EP1195848B1

EP1195848B1 - Microwave absorber wall

Info

Publication number: EP1195848B1
Application number: EP00870225A
Authority: EP
Inventors: August Timmerman; Gil Cottard
Original assignee: Emerson and Cuming Microwave Products Inc
Current assignee: Laird Technologies Inc
Priority date: 2000-10-05
Filing date: 2000-10-05
Publication date: 2006-10-25
Anticipated expiration: 2020-10-05
Also published as: HK1046990A1; EP1195848A1; ATE343857T1; HK1046990B; DE60031543D1

Abstract

Microwave absorber wall (1) comprising a metal wall (3) onto which a plurality of absorber elements (2, 2') are mounted, each of these elements having a base (6, 6') facing the metal wall (3), wherein the bases (6) of a first number of absorber elements (2) are located in a first plane ( alpha ) and that the bases (6') of a second number of absorber elements (2') are located in a second plane ( beta ) parallel to the first plane ( alpha ), the first plane ( alpha ) being shifted in height direction of the absorber elements (2, 2') with respect to the second plane ( beta ), the second plane ( beta ) being further from the metal wall (3) than the first plane ( alpha ). <IMAGE>

Description

The invention relates to a microwave absorber wall comprising a metal wall onto which a plurality of absorber elements are mounted, each of these elements having a base facing the metal wall.
From Gau J-R J. et al. : « Chebyshev Multilevel Absorber Design Concept », IEEE Transactions on antennas and propagation, US, IEEE Inc. New York, vol. 45, no. 8, 1 August 1997, pages 1286-1293, a microwave absorber wall is known which shows all the features of the preamble of claim 1.
The absorber elements in the microwave absorber wall of the invention can be flat or tapered absorber elements, or any kind of absorber elements known to the person skilled in the art. Tapered absorber elements can have the shape of a pyramid, a wedge, a cone, a tetrahedron or any type of polygon or tapered shape known to the person skilled in the art.
Microwave absorber walls with tapered absorber elements mainly find application in anechoic chambers. An anechoic chamber is a room in which for example the following measurements can be conducted:

antenna pattern measurements, in which the radiation pattern of an antenna can be determined,
radar cross section (RCS) measurements, in which an object is irradiated by a radar or a transmitter/receiver coupled to a transmitting antenna for determining the radar reflection image of the object,
electromagnetic compatibility (EMC) measurements, in which the radiation of a device is measured in order to determine whether the radiation emitted by the apparatus or device is below prescribed limitations,
electromagnetic interference (EMI) measurements, in which a device is irradiated with electromagnetic energy from a radiation source to determine whether the functioning of the device is disturbed by the irradiated electromagnetic energy.

To obtain reliable measurement results in an anechoic chamber, the measurements should be conducted under free space conditions, which means that outside interference and reflections and standing waves at the inside of the anechoic chamber should be avoided as much as possible. To shield the anechoic chamber off from outside interference, the anechoic chamber is surrounded by a metal cage. Because of the presence of this metal cage, unwanted reflections and standing waves could occur between a radiation source and the metal walls of the cage. To prevent the occurrence of reflections and standing waves at the inside of the metal cage, the walls, the ceiling and the floor of the metal cage are provided with microwave absorber walls.
The function of the absorbers in anechoic chambers is to ensure that the incidence of reflected waves in the area where the measurement takes place (the so-called quiet zone) remains below a given value (the performance). The object, apparatus or antenna on which the measurement is conducted is placed in the quiet zone. The performance in the quiet zone is the ratio of the amount of reflected energy entering the quiet zone to the amount of direct energy entering the quiet zone, expressed in dB. It is said that an anechoic chamber has a good performance if the total amount of reflected energy entering the quiet zone is below 0.01% of the total amount of direct energy entering the quiet zone. This corresponds to a performance of -40 dB: $P = 10 \log (E_{ref} / E_{dir}) = 10 \log 10^{- 4} dB = - 40 dB$

with P: performance in the quiet zone,
E_ref: total amount of reflected energy entering the quiet zone,
E_dir: total amount of direct energy entering the quiet zone.

