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Patent Searching and Data


Title:
A RADAR ABSORBER
Document Type and Number:
WIPO Patent Application WO/2016/209181
Kind Code:
A1
Abstract:
With the present invention, a broadband and low-weight radar absorber (R) is provided which absorbs radar signals. Said radar absorber (R) comprises at least one base layer (1); at least two spacing layers (2) located on the said base layer (1), in the form of a glass fabric, each having thereon at least one frequency selective surface (3); and at least one filler layer (4) interposed between said spacing layers.

Inventors:
YILDIRIM EGEMEN (TR)
INAL MEHMET ERIM (TR)
DOGAN OGUZ (TR)
Application Number:
PCT/TR2016/000079
Publication Date:
December 29, 2016
Filing Date:
June 03, 2016
Export Citation:
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Assignee:
ASELSAN ELEKTRONIK SANAYI VE TICARET ANONIM SIRKETI (TR)
International Classes:
H01Q17/00; H01Q15/00
Domestic Patent References:
WO1993023893A11993-11-25
Foreign References:
EP2096711A12009-09-02
Other References:
None
Attorney, Agent or Firm:
CAYLI, Hulya (Koza Sokak 63/2 G.O.P, Ankara, TR)
Download PDF:
Claims:
CLAIMS

1. A radar absorber (R), characterized by comprising:

at least one base layer (1);

- at least two spacing layers (2) located on the said base layer (1 ), in the form of a glass fabric, each having thereon at least one lossy frequency selective surface (3); and

- at least one filler layer (4) interposed between said spacing layers. 2. A radar absorber (R) according to claim 1 , characterized in that the pattern of the lossy frequency selective surface (3) on each spacing layer (2) is different.

3. A radar absorber (R) according to claim 1 , characterized in that the pattern of the lossy frequency selective surfaces (3) on at least two spacing layers (2) is the same.

4. A radar absorber (R) according to claim 1 , characterized in that said base layer (1 ) is in the form of a carbon fabric. 5. A radar absorber (R) according to claim 1 , characterized in that said filler layer (4) contains glass fiber woven thermoset resin fabric.

6. A radar absorber (R) according to claim 1 , characterized in that said filler layer (4) contains a closed cell structured foam.

7. A radar absorber (R) according to claim 1 , characterized in that said filler layer (4) contains glass fiber woven thermoset resin fabric containing at least one magnetic/dielectric loss filler. 8. A radar absorber (R) according to claim 1 , characterized by comprising at least one outer layer (5).

9. A radar absorber (R) according to claim 8, characterized in that said outer layer (5) contains glass fiber woven thermoset resin fabric.

Description:
A RADAR ABSORBER

Technical Field

The present invention relates to a multi-layered radar absorbing structure for absorbing radar signals. Background Art

A radar absorbing structure/material is a structure that reflects incident electromagnetic wave, as low as possible, over operating frequencies and converts a large part of the electromagnetic energy into heat so as to absorb it. Radar absorbers may be used in air, land and sea platforms wherein a small radar cross-section is of critical importance. By using radar absorbers on an outer surface of said platforms, radar signals reaching the surface of the platform are highly absorbed and the target detection range of the radar systems may be considerably reduced. In order to reduce the radar trace of the platforms where they are used with respect to various radar systems, radar absorbers must operate over a broad bandwidth (e.g. absorb signals over a broad range of frequencies). In addition, radar absorbers should be very light in order not to apply additional weight to the platform where they are used, which is of great importance especially for air platforms.

