WO2007080368A1

WO2007080368A1 - Absorber

Info

Publication number: WO2007080368A1
Application number: PCT/GB2006/004896
Authority: WO
Inventors: Barry Chambers; Lee Ford
Original assignee: University of Sheffield
Current assignee: University of Sheffield
Priority date: 2006-01-13
Filing date: 2006-12-21
Publication date: 2007-07-19
Anticipated expiration: 2008-07-13

Abstract

Embodiments of the present invention relate to an absorber comprising at least one or a first plurality of geometric transition absorbers (202 - 210) arranged to influence an incident electromagnetic wave and at least one a frequency selective filter (212) realised using at least one or a plurality of electrically conductive or resistive elements (216) .

Description

ABSORBER

Field of the Invention

The present invention relates to an absorber and, more particularly, to an electromagnetic wave absorber.

Background to the invention

There is a legal requirement for all electrical equipment in the EU and elsewhere to be compliant with regard to the emission of or susceptibility to electrical interference. Compliance is tested in an anechoic facility that comprises a room having walls coated with radio wave absorbent material so as to simulate a non reflecting (free-field) environment. The required electrical thickness of the absorbent material varies with the frequencies of interest. Hence, the physical thickness of many absorbent coatings is very large at the lowest testing frequencies. For example, a typical thickness of several metres might be required for investigations relating to frequencies of the order of 30 MHz. It will be appreciated that this reduces the space available in the measurement facility for the actual test equipment and the equipment under test.

Geometrically profiled absorbers are widely used to reduce the reflection and scattering from surfaces in an anechoic test chamber. They are often made from a polymer foam material that is loaded with an electrically conducting ink or powder such as carbon. Loaded foam slabs are cut with a wire saw to form pyramids and other conical profiles, wedges, flat sheets or slabs and other geometrical shapes, known as geometric transition absorbers or conical structures. Geometric transition absorbers or conical structures form a gradual transition from free-space to a continuous loaded slab of material. Most of an incident radiowave signal is absorbed rather than being back- scattered if this geometrical transition takes place over a distance corresponding to several wavelengths of the incident radiowave signal. Furthermore, the absorber is effective over a wide range of wavelengths (or frequencies). Critically, it is the thickness of the absorber which determines the performance at the lowest operating frequency (longest wavelength). To be effective at a frequency of 30 MHz, which corresponds to a wavelength of 10 metres, absorber cones might need to be 2 to 4 metres in length. This requires a large amount of material for fabrication, which, in turn, is costly as well as being difficult to handle and install. Furthermore, for a given size of anechoic chamber, much of the interior volume is taken up by the absorber, which greatly restricts the space available for equipment testing purposes. The prior art has concentrated on varying the exact shape and form of the "absorbers" and on techniques for controlling the variation of loading conductivity, e.g. through the cone transition region. Often this has been done to improve the performance of the absorber at the high frequency end of its operating range.

In an attempt to improve the performance of the absorber at lower frequencies such as, for example, 26 MHz, hybrid designs are available commercially from, for example, Emerson & Cumming and Rantec that comprise a pyramidal foam absorber that is placed on top of a flat ferrite (ceramic) tile. The latter is expensive to make due to a high reject rate, is very heavy compared to foam and typically only attenuates incident radio waves of the order of less than 100 MHz to several hundred MHz by 15 - 20 dB .

It is an object of embodiments of the present invention to at least mitigate one or more of the problems of the prior art.

Summary of invention

Accordingly, embodiments of the present invention provide an absorber comprising at least one or a first plurality of geometric transition absorbers arranged to influence an incident electromagnetic wave and at least one frequency selective filter realised using at least one or a plurality of electrically conductive or resistive elements.

Advantageously, embodiments allow a reduction in the thickness and weight of the material to be realised.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a prior art absorber,

Figure 2 depicts an absorber according to an embodiment,

Figure 3 illustrates an absorber according to a further embodiment,

Figure 4 shows a first embodiment of elements of a frequency selective surface or filter, Figure 5 illustrates a second embodiment of elements of a frequency selective surface or filter,

Figure 6 depicts a third embodiment of elements of a frequency selective surface or filter,

Figure 7 shows a fourth embodiment of elements of a frequency selective surface;

Figure 8 depicts a fifth embodiment of elements of a frequency selective surface;

Figure 9 illustrates a fifth embodiment of an element of a frequency selective surface;

Figure 10 shows a sixth embodiment of elements of a frequency selective surface;

Figure 11 illustrates a cell of an element of a frequency selective surface according to an embodiment;

Figures 12a and 12b show embodiments of lossy slabs bearing frequency selective surface elements;

Figure 13 depicts a graph of variation of reflectivity with frequency for an absorber according to various embodiments;

Figure 14 shows a graph of variation of reflectivity with frequency for an absorber according to various embodiments;

Figure 15 illustrates an embodiment of an absorber used in producing the graphs of Figures 13 and 14;

Figure 16 shows an embodiment of a frequency selective element for the absorber used in producing the graphs in Figures 13 and 14;

Figure 17 depicts a further embodiment of a geometrical transition absorber according to an embodiment;

Figure 18 illustrates a still further embodiment of a geometrical transition absorber according to an embodiment;

Figure 19 shows a yet