FR2560444A1 - Matched radome for the mechanical protection of a microwave antenna and its method of implementation. - Google Patents

Abstract

The invention relates to the mechanical protection of a microwave antenna. In order afford protection to an antenna behind a radome of appropriate thickness, the radome is electromagnetically matched by means of self-inductive shields 12, 13 and/or capacitive shields 15 placed outboard and/or inboard of the radome and suitably sized as a function of thickness, nature of the thick radome 11 and working characteristics of the antenna. The invention applies to the protection of "hardened" radar or telecommunication antennas.

Description

The invention relates to a radome adapted for protecting an antenna

  microwave and its method of

Implementation.

  In the context of the protection to certain

  mechanical, or hardening, antennashyperfrequency and in particular overhead devices radar system or telecommunication, it is necessary to protect these airborne devices by thick radomes. These radomes consist of

  a dielectric material, such as a plastic, honeycomb

  whether or not of appropriate quality. The thickness of the radome must of course be sufficient for the radome to have the qualities of mechanical strength desired, and its thickness must

  on the other hand, be adapted so as not to disturb the

  sion and reception of electromagnetic waves in the

of work of the antenna.

  A classic solution is to define a thick

  radome compatible with both the mechanical constraints

  and with the radioelectric constraints, so as to create the minimum disturbance to the crossing of the waves in the

working band of the antenna.

  To obtain a suitable "transparency" in the working band of the antenna, one must satisfy the equation: e g =) Ag1) in which, n is a whole number

  2; Ago is the wavelength in the dielectric medium

  the central frequency of the working band of the antenna,

e is the thickness of the radome.

  It appears, therefore, that in general

  squeeze the thickness of the radome to fall on a whole number

  of half-wavelengths, otherwise the electro-

magnetic is not obtained.

  In addition to this oversizing, this process also

  the disadvantage that the electromagnetic adaptation is not

  3 is not satisfactory as soon as one departs significantly from

  the average wavelength of the working frequency band.

  In other words, the self-winding electromagnetic window

  is very narrow, the transmission / reception band being

  about 5% around the central wavelength and even

  me unless n becomes greater than 3.

  Another disadvantage is that for antennas with large aperture, the adaptation becomes poor when the angle of incidence of the radiation is inclined too much with respect to the central axis of the antenna, for example for an incidence greater than 30 .

  The object of the invention is to solve these difficulties.

  your. For this purpose, the method of protection by radome

  of a microwave antenna is characterized by dissolving

  the mechanical protection of electromagnetic adaptation

  of the radome, by associating with a radome of mechanical characteristics

  calculated standards required, at least one inductive screen or

  citif placed near the radome and ensuring its adaptation

  to the operating frequency band of the antenna.

  The invention also relates to a radome adapted for protecting a microwave antenna, in particular

  radar or telecommunication system, characterized in that

  takes, associated with a radome of mechanical protection of thick-

  useful, at least one inductive or capacitive screen for adapting

  the operating frequency band of the antenna, which screen is placed against the outer surface and

inside the radome.

  It appears that in this way we can determine

  and build the radome of mechanical protection in the calculation

  simply by taking into account the mechanical

  to be able to withstand, while the electronic adaptation

  magnetic is done independently. In this way it is no longer necessary to oversize the thickness of the radome, and secondly the electromagnetic adaptation can be done in much better conditions, so as to open a wider electromagnetic window and avoid distortions too

  radiation emitted or received even from

important importance.

  The invention and its implementation will appear more

  clearly with the help of the following description made in

  reference to the accompanying drawings, in which: FIG. 1 schematically shows an inductive adaptation grid that can be used in accordance with the invention, FIG. 2 shows like FIG. 1 another variant, FIG. 3 is a sectional view made of according to the lines III-III of FIGS. 1 or 2, FIG. 4 schematically shows the electromagnetic adaptation of a radome according to the invention, FIGS. 5 and 6 are schematic examples of capacitive adaptation screens, Figure 7 shows a construction variant of a

  adapted radome according to the invention.

  According to the invention, the thickness of the radome necessary for obtaining the protection is first calculated.

  desired mechanics. We choose simultaneously the nature of the material

  which defines the value of the dielectric constant

electric.

  Thus, in FIG. 4 is schematically represented in section the thickness e of a radome constituted by a dielectric material of constant s determined (for example υ = 3.8

  when the radome is constructed of glass-laminated material

resin of a certain type).

  The adaptation to the working frequency band of the antenna protected by radSme is carried out separately at

  neighborhood of each radome interface by an adjustment screen.

  such as a metal grid 2,3, whose characteristics

  can be determined by calculation or experience.

  In FIG. 1, a grid 4 is shown

  catrées with wire of rectangular section

  which can constitute an inductive screen, particularly in

  the case of substantially planar radomes. When the radome is

  a relatively large curvature surface, it is pre-

  can replace the square mesh grid 4 with a grid

  the round mesh such as 5 illustrates in Figure 2 formed

  square section metal wires, suitably juxtaposed.

  Such a grid with round meshes can indeed be shaped according to any desired curvature without introducing deformation

  crossing the electromagnetic beam.

  In short, the inductive shields 2 and 3 are intended to counterbalance the capacitive effect of the thickness e of the radome

1 of dielectric constant s.

