric material, preferably polypropylene (PP). The radome 20 may especially be of the "honeycomb" type whose cells have a substantially conical shape or of the "honeycomb" type.

For the purposes of the following description, whatever the structure of the first monolayer or multilayer layer 24 of the radome 20, it will be named in the singular "layer". The composition of the first layer 24 is well known to those skilled in the art and will not be further detailed.

In order to improve the performance of the low frequencies, a second layer 30 (visible in transparency) is fixed to said first layer 24. Preferably, the second layer 30 is arranged so as to be in front of the guide of FIG. wave of the antenna 10, that is to say on the inner surface of the radome 20 when it is mounted on the reflector 14.

The goal is to make the radome transparent to the waves. When a wave meets a radome, there is of course a transmitted wave but also a partially reflected wave. By combining different layers with very precise thicknesses, this reflected wave can be canceled over a defined bandwidth. Indeed the waves reflected on the different layers are superimposed and can cancel if the sum of the phases is zero.

We will now consider the figure 2 which illustrates in more detail the internal face of the radome 20 comprising the first layer 24. A second layer 30 is fixed to the first layer 24.

The second layer 30 is preferably made of materials of the same nature as for the layer 24. However, since it does not directly undergo the external influences, that is to say the wind and the UV radiation, its mechanical characteristics can be eased.

The area of the second layer 30 should be strictly less than 100% of the area of the first layer 24, and preferably the area of the second layer 30 should have at least 50%, and preferably at least 70%, of the the area of the first layer 24 to have a significant radioelectric impact.

The second layer 30 may furthermore have any shape, and preferably polygonal shapes such as a hexagon, triangle or quadrilateral such as a parallelogram, a square, a rectangle, a rhombus, a trapezium, etc., although circular or oblong shapes are allowed. Preferably, a square shape is from the point of view simplicity preferable because easier and less expensive to produce. However, any other form is possible if the minimum ratio of areas is greater than about 50%.

The second layer 30 may be fixed to the first layer 24 in any manner and using all the means available for this purpose according to the constraints known to those skilled in the art.

The second layer 30 may in particular be attached to attachment points 32 as illustrated, or plated continuously over the entire surface of the first layer. In addition, the attachment points 32 can fix the second layer 30 in contact with the first layer 24, just as these attachment points 32 can also keep the second layer 30 spaced from said first layer 24. In addition, the spacing between the first and second layers may vary and not remain constant.

Preferably, the attachment points 32 are preferably made of UV-resistant plastic materials, and of a structure sufficiently strong to hold the two layers in place. In addition, the fixing points have a small diameter so that the set of attachment points 32 represents an area less than 0.05% of the surface of the radome.

With reference to the figure 3 , and in a non-limiting variant, these attachment points 32 between the second layer 30 and the first layer 24 of the radome 20 may in particular be fastening clips, allowing easy attachment of the second layer 32 to the radome 20. These clips 32 can thus allow easy and quick disassembly of the second layer 32.

The Figures 4 to 6 illustrate possible alternative arrangements of the first and second layers 24, 30, i.e., a radome according to various embodiments.

The figure 4 illustrates a variant in which the second layer 30 is fixed to the main layer 24 of the radome by attachment points 32 on the periphery of said second layer 30, as well as by one or more central attachment point (s) 32. In addition, spacers 44 of equal thickness are mounted co-axially with the attachment points 32 to maintain a constant spacing between said first and second layers. The spacers 44 preferably have a height of between 10 mm and 3 mm.

The figure 5 illustrates a variant in which the second layer 30 is fixed to the first layer 24 of the radome by attachment points 32 on the periphery of said second layer 30, as well as by one or more central attachment point (s) 32. In addition, one or more struts 54 are mounted co-axially with the attachment points 32 only on the central portion of the first and second layers 24, 30 to have a curved shape, and more particularly to give a convex surface to the second layer 30 with respect to the flat surface of the first layer 24. The struts 54 of the central portion preferably have a height of between 10 mm and 3 mm.

The figure 6 illustrates a variant in which the second layer 30 is fixed to the first layer 24 of the radome by attachment points 32 on the periphery of said second layer 30, as well as by one or more central attachment point (s) 32. In addition, one or more struts 64 are mounted co-axially with the attachment points 32 only on the peripheral portion of the first and second layers 24, 30 to give the second layer 30 a curved shape, and more particularly so that it has a concave surface with respect to the flat surface of the first layer 24. The struts 64 preferably have a length of between 10 mm and 3 mm. It should be noted that the material of the second layer 30 must have some resilience to bending to allow a curved shape.

The variants illustrated in Figures 5 and 6 allow to widen the bandwidth. Indeed, as the difference between the first and second layers 24, 30 affects the frequency of adaptation of the radome 20, a variable difference between the first layer 24 and the second layer 30 slightly degrades the performance on the central frequency but allows 'broaden the bandwidth. In addition, a nonplanar shape of the radome deflects the reflections outside the primary source situated at the focal point of the parabola 14.

Indeed, as an example, Figures 7 and 8 illustrate the value of the reflection coefficient R in dB, or standing wave ratio, as a function of the frequency F in GHz of the wave incident on the radome over a frequency range from 6 to 40 GHz, for this radome ( figure 8 ) and a known radome ( figure 7 ).

The known radome, adapted for operation in the frequency range 20-30 GHz, corresponding to the performances of the figure 7 (curve 107) has a single layer of thickness 3.7 mm. The frequency performance is very good around 25 GHz, but only acceptable in the rest of the field.