As is disclosed in the article "Microwave Absorbers and Anechoic Chambers" in Mikrowellen Magazin, Vol. 11, No. 2, 1985, an absorber has to provide a good transition between the two media (air in chamber and metal wall), to ensure a good absorption of incident waves and as little reflection of incident waves as possible. For this purpose, the absorber is constructed such that its impedance changes gradually from the intrinsic impedance of the free space (377 Ω) to the input impedance of the metal wall (0 Ω = electric short circuit) onto which the absorber is mounted. In other words, it is advisable that the absorber has a varying dielectric constant ε which gradually increases from the side facing the inside of the chamber to the side facing the metal wall. In this way, an incident wave is not reflected upon entering the absorber, as the impedance of the side facing the inside of the chamber equals the impedance of the air in the chamber, and an incident wave travelling through the absorber can be attenuated to a negligible amplitude before it reaches the metal wall. The energy present in the wave which is absorbed by the absorber, is converted to heat, which heat is dissipated on the external surfaces of the absorber into the air and is transferred from the base of the absorber to the metal wall, which dissipated the heat towards the outside of the chamber.
To obtain a gradually decreasing impedance from the side facing the inside of the chamber to the side facing the metal wall, the absorber elements preferably have a tapered shape, with a base facing the metal wall and a top pointing away from the metal wall. It is known that the larger the wavelength λ (or the lower the frequency f) of the electromagnetic energy to be absorbed, the larger the tapered absorber elements should be. As a result, the tapered absorber elements should have a substantial height, for example 2 m, to obtain an absorber wall with broadband absorption and a good performance in the lower frequency range of 30 to 100 MHz. This is needed as EMC and EMI measurements typically require a good performance in a broadband frequency range, namely 30 to 1000 MHz. As used herein, the term "microwave" is intended to mean an electromagnetic wave with a frequency in the range of 30 to 1000 MHz, even if this term also concerns frequencies between 1000 MHz and 300 GHz.
Since the tapered absorber elements are usually made in expensive material, larger absorber elements are used in the critical areas only, whereas smaller absorber elements are used in the non-critical areas. The critical areas are those where so-called specular reflection occurs. The term "specular reflection" means reflection which is the result of waves which are reflected only once between the source and the target, and which follow the laws of optical reflection on surfaces. The non-critical areas are those where the waves are reflected several times between the source and the target.
It is an aim of the present invention to provide a cheaper anechoic chamber, in particular to reduce the size of the tapered elements of a microwave absorber wall.
This aim is achieved according to the invention in that the bases of a first number of absorber elements are located in a first plane and that the bases of a second number of absorber elements are located in a second plane parallel to the first plane, the first plane being shifted in height direction of the elements with respect to the second plane.
The invention is mainly applied in the critical areas in an anechoic chamber, preferably using tapered absorber elements. However, tapered or other absorber elements may also be mounted in this way in the non-critical areas of an anechoic chamber or in any other absorber walls. By shifting the first number of absorber elements in height direction it can be achieved that, for microwaves with a given wavelength λ for which the distance between the first plane and the second plane equals e.g. λ/4, a first part of the reflected waves will have travelled a distance λ/2 longer in front of the absorber wall than a second part of the reflected waves. As a consequence, the first part is reflected in phase opposition with the second part, giving rise to destructive interference and thus to attenuation and a reduction of the reflected microwave energy at wavelength λ. In an anechoic chamber, this can lead to a reduction of the total amount of reflected microwaves entering the quiet zone.
For a given distance dh between the first plane and the second plane, one would expect destructive interference to occur only for microwaves of which the wavelength λ₀ equals four times said distance dh (λ₀/4 = dh) and of which the angle of incidence θ₀ is approximately perpendicular to the first and second planes (θ₀ = 0°). Surprisingly however, it has now been found that destructive interference also occurs for microwaves of which the wavelength λ varies up to about 40% from λ₀ and of which the angle of incidence θ varies up to about +10° or -10° from θ₀.
