Device for reducing the radome effect with a surface-radiating wideband antenna and reducing the radar cross section of the assembly

A layer (30) absorbing transmitted radiation, placed between an antenna (10) and a radome (20) and extending parallel to the surface of the antenna at a close distance thereto, the absorption coefficient of said absorbing layer varying between a minimum value in the center of the radiating surface and a maximum value at the periphery of said radiating surface. The absorbing layer may in particular be formed by a central area (31) with a zero or virtually zero absorption coefficient surrounded by a peripheral area (32) with a constant absorption coefficient. It may also be formed by a succession of concentric areas exhibiting respective absorption coefficients increasing from the center to the periphery. In addition to the reduction of the radome effect, said structure reduces significantly the radar cross section of the assembly when the latter is the target of a radar.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for reducing the disturbing 
effect produced by the reflections of waves on a radome protecting a 
surface-radiating wideband antenna, and for reducing the radar cross 
section of the assembly. 
2. Description of the Prior Art 
As a matter of fact, with these antennas (that are commonly used for 
countermeasures), it is well known that the radome disturbs the radiation 
pattern of the antenna it protects. 
This disturbing phenomenon is the more significant as the wall of the 
radome is thick and the operating frequencies are high. 
Now, it must be possible, and it is often necessary, to have relatively 
thick radomes capable of withstanding rain erosion, hydrostatic pressure, 
etc., as the case may be, without too much impairing the performance of 
the antenna, in particular at high frequencies. 
FIG. 1 illustrates this disturbing phenomenon due to the wave reflections 
on the radome. 
The reference numeral 10 denotes the antenna, which is of the type with 
essentially surface radiation and which is on this drawing a flat antenna, 
although other shapes could be envisaged as well (cylindrical antenna, 
spherical antenna, etc.). 
In addition, at the frequencies of interest, it will be assumed that the 
propagation of the waves is a propagation of the optical or quasi-optical 
type. 
An incident ray OR1 coming from a point O of the antenna will go through 
the radome 20 at the point 21, but a portion of its energy will be 
reflected in the air/radome interface at this point. The reflected ray, 
reflected in the direction of the antenna, will again be reflected at the 
point 11 on the surface of the antenna 10, thus giving rise to a new 
incident ray R2 that will go through the radome but with a partial 
reflection in the radome/air interface at the point 22, that will be 
followed, as previously, by a further reflection on the antenna at the 
point 12, and so on. 
Due to this process, in addition to the transmission loss inherent to the 
presence of the radome (a loss that depends on the material and the 
thickness of the radome), there will be an additional disturbance due to 
interferences between the main ray R1 and the parasitic ray R2 and the 
other rays produced by the successive multiple reflections, all 
phase-shifted relative to R1; these interferences will result in 
significant irregularities of the radiation pattern of the antenna, these 
irregularities being the more significant as the wall of the radome is 
thick and the working frequencies are high. 
When the bandwidths are very wide (f.sub.max /f.sub.min ratios&gt;10) and the 
working frequencies are high (f.sub.max of about 20 GHz), if the minimum 
thickness of the radome is higher than 1 mm, it is virtually impossible to 
achieve a reflection coefficient lower than 0.2 for the radome even if an 
optimized structure (multilayer radome, sandwich radome) is used for the 
latter to reduce the reflection coefficient without reducing too much the 
transmission coefficient. 
Thus if it is desired to radiate, for example, in the 2-20-GHz = band, the 
apparent area of the antenna in the case of a spiral antenna, for example, 
is given by the Formula S.sub.a =(.lambda..sub.max /.pi.).sup.2 .pi., 
where .lambda..sub.max /.pi. is the diameter of the radiating area at the 
lowest frequency; for f.sub.min =2 GHz, we thus have S.sub.a =71 cm.sup.2. 
This figure will be compared with the equivalent area at the highest 
frequency given by S.sub.e =.lambda..sub.min.sup.2 (G/4.pi.), that is 
S.sub.e =0.36 cm.sup.2 for an antenna gain G of 3 dB at 20 GHz. 
We thus have S.sub.a /S.sub.e =200, so that with the prior art 
configuration shown in FIG. 1, the energy reflected by the radome will be 
almost entirely reflected again by the antenna since the peripheral 
surface, not active at the highest frequencies, will be seen as a 
reflecting plane extending to infinity by the active central portion. 
SUMMARY OF THE INVENTION 
A purpose of the present invention is accordingly to minimize this 
phenomenon and consequently to allow the use of relatively thick radomes 
without impairing the performance at the highest frequencies and without 
having recourse to complex structures for the radome. 
To solve this problem, the present invention is based on the observed fact 
that in the surface-radiating wideband antennas generally used (spiral 
antennas, log-periodic antennas and similar antennas), the radiating areas 
are essentially located toward the center for the highest frequencies and 
essentially toward the periphery for the lowest frequencies. 
Taking into account this property, the present invention proposes to place 
between the radome and the antenna a lossy dielectric that acts in a 
selective manner between the center and the periphery. 
More precisely, there is provided a layer for absorbing the transmitted 
radiation, placed between the antenna and the radome and extending 
parallel to the surface of the antenna at a close distance thereto, the 
absorption coefficient of this absorbing layer varying between a minimum 
value at the center of the radiating surface and a maximum value at the 
periphery of this radiating surface. 