This is due to the increasing frequency difference with respect to the half-wavelength at the central operating frequency for which the antenna is dimensioned. Deviations from the results obtained with a radome of suitable thickness result in a radome reflection coefficient reaching levels above -20 dB over most of the domain, which can lead to problems with vis global performances of the antenna in terms of losses by reflection ("return loss" in English). Indeed, the limit of acceptable performance for a radome corresponds to a reflection coefficient of about -20 dB materialized by the reference line 109. Above this value, the antenna radiation pattern and the ratio of stationary wave may be disturbed.

The radome corresponding to the curve 108 of the value of the reflection coefficient illustrated in FIG. figure 8 comprises a first layer 24 with a thickness of 3.7 mm, and a second layer 30 with a thickness of 3.7 mm, with a spacing between the first and second layers 24 and 30 of 5 mm. It can be seen that an antenna tuned for an optimum response around 25 GHz, including the radome described above, has acceptable performance for frequencies between 6 and 12 GHz.

As another example, the figure 9 illustrates a curve 110 of the loss P of reflection radiation ("return loss" in English), in dB, as a function of the frequency F in GHZ of the wave incident on the radome over a frequency range from 7.1 GHz at 8.5 GHz, for a parabolic antenna 1 meter in diameter with a radome having a single layer structure "sandwich" 3.7 mm thick already detailed above.

It can be seen that the effectiveness of the known radome is limited. Indeed, the radome is not very transparent since the curve 110 has exceedances of the reference line 111, corresponding to the limit value of 18 dB, which occur at different points of the curve 110.

In the case of this radome comprising a first layer 24 and a second layer 30 with a thickness of 3.7 mm, with a spacing between the first and second layers 24, 30 of 5 mm (similar to the radome of the figure 2 ), the performance over the frequency range 7.1 to 8.5 GHz is quite satisfactory, as illustrated in figure 10 (curve 112). A reflection loss of less than 18 dB is observed over the entire frequency band explored by curve 112.

It is clear that the performance of the antenna is significantly improved. An average gain of 3 to 4 dB is observed on the peaks of the curve 110 which exceeded the limit of 18 dB, so that on the curve 112 quite acceptable values for these peaks are found, lying around 19 dB. dB. It is understood that the radome, according to the embodiment just described, is effective on a lower frequency band than known radomes.

• de donner différentes formes de la deuxième couche du radôme,
• d'employer différents matériaux et structures pour les couches du radôme,
• de ménager un espacement entre les première et deuxième couches qui peut être réalisé aussi à l'aide d'une feuille de mousse de faible densité qui permettrait une meilleure régularité de l'espacement.

Of course, the present invention is not limited to the described embodiments, but it is capable of many variants accessible to those skilled in the art without departing from the spirit of the invention. In particular, without departing from the scope of the invention, it will be possible to provide:

• to give different shapes of the second layer of the radome,
• to use different materials and structures for the layers of the radome,
• to provide a spacing between the first and second layers that can be achieved also using a low density foam sheet that would allow a better regularity of the spacing.

Claims (12)

1. A radome (20) intended to be installed on an antenna (10) with a concave reflector, comprising:

   - a first layer (24),

   - at least one second layer (30) fastened to said first layer in such a way as to increase the effective thickness of said radome (20),

   characterized in that

   - the surface area of said second layer (30) is less than 100% of the surface area of said first layer (24),

   - the surface of the second layer (30) has a different shape from the surface of said first layer (24),

   - the second layer (30) is fastened to the first layer (24) by means of a spacer (44) so as to space said second layer (30) from said first layer (24) at least on part of it.
2. A radome according to claim 1, wherein the second layer (30) is formed of a material of the same type as for the first layer (24).
3. A radome according to one of the claims 1 and 2, wherein the spacer enables a spacing of the first and second layers (24, 30) comprised between 10 mm and 3 mm.
4. A radome according to any one of the claims 1 to 3, wherein the surface of the second layer (30) of the radome is curved and the surface of the first layer (24) of the radome is flat.
5. A radome according to any one of the claims 1 to 3, wherein the surface of the second layer (30) of the radome is flat and the surface of the first layer (24) of the radome is curved.
6. A radome according to one of the preceding claims, wherein the second layer (3) has a polygonal-shaped surface or one bounded by a closed curve.
7. A radome according to one of the preceding claims, wherein the surface area of the second layer (30) covers at least 70% of the surface area of said first layer (24), preferably at least 50% of the surface area of said first layer (24).
8. A radome according to one of the preceding claims, wherein the second layer (30) is removably fastened to the first layer (24).
9. A radome according to one of the preceding claims, wherein the space between the first and second layers (24, 30) is filled with low-density foam.
10. An antenna (10) comprising a radome (20) according to one of the preceding claims, wherein the second layer (30) is arranged so as to face a reflector of said antenna.
11. An antenna (10) according to claim 10, wherein the second layer (30) is arranged on the internal surface of the radome that faces the waveguide.
12. A method for manufacturing a radome (20) according to any one of the claims 1 to 9, comprising the following steps:

    - providing a first layer of a radome,

    - fastening a second layer onto the surface of said first layer so as to increase the effective thickness of said radome,

    characterized in that the surface area of said second layer is less than 100% of the surface area of said first layer,

    - the surface of the second layer (30) has a different shape from the surface of said first layer (24),

    - the second layer (30) is fastened to the first layer (24) by means of a spacer (44) so as to space said second layer (30) from said first layer (24) at least on part of it.

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