In the microwave absorber wall of the invention, the first number of absorber elements and the second number of absorber elements preferably have substantially the same size and shape, and are preferably constructed in the same material. This has the advantage that the same type of absorber elements can be used for all absorber elements of the microwave absorber wall, which can lead to a simpler construction and a reduction in the cost of the absorber wall. However, if the person skilled in the art deems it appropriate, the first number of absorber elements can be constructed in a size and/or shape different from the size and/or shape of the second number of absorber elements. Also, the first and second number of absorber elements can be constructed in a different material. It is however advisable that the person skilled in the art makes sure that, for the incident microwaves of wavelength λ and angle of incidence θ for which destructive interference is desired, the first number of absorber elements reflect the microwaves in phase opposition with the incident microwaves reflected by the second number of absorber elements.
In the microwave absorber wall of the invention, the distance between the first and the second plane is preferably chosen such that destructive interference is obtained for microwaves in the lower part of the microwave frequency range, e.g. for microwaves between 30 and 100 MHz. This has the advantage that the size of the absorber elements can be reduced, as the attenuation of microwaves in the lower part of the microwave frequency range is obtained by the destructive interference and only microwaves at higher frequencies have to be attenuated by the absorber material in itself. So the invention allows reducing the size of the tapered absorber elements and thus the costs of the absorber wall, without adversely affecting the performance of the absorber wall.
In a preferred embodiment of the microwave absorber wall of the invention, a metal plate is preferably attached to the base of each absorber element of the second number of absorber elements. As the bases of the second number of absorber elements are located in the second plane at distance dh from the metal wall, the second number of absorber elements do not have their bases in contact with the metal wall. The metal plate thus serves to provide a metal surface at the base of each of the second number of absorber elements. Providing the metal plate is preferred as this gives the second number of absorber elements substantially the same reflection coefficient as the first number of absorber elements, which means that incident microwaves of wavelength λ are reflected by them in the same manner, only in phase opposition with the incident microwaves of wavelength λ reflected by the first number of absorber elements.
In the microwave absorber wall of the invention, a spacer is preferably provided between the metal wall and the base of each absorber element of the second number of absorber elements. This spacer is firstly provided for construction purposes, namely to provide a mounting base for each of the second number of absorber elements. The spacer is preferably constructed in absorber material, but may also be constructed in any other material known to the person skilled in the art. Providing a spacer in absorber material has the advantage that microwave energy which leaks through the metal plate at the base of each of the second number of absorber elements can be absorbed. In this way, providing the spacer can prevent that the microwave energy which leaks through is reflected on the metal wall and emitted back into the anechoic chamber. Providing spacers in absorber material has the advantage that the overall performance of the absorber wall can be enhanced.
In the absorber wall of the invention, the absorber elements are preferably arranged in first and second groups, each group comprising an equal amount of absorber elements, each first group having bases located in the first plane and each second group having bases located in the second plane, the first groups alternating with the second groups. With this arrangement it can be achieved that microwaves which are reflected via the first groups alternate with the microwaves reflected via the second groups, so that for microwaves with wavelength λ for which destructive interference occurs, the destructive interference can be enhanced and the amount of microwave energy at wavelength λ which is reflected by the absorber wall into the quiet zone can be minimised.
In a preferred embodiment of the microwave absorber wall of the invention, the bases of each first and second group of absorber elements are arranged in squares. In this way, the first and second groups can be substantially uniformly distributed over the absorber wall to achieve a substantially uniform attenuation of the incident electromagnetic wave energy.
The absorber elements of the microwave absorber wall according to the invention are preferably made out of polyurethane, polyurethane ester or ether foam impregnated with an electrically conductive material or any other suitable material known to the person skilled in the art. This electrically conductive material is for example carbon black, graphite or any other electrically conductive material known to the person skilled in the art. To reduce inflammability, the absorber material is preferably impregnated or coated with a fire retardant material, such as phosphor compounds or other.
The metal wall of the microwave absorber wall according to the invention is preferably constructed in an iron, copper or steel alloy, or in any other metal or alloy with the characteristics of good electrical and thermal conductivity known to the person skilled in the art.
The invention will be further elucidated by means of the appended figures and the description given below, in which the same reference numbers always refer to the same parts.