According to a number of advantageous embodiments, said minimum value may 
be a zero or virtually zero value; the absorbing layer may in particular 
be formed by a central area with a zero or virtually zero absorption 
coefficient surrounded by a peripheral area with a constant absorption 
coefficient, and the central area may in particular be formed by a hollow 
in the absorbing layer. 
The absorbing layer may also be formed by a succession of concentric areas 
exhibiting respective absorption coefficients increasing from the center 
to the periphery. 
As to the materials to be used, 
the absorbing layer may be a dielectric layer including carbon particles 
dispersed in a cellular material; or 
it may also be a layer including a ferromagnetic material; in this case, 
the thickness of the layer including the ferromagnetic material being 
chosen so as to correspond, taking into account the permittivity and the 
magnetic permeability of the ferromagnetic material, to the resonance or 
to the vicinity of the resonance at the frequency at which the maximum 
absorption effect is desired.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIGS. 2 to 4, the reference numerals 10 and 20 denote respectively, as 
in FIG. 1, the antenna and the radome that protects it. 
In a manner characteristic of the present invention, there has been placed, 
parallel to the plane of the antenna and at a short distance therefrom, an 
absorbing layer 30 whose attenuation is not the same in the center and at 
the periphery. 
By "short distance" it will be understood a distance between the antenna 
and the absorbing layer as short as possible but nevertheless sufficient 
not to substantially affect the propagation of the currents in the 
antenna. 
In the case of FIG. 2, which is the simplest case, the absorbing layer 30 
is a uniform layer exhibiting a central hole 31 so as to leave only a ring 
32 covering the peripheral portion of the antenna 10. 
In this way, as it is known that absorbing materials, in a general manner, 
exhibit a transmission loss proportional to frequency, an absorbing 
material located over the peripheral portion of the antenna will have 
little effect on the energy radiated by the latter in this area 
corresponding to the lowest frequencies, this radiation being illustrated 
in FIG. 2, for example, by the ray R2 coming from the point O' located in 
the peripheral radiating area. 
On the other hand, this material will exhibit a high attenuation for the 
highest frequencies. As these high frequencies radiate from the central 
area of the antenna (point O and radius R1 in FIG. 2), the waves radiated 
directly from this area are not attenuated due to the fact that the 
central area is facing the hole 31, but the reflected rays (for example 
the ray reflected at point 21) encounter the absorbing peripheral ring 32 
and ace thus almost fully eliminated. 
As a variant, there can be provided, as shown in FIGS. 3 and 4, a 
progressive increase of the absorption coefficient from the center to the 
periphery. 
This variation is obtained, for example, by providing a plurality of 
concentric areas 31 to 36 with increasing diameters D1 to D5 and 
exhibiting an increasing absorption from the center to the periphery. 
In FIG. 3, the absorption coefficient is varied by increasing the thickness 
h of the absorbing layer from the center to the periphery; conversely 
(FIG. 4) and with the same result, it is possible to use an absorbing 
material with an absorption coefficient varying with the diameter while 
the thickness h remains constant. 
In any case, it is desirable to choose for the central area 31 a material 
or a thickness allowing the lowest possible absorption so as not to act on 
the high frequencies. 
As to the material of the absorbing layer 30, it is possible to use, for 
example, a dielectric absorbent based on carbon dispersed in a cellular 
material. 
The reflection coefficient of such a material is low, which permits 
attenuation without reflecting (as a matter of fact, if the material was 
reflecting, the phenomenon illustrated in FIG. 1 occurring between the 
antenna 10 and the radome 20 would occur again between the antenna 10 and 
the absorbing layer 30). However, such a type of absorbent requires 
relatively significant thicknesses up to 5 to 10 mm depending on the 
frequency band. 
It is also possible to use an absorbent based on resin and powdered iron. 
However, with this type of material, the absorbent/air interface exhibits a 
non-negligible reflection. To remedy this disadvantage, the 
characteristics of the material and the thickness of the absorbing layer 
are chosen so as to correspond to the resonance or to the vicinity of the 
resonance i.e., with an attenuation such that the energy reflected after a 
round trip in the absorbing layer be substantially equal to the energy 
reflected in the air/absorbent interface: the two waves being in phase 
opposition, the effect of the reflection is virtually cancelled. 
This resonance condition is satisfied when the thickness e is chosen such 
that e(.mu..epsilon.).sup.1/2 =.lambda./4, where .mu. is the magnetic 
permeability and .epsilon. is the permittivity of the absorbent. 
The advantage of this last type of absorbent is that it permits use of a 
small thickness, about 1 to 2 mm. Additionally, its absorption coefficient 
is of about 3 dB/mm at 10 GHz, and the attenuation of the energy reflected 
by the radome at high frequencies will be very significant. 
Of course, although the present invention has been described for a flat 
antenna protected by a flat radome, this configuration is not limiting and 
the invention can as well be applied to a flat antenna protected by a 
cylindrical radome, a conical radome, an hemispheric radome or other 
radome, to a cylindrical antenna protected by a cylindrical radome, to a 
spherical antenna protected by a spherical radome, etc. 
Furthermore, in addition to the fact that it permits considerable reduction 
in the radome effect, the structure according to the present invention has 
also the advantage of reducing in a significant manner the radar cross 
section of the antenna-radome assembly, thanks to which when such a 
structure is the target of a radar, its radar cross section as seen by the 
radar will be considerably reduced due to the absorbing layer that has 
been added.