Radar absorbing structures/materials (RAS/RAM) are such structures that absorb incident electromagnetic energy and reflect a very small part thereof. The main principle is to keep the input impedance of the structure as close as possible to the intrinsic impedance value of the electromagnetic wave in air by combining materials and/or lossy surfaces having different magnetic and electrical features, so as to absorb incident wave without a substantial reflection. In prior art, radar absorbers are mainly divided into four groups based on their structures. Said groups are classified as gradual- transition absorbers, resonant absorbers, magnetic absorbers, and circuit analog absorbers. In the gradual- transition absorbers, the impedance transition is achieved using geometrical shaping or gradual transition in electrical features of the material. In absorbers with geometrical shaping, there exists no change in the electrical feature of the material through the structure, and homogenous materials are shaped geometrically in order to prevent reflection. In order to achieve acceptable reflection values, the length of such absorbers is kept long up to several wavelengths at the lower limit of the operating frequency. This results in a heavy, large and fragile structure of such absorbers. While exhibiting a superior performance in terms of absorbance characteristics and bandwidth as compared to other types of absorbers, said gradual- transition absorbers with geometrical shaping are not suitable for outdoor applications. They are especially preferred in EMI/EMC test fields and antenna measurement chambers. An impedance transition between the impedance value of an electromagnetic wave in air and the short- circuit value of 0 ohm may also be accomplished by imposing a gradual transition in the electrical features of the material used. Absorbers that are obtained by gradual feeding of a lossy filler into a lossless or low loss base material (with a concentration increasing from the surface to the base) constitute another field of application of such gradual- transition absorbers . With a gradual increase in the concentration of the filler, a gradually-changing impedance value is obtained so that broadband impedance conversion is achieved. It is not easy to manufacture such absorbers, and also high thickness values are required for broadband characteristics.

Unlike the gradual - transition absorbers, absorbance principle of resonant absorbers relies on accumulation of multiple reflections created in the structure such that they dampen each other. As it is the case for other types of absorbers, these structures should also include a highly conductive layer at the lowermost layer. However, in resonant absorbers, this conductive layer has a high effect on the performance of the structure. Since the resonant absorbers are thinner than the gradual- transition absorbers, the amplitude of the sign reflected from the conductive layer is relatively large and its role in the total reflection coefficient is high. The electromagnetic wave incident on the resonant structures experiences a first reflection on the surface of the structure and penetrates thereto. The wave passing through the structure is reflected from the lowermost conductive plate in single-layer structures whereas in multi-layer structures, it is reflected from each surface where electrical discontinuity is available. By adjusting the phase and amplitude values of these multiple reflections, electrical features of the layers used in the structure and cross-layer distance, it is aimed at having an equivalent reflection on the surface of the structure as small as possible. The Salisbury screens, Dallenbach layers and Jaumann type absorbers are typical resonant absorbers. In resonant absorbers, broadband characteristics and a high damping rate may be obtained only by using a plurality of layers. In this case, since the distance between the successive layers is one quarter-wavelength, the total thickness of the structure is greatly increased.

Magnetic absorbers are formed by combining a layer or layers having a magnetic constant different from 1. Along with the dielectric constant, the fact that magnetic constant is controllable provides for design flexibility, and structures with no need for a high thickness may be provided with acceptable absorbance characteristics. Artificial materials with magnetic features are obtained by scattering particles of iron and ferrite into dielectric materials. Therefore, such absorbers may be thin, but they are also very heavy. Furthermore, such absorbers are usually preferred in low frequency applications since the magnetic features of the materials used are effective at low frequencies. Magnetic constants of ferrites are rapidly reduced as the frequency increases, but electrical constants remain unchanged to a considerable extent. Thus, electrical losses take place at high frequency regions.

The last class of radar absorbing structures/materials is Circuit -Analog Radar Absorbing Structures (CA RAS). Similar to resonant absorbers, the equivalent reflection is minimized using multiple reflections. In resonant absorbers, the mechanism of loss is provided by lossy surfaces present in the structure. These lossy surfaces are patternless, which constitute layers that may be modeled by an equivalent resistance. The lossy layers used in CA RAS's are lossy Frequency Selective Surface (FSS) structures having periodic conductive patterns thereon. Said patterns are consisted of lossy surfaces and the values of the surface resistance may be varied from layer-to-layer. The fact that the layers of the structure have reactive features arising from periodic patterns confers a substantial flexibility to the design. The lossy surfaces may be modeled using an equivalent capacitor, coil and resistance so that equivalent circuit techniques may be used in the design of the absorbing structure. Therefore, such absorbers are called as circuit analog radar absorbing structures. In conventional circuit analog radar absorbers, the distance between the successive lossy FSS layers is equal to a quarter-wavelength. Broadband characteristic structures may be obtained only by increasing the number of the layers, which results in very thick structures as it is in the resonant absorbers.