Figure 1 shows a schematic cross-sectional view of the microwave absorber wall according to the invention.
Figure 2 shows a schematic front view of the microwave absorber wall according to the invention.

The microwave absorber wall 1 shown in figure 1 comprises a metal wall 3 onto which a plurality of absorber elements 2, 2' are mounted. Each absorber element 2, 2' comprises a base 6, 6' which faces the metal wall 3. A first number of absorber elements 2 has their bases 6 located in a first plane α and a second number of absorber elements 2' has their bases 6' located in a second plane β. The planes α and β are substantially parallel to each other. The second plane β is located a distance dh further from the metal wall than the first plane α.
The absorber elements 2, 2' of the microwave absorber wall 1 shown in figure 1 have a tapered shape. This tapered shape can be a pyramid, a wedge, a cone, a tetrahedron or any type of polygon or tapered shape known to the person skilled in the art. Apart from tapered absorber elements, also flat or any other absorber elements known to the person skilled in the art can be used.
Each tapered absorber element 2, 2' in figure 1 comprises a tapered member 4, 4' and a base member 5, 5' which are constructed in one piece. The base member 5, 5' of each absorber element 2, 2' has a base 6, 6' which faces the metal wall 3. The tapered member 4, 4' of each absorber element 2, 2' has a top 7, 7' which points away from the metal wall 3. The bases 6 of the tapered absorber elements which are indicated with reference numeral 2 are in contact with the metal wall 3, whereas the bases 6' of the tapered absorber elements which are indicated with reference numeral 2' are located on a distance dh from the metal wall. In other words, the bases 6' of absorber elements 2' are located in the first plane α and the bases 6 of absorber elements 2 are located in the second plane β.
Preferably, a metal plate 11 is attached to the base 6' of each of the second number of absorber elements 2'. As the bases 6' of the second number of absorber elements 2' are located in the second plane β at distance dh from the metal wall, the second number of absorber elements 2' do not have their bases 6' in contact with the metal wall 3. The metal plate 11 thus serves to provide a metal surface at the base 6' of each of the second number of absorber elements 2'. Providing the metal plate 11 is preferred as this gives the second number of absorber elements 2' substantially the same reflection coefficient as the first number of absorber elements 2.
In order to obtain that the absorber elements 2' have bases 6' located on a distance dh further from the metal wall 3 than the bases 6 of absorber elements 2, each base 6' is preferably mounted on a spacer 8. In other words, the spacer 8 bridges the distance dh between the first plane α and the second plane β.
In figure 1, the first and second planes α, β are parallel to the metal wall 3, the second plane β being located on a distance dh from the metal wall 3 and the first β being in contact with the metal wall 3. The planes α and β may however also enclose an angle with the metal wall 3. Furthermore, the first plane α may be located on a second distance from the metal wall 3, for example when an additional layer is provided between the absorber layer formed by the tapered absorber elements 2, 2' and the metal wall 3. Such an additional layer can for example be provided for attenuating microwaves of a frequency between 30 and 100 MHz. This additional layer is then made in an absorber material which attenuates microwaves with a frequency between 30 and 100 MHz, for example ferrite or any other absorber material known to the person skilled in the art. The additional layer is preferably constructed as a plurality of ferrite spacers, which are attached between the bases 6 of tapered absorber elements 2 and the metal wall 3 and between the spacers 8 and the metal wall 3. Providing such an additional absorber layer has the advantage that the absorption of microwaves at lower frequencies, e.g. between 30 and 100 MHz, can be achieved in this additional layer, so that the tapered absorber elements 2, 2' can have a smaller height. This reduction in height can be allowed, since the tapered absorber elements 2, 2' only have to attenuate microwaves with frequencies above the frequencies which are absorbed by the additional layer. As a result, providing the additional absorber layer between the absorber layer formed by the tapered absorber elements 2, 2' and the metal wall 3 can reduce the cost of the absorber wall 1.