  Good results are obtained by placing the metal grids 2 and 3 at a distance from the interface 6.7 with the radome air equal to X / "(where Xq is the average wavelength of the working frequency band in the dielectric material of constant ú), the opening of the meshes or the diameter of the rings as well as the section of the wire constituting the inductive screens 2, 3 are related to the dielectric constant E of the radome and to the wavelength As a non-limiting example, a grid consisting of a stack of rings according to the device of the diagram of Figure 2, whose average diameter is 24.9 millimeters and the square section to 0.5 x 0.5 mm, arranged at 4.9 millimeters from the airdielectric interface

  a radome in a glass-resin complex of dielectric constant

  that s = 3.8, ensures an adaptation of this interface in the frequency band going from 4.4 to 5 GHz, for incidences varying from 0 to 30, and for any polarization of the wave, adaptation characterized by a ratio stationary wave

(TOS) less than 1.2.

  The same performance will be achieved with a 19.5 mm square mesh grid made of

  metal sections 0.6 mm in diameter and arranged at the same

  this of the air-dielectric interface. In the case of the circular mesh grid, there is no difficulty in covering a spherical cap 3.5 meters wide and 7.25 meters in radius of curvature, while a square grid in a square similar use would present on the edges

  of this cap a defect of orthogonality of 7, very prejudi-

  Desirable for the good transmission of the polarization of the wave.

  The adaptation can be further improved and the frequency band widened by further using capacitive screens which can be constituted: either by a thin blade of dielectric material

  maintained in front of the air-dielectric interface by a

  tercalaire honeycomb, - is still a metal grid different from

  in that it consists of metallic obstacles

  non-contiguous, such as crosses, as schematizes the screen 9 of Figure 5 or circular pellets, as schematically the screen 10 of Figure 6, which crosses and pellets met-lliques may be deposited on a Esu pport thin, by any appropriate technique, for example

photoengraving.

  An example of mixed adaptation is illustrated in

  7. Mechanical protection is essentially achieved by a thickness e dielectric material It forming the mechanical core of the radome. In front of this layer 11, ie towards the outside of the antenna, two inductive metal grids 12, 13 have been arranged. Behind the layer he is

  10 to say towards the inside of the radome.

  it is reduced by a dielectric constant close to that of

  the air is equivalent to a blade of air, and behind which one finds

  ve a thin layer 15 of constant dielectric material

  superior forming capacitive screen.

  By way of example, for the above-mentioned frequency band, the following characteristics are selected: grid 12: pitch 14 mm, wire diameter 0.2 μm, distance d 1 from the interface before 16 mm. layer 11 - 7.85 mm; gate 13: pitch 28 mm, diameter of the wires 0.2 mm; resistance d 2 of the front face 16: 22.8 mm; e {9th thickness of the layer 11): any in -; c ti-? of the desired nmechanical 1-p-rotection, e = 3.8 in the blade 4: 9 Im; s = Mn1:

  the blade 15: e = 4 m constant

zs = (ii drome e rees).

  Such a radmoe has a TOS of less than 1775 in

  ..... uncsJ 2O% up to 50 '... DTD: as3 gz .. ç. Sci. Tques 12, 13 on the e-side

  i ::, n: eten: d eiv s -rformances in a larger band

  - .. it is possible to make rings, of which

  sicns: es;. s ss alers r-V-ie 2} iz of rings = 1783 mm ie 1r, r in the case of -ca01,2 rn ^ .. to 5 9th ta-ndis that the l 2 dielectric 3 has it ls - l - - - marry perfectly the curvature - ..: - iG uela i exe 14 can be done by

  4 * Example sn, n alveolar ma-tlia.

Claims (10)

  1. Radiation protection method for a microwave antenna characterized in that the mechanical protection of the electromagnetic adaptation of the radome is dissociated by associating at least one inductive or capacitive screen (2, 3,12,13,   placed near the radome (1,11) and ensuring its adaptability   to the operating frequency band of the antenna.   2. Radome adapted for the protection of a microwave antenna, in particular radar or telecommunication,   characterized in that it comprises associated with a radome protector   mechanical thickness of useful thickness e, at least one inductive or capacitive screen (2, 3, 12, 13, 15) adapted to the operating frequency band of the antenna, which screen is   placed against the outer and / or inner surface of the radome.   3. Radome according to claim 2 characterized in   the inductive screen (2,3,12,13) comprises a metal grid   lic. 4. Radome according to claim 3 characterized in that the metal grid is a square mesh grid (4). 5. Radome according to claim 3 characterized in   what the metal grid is a circular mesh grid re (5).   6. RadOme according to claim 2 characterized in that the capacitive screen is formed of a thin sheet of dielectric (15).   7. Radome according to claim 2 characterized in that the capacitive screen is formed of a thin carrier sheet bearing electrical conductive patterns such as   dipoles, crosses (9) or pellets (10).   8. Radome according to one of the claims 2 = 5   characterized in that the screen 2s-à is> ... 'u- the surface of the radame (11).   9. Radcls lnclude claim 2, where it is possible for the screen (2,15) to be placed on the screen. surface of the radome Â1,11). -7   10. Radome according to one of claims 2 to 9   characterized in that comprising at least one inductive screen (12,13) and at least one capacitive screen (15), the capacitive screen (15) is placed inside the radome and the inductive screen (12,13) is placed outside.