In the prior art radar absorbers, the number of the layers in the radar absorber should be very high in order to achieve the desired bandwidth. Thus, in addition to their thickness, the prior art radar absorbers are quite heavy, and accordingly, they are not preferred in air and sea platforms, in particular, where weight is of critical importance.

Brief Description of the Invention

With the present invention, a radar absorber is provided which absorbs radar signals. Said radar absorber comprises at least one base layer with a high conductivity; at least two spacing layers located on the said base layer, in the form of a glass fabric, each having thereon at least one frequency selective surface; and at least one filler layer interposed between said spacing layers. In the radar absorber according to the present invention, after the frequency selective surfaces are designed based on the desired bandwidth, they are applied on a spacing layer made of glass fabric by means of a printing method. A filler layer made of different materials is disposed between the spacing layers. In this way, the radar absorber operates over the desired frequency range, and thanks to the low density fillers used, its weight is kept very low. Furthermore, since the frequency selective surface is coated on the spacing layer by a printing method, the radar absorber is produced in an easy manner.

Object of the Invention An object of the present invention is to provide a radar absorber having a high bandwidth (3,4 octaves; 1 decade).

Another object of the present invention is to provide a radar absorber having a low weight (0.9 g/cm 3 ). Another object of the present invention is to provide a thin (~λ/20) radar absorber.

Another object of the present invention is to provide a radar absorber suitable for use in different platforms.

Another object of the present invention is to provide a radar absorber which is easy-to- manufacture, durable and reliable. Still another object of the present invention is to provide a radar absorber having superior absorbance features (>15 db across the band, >20 dB on particular frequency points).

Description of the Drawings The illustrative embodiments of the radar absorber according to the present invention are illustrated in the enclosed drawings, in which:

Figure 1 is a side sectional view of the radar absorber.

Figures 2a-2e are exemplary views of the different patterns of frequency selective surfaces provided in the radar absorber.

All the parts illustrated in the drawings are individually assigned a reference numeral and the corresponding terms of these numbers are listed as follows.

Radar absorber (R)

Base layer (1 )

Spacing layer (2)

Frequency selective surface (3)

Filler layer (4)

Outer layer (5)

Description of the Invention Radar absorbers constitute one of the primary techniques used to reduce the radar traces of air, land and sea platforms. The radar absorbing structures/materials used for such purposes are applied on outer surfaces of the platforms, and they absorb incident electromagnetic wave at a high level, and reflect part of it that has a very low energy back to the energy source. In order for the radar absorbers to operate over a broad bandwidth, the number of the layers and thus the total thickness thereof should be high. Therefore, in case high density materials are used, radar absorbers having a high bandwidth become quite heavy. The radar absorbers having a high weight cannot be utilized in those platforms where weight has a critical importance. Therefore, with the present invention, there is provided a radar absorber which is quite light though its bandwidth is high.

The radar absorber (R) according to the present invention, as illustrated in figures 1-2, comprises at least one base layer (1 ) which is preferably fixed to a surface (i.e. an air platform); at least two spacing layers (2) located on the said base layer (1 ), in the form of a glass fabric, each having thereon at least one lossy frequency selective surface (3); and at least one filler layer (4) interposed between said spacing layers. In the radar absorber (R) according to the present invention, the lossy frequency selective surfaces (3) as illustrated in figure 2 are designed based on the desired bandwidth, and they are applied on the spacing layer (2) made of glass fabric preferably by means of a printing method. The pattern of the frequency selective surface (3) on each spacing layer (2) may be different, or the pattern of the frequency selective surface (3) on at least two spacing layers (2) may be the same. By designing the said frequency selective surfaces (3) with the desired features and printing same on the spacing layer (3), such layers with reactive impedance characteristics are obtained and the radar absorber (R) is made thin and lightweight even in the targeted bandwidth.