The absorber elements 2, 2' and preferably also the spacers 8 are made in an absorber material, for example polyurethane, polyurethane ester or ether foam or any other suitable material known to the person skilled in the art. The absorber material is preferably impregnated with an electrically conductive material, which can for example be carbon black, graphite or any other electrically conductive material known to the person skilled in the art. To reduce inflammability, the absorber material is preferably also impregnated or coated with a fire retardant material, such as a phosphor compound or any other. The spacers 8 can be constructed in the same absorber material as the absorber elements 2, 2', or in a different absorber material.
Constructing the spacers 8 in an absorber material has the advantage that the spacers 8 are able to absorb microwave energy which leaks through the metal plates 11. The spacer 8 may however also be constructed in any material deemed suitable by the person skilled in the art.
The metal wall 3 and the metal plates 11 are preferably constructed in an alloy of iron or copper, or in any other metal or alloy with the characteristics of good electrical and thermal conductivity known to the person skilled in the art.
The absorber elements 2, 2' are preferably arranged in first and second groups 9, 9'. Each group preferably comprises an equal amount of absorber elements, but the first groups 9 may also have an amount of absorber elements 2 different from the amount of absorber elements 2' in the second groups 9'. The absorber elements 2 of the first groups 9 have bases 6 located in the first plane α, whereas the absorber elements 2' of the second groups 9' have bases 6' located in the second plane β. The first groups 9 preferably alternate with the second groups 9' in order to obtain a substantially uniform absorption of the incident microwaves. The bases 6 of the first groups 9 of absorber elements 2 and the bases 6' of the second groups 9' of absorber elements 2' can be arranged in squares, triangles, rectangles, hexagons, circles or any other arrangement known to the person skilled in the art.
Figure 2 shows a preferred embodiment of the microwave absorber wall 1 according to the invention. In this embodiment each absorber element 2, 2' has a pyramidal shape with a square base 6, 6'. The tapered absorber elements 2 are arranged in first groups 9 and the tapered absorber elements 2' are arranged in second groups 9'. Each group 9, 9' comprises an equal amount of absorber elements (nine absorber elements in figure 2) which are arranged in a square. The first groups 9 alternate with the second groups 9'. Each absorber element 2 of the first group 9 has a base 6 which is located in the first plane α in contact with the metal wall 3. Each absorber element 2' of the second group 9' has a base 6' which is located in the second plane β on a distance dh from the metal wall 3. In figure 2, the shaded squares represent tapered absorber elements 2 of first groups 9, which have their bases 6 in contact with the metal wall 3, whereas the white squares represent tapered absorber elements 2' of second groups 9', of which the bases 6' are located on a distance dh from the metal wall.
Figure 1 further shows a device 10 irradiating the microwave absorber wall 1 with electromagnetic microwave energy. The microwave energy has a certain wavelength λ and an angle of incidence θ. The angle of incidence θ is the angle between the propagation direction of the microwaves and an axis perpendicular to the microwave absorber wall. In figure 1, the propagation direction coincides with the axis perpendicular to the microwave absorber wall, so the angle of incidence θ = 0°. The angle of incidence may however vary, which is indicated by the arrow in figure 1.