In a preferred embodiment of the invention, said base layer (1 ) is in the form of a carbon fabric (carbon fiber woven fabric). With the high conductivity value of the base layer (1), the radar absorber (R) is prevented from being affected by electrical features of the surface of the platform where it is used.

The layers of the lossy frequency selective surface (3) are obtained by applying a radar- absorbing paint on a fiber-reinforced plastic or thin base material in accordance with periodic patterns. Between the successive layers of lossy frequency selective surface (3), different fillers (4) may be used. In structures with a plurality of filler layers (4), the structure of the filler layers (4) may be the same, or different structures may be employed for different filler layers (4). These materials are:

• glass fiber woven thermoset resin fabric,

• closed- cell foam structures,

• glass fiber woven thermoset resin fabric containing at least one magnetic/dielectric loss filler.

In another preferred embodiment of the invention, said radar absorber (R) comprises at least one outer layer (5) that is located at the top surface thereof and protects the underlying layers from external effects. Said outer layer (5) preferably contains glass fiber woven thermoset resin fabric. When the radar absorber (R) is used on a platform, said outer layer (5) is the outermost surface surrounding the platform. Since the outer layer (5) is made from a robust material such as glass fiber woven thermoset resin fabric, the radar absorber (R) may be used for a long period of time with low maintenance cost and without experiencing any performance loss under harsh environmental conditions. In an illustrative embodiment of the invention, the radar absorber (R) comprises a base layer (1 ) located at the undermost side and made of carbon fabric; five spacing layers (2) made of a glass fabric, each having thereon one frequency selective surface (3); a filler layer (4) interposed between the spacing layers (2) and made of a closed cell structured foam; and an outer layer (5) located at the uppermost side and made of glass fiber woven thermoset resin fabric. After the said layers are assembled, they are cured at different temperature values and under pressure and/or vacuum. In this way, the radar absorber (R) is formed as a composite structure. In this embodiment, the absorption rate of the radar absorber (R) over a broadband is higher than 15 dB, and at spot frequencies it is higher than 20 dB. The weight of the radar absorber (R) is 0.9 g per cubic centimeter. With this production method, the invention may be used as a structural material for different platforms (land, air, sea, submarine, etc.) and with different geometric shapes.

The electromagnetic wave incident on the structure experiences a first reflection on the outer surface of the structure. Said electromagnetic wave that has lost a small part of its energy due to said reflection penetrates into the structure. A new reflecting component is generated within the structure at each point having discontinuity. Said reflection points are those surfaces where lossy frequency surface (3) layers are located. Reflective characteristics of these layers are closely related with the geometric patterns of the surfaces and the surface resistance values. Therefore, the layers with different reflective and conductive characteristics may be obtained by adjusting the geometric patterns and surface resistance values. Multiple reflections generated inside the structure, a reflection arising from the undermost conductive layer as well as a reflection arising from the outer layer accumulate on the structure and constitute equivalent reflection characteristics thereof. Therefore, the absorbance performance of the structure is defined by the structural properties of the lossy frequency selective surface (3) layers and the distance between said layers. With these parameters, high broadband absorbance performances may be obtained with low reflection values. Moreover, with the invention, an absorbance performance having very low reflection values (<-25 dB) may be obtained at specific frequency points, along with the reactive impedance induced flexibility of the patterned FSS layers. Another significant aspect of the invention is that it is not necessary to use resonating surfaces in selecting lossy FSS as it is the case in conventional circuit analog radar absorbing structures. The distance between the layers may be greatly reduced by using capacitive or inductive characteristic surfaces, whereby very thin absorbing structures may be designed.

In the radar absorber (R) according to the present invention, after the lossy frequency selective surfaces (3) are designed based on the desired bandwidth, it is obtained by applying a lossy material on the spacing layer (2) made of glass fabric by means of a printing method, in accordance with the desired geometrical form. The filler layer (4) made of different materials is disposed between the spacing layers (2). In this manner, the radar absorber (R) operates at the desired frequency range and it is made light-weight. Furthermore, since the frequency selective surface (3) is coated on the spacing layer (2) by a printing method, the radar absorber is produced in an easy and repeatable manner.