The tapered absorber elements 2, 2' form a transition between two media: the air through which the microwaves propagate and the metal wall 3 of the microwave absorber wall 1. Because of their tapered shape, the absorber elements 2, 2' form a gradually decreasing impedance from the side of the absorber wall 1 facing the device 10 to the side of the absorber wall 1 facing the metal wall 3. In other words, the tapered absorber elements 2, 2' constitute a microwave absorber with decreasing impedance from the intrinsic impedance of the air (377 Ω) to the input impedance of the metal wall (0 Ω). Because of this gradually decreasing impedance, reflection of the incident microwaves upon entering the absorber 1 can be counteracted, as the impedance of the side of the absorber wall facing the device 10 equals the impedance of the air, and the microwaves are attenuated to a negligible amplitude before reaching the metal wall 3. In this way, the reflection of the microwaves on the metal wall is reduced. The energy which is present in the microwaves upon incidence and which is absorbed by the absorber wall 1 is converted to heat. This heat is dissipated by the external surfaces of the absorber elements 2, 2' into the air and transferred to the metal wall 3, which dissipates the heat towards the outside.
The absorption of microwaves by the microwave absorber wall according to the invention can be explained as follows. At higher frequencies of for example 300 to 1000 MHz the incident wave energy is reflected several times from the side planes of the tapered absorber elements 2, 2' before finally being reflected back in a direction away from the absorber wall 1. At each one of these reflections or bounces on the flat surface of the absorber elements 2, 2', a fraction of the incident wave energy is absorbed. Thus, the microwaves are attenuated to a negligible amplitude before they are reflected on the metal wall 3. At lower frequencies of for example 30 to 300 MHz however, the wavelength λ becomes much longer than the spacing between adjacent pyramidal absorbers 2, 2'. The depth of penetration of the electromagnetic waves (the so-called "skin depth") into the absorber material becomes likewise long compared to the size of the pyramids. So at these lower frequencies, the reflection cannot be modelled in terms of successive reflections from individual surfaces of the tapered elements 2, 2'. The absorption of microwaves at lower frequencies is rather a case of absorption in the absorber material itself as the wave travels through this material. As a result, the tapered absorber elements 2, 2' need to have a substantial height in order to be able to absorb enough of the electromagnetic wave energy before it is reflected back on the metal wall 3.
With the microwave absorber wall 1 of the invention, it is however possible to limit the height of the absorber elements 2, 2', without losing absorption performance. This is because, in the microwave absorber wall 1 of the invention, use is made of destructive interference to attenuate the microwaves at lower frequencies of for example 30 to 100 MHz. This destructive interference is obtained by shifting the second number of absorber elements 2' over a distance dh in height direction of the absorber elements 2, 2' with respect to the metal wall 3, so that the bases 6' of the absorber elements 2' are located in the first plane α. By shifting the absorber elements 2' over a distance dh in height direction, destructive interference is obtained for microwaves with a wavelength λ = 4 dh and an angle of incidence θ = 0°. This destructive interference occurs because these microwaves travel a distance 2 dh longer in front of the absorber wall 1, before being emitted by the microwave absorber wall back into the air. As a result, a first microwave with wavelength λ = 4 dh and angle of incidence θ = 0° which travels through the absorber wall 1 via an absorber element 2' is emitted in phase opposition with a second microwave with equal wavelength and angle of incidence which travels through the absorber wall via an absorber element 2. Due to this phase opposition, the first microwave and the second microwave interfere destructively. As a result, both the first and the second microwave are substantially completely attenuated. The electromagnetic energy which was contained in both microwaves is converted to heat, which heat is dissipated on the external surfaces of the absorber elements 2, 2' and is transferred by the microwave absorber wall 1 to the metal wall 3 and dissipated by the metal wall 3 towards the outside.
The relation between the frequency f₀ at which destructive interference is desired and the distance dh can be expressed as follows: $f_{0} = \frac{c}{λ} = \frac{c}{4 d h}$
with:

f₀ =: frequency for which destructive interference is desired,
c =: the propagation speed of the microwave, i.e. the speed of light (c = 299.722 km/s),
λ₀ =: wavelength corresponding to the frequency f₀,
dh =: distance between the first plane and the second plane.

₀

dh.

₀

dh

₀

dh

₀

dh

a microwave with a frequency f₀ = 50 MHz has a wavelength λ₀ = c / f₀, so: $λ_{0} = \frac{c}{f} = \frac{2.99 \times 10^{8} m / s}{50 \times 10^{6} s^{- 1}} = 6 m$
this leads to a distance dh = λ₀ / 4 = 1.5 m.

For the distance dh between the first plane α and the second plane β, one would expect destructive interference to occur only for microwaves of which the wavelength λ equals λ₀ and of which the angle of incidence θ approximates the axis perpendicular to the first and second planes α, β (θ = θ₀ = 0°). Surprisingly however, it has now been found that destructive interference also occurs for microwaves of which the wavelength λ varies up to about 40% from λ₀ and of which the angle of incidence θ varies up to between about +10° and -10° off θ₀. In the example given above, this means that destructive interference occurs for microwaves with a wavelength λ between 30 and 70 MHz and with an angle of incidence θ between +10° and -10° (with respect to the axis perpendicular to the first and second planes α, β). As a result, the height of the tapered absorber elements 2, 2' can be reduced, because less absorption of microwaves at lower frequencies is needed. So the absorber wall 1 of the invention allows reducing the size of the absorber elements 2, 2' without adversely affecting the performance of the absorber wall 1. In the example given above, the height of the tapered absorber elements can be reduced from 2 m (without height shift dh) to 1 m (with height shift dh), without adversely affecting the absorption performance of the microwave absorber wall 1.
Due to the reduction in height of the tapered absorber elements 2, 2', less absorber material is needed for constructing the microwave absorber wall 1. As a result, the cost of the microwave absorber wall 1 can be reduced.
The microwave absorber wall 1 of the invention can be applied in an anechoic chamber. More particularly, the microwave absorber wall 1 of the invention can be advantageous in the critical areas of an anechoic chamber, i.e. the areas where specular reflection occurs. Specular reflection means that the microwaves are reflected only once between the source and the target. In state of the art anechoic chambers the tapered absorber elements in the critical areas have a substantial height, as approximately all the incident microwave energy has to be absorbed in one turn. Using the microwave absorber wall of the invention in the critical areas of an anechoic chamber makes it possible to reduce the height of the tapered absorber elements in these critical areas, which can lead to a reduction of the cost of the anechoic chamber. The microwave absorber wall of the invention can however also be applied to the non-critical areas in an anechoic chamber, i.e. the areas where the incident microwaves are reflected more than once between the source and the target, or to any other microwave absorber walls.

Claims

Microwave absorber wall (1) comprising a metal wall (3) onto which a plurality of absorber elements (2, 2') are mounted, each of these elements having a base (6, 6') facing the metal wall (3), the bases (6) of a first number of absorber elements (2) being located in a first plane (α) and the bases (6') of a second number of absorber elements (2') being located in a second plane (β) parallel to the first plane (α), the first plane (α) being shifted in height direction of the absorber elements (2, 2') with respect to the second plane (β), the second plane (β) being further from the metal wall (3) than the first plane (α), characterised in that a metal plate (11) is provided on the base (6') of each of the second number of absorber elements (2').
Microwave absorber wall according to claim 1, characterised in that the distance (dh) between the first plane (α) and the second plane (β) equals % of a wavelength (λ) of incident microwaves on the absorber wall (1).
Microwave absorber wall according to claim 1 or 2, characterised in that the first and second number of absorber elements (2, 2') have substantially the same shape and size, and that they are constructed in the same material.
Microwave absorber wall according to any one of claims 1-3, characterised in that the absorber elements (2, 2') have a tapered shape with a top (7, 7') pointing away from the metal wall (3).
Microwave absorber wall according to any one of claims 1-4, characterised in that the distance (dh) between the first plane (α) and the second plane (β) is between 0.75 and 2.5 m.
Microwave absorber wall according to any one of claims 1-5, characterised in that a spacer (8) is located between the metal wall (3) and the metal plate (11) on the base (6') of each of the second number of absorber elements (2').
Microwave absorber wall according to any one of claims 1-6, characterised in that the absorber elements (2, 2') are arranged in first and second groups (9, 9'), each group comprising an equal amount of absorber elements, each first group (9) having bases (6) located in the first plane (α) and each second group (9') having bases (6') located in the second plane (β), the first groups (9) alternating with the second groups (9').
Microwave absorber wall according to claim 7, characterised in that the bases (6, 6') of each first and second group (9, 9') of absorber elements (2, 2') are arranged in a square.
Microwave absorber wall according to any one of claims 1-8, characterised in that an additional layer is provided between the absorber layer formed by the absorber elements (2, 2') and the metal wall (3), the additional layer being of an absorber material which attenuates microwaves with a frequency between 30 and 100 MHz.
Anechoic chamber comprising at least one microwave absorber wall as claimed in any one of claims 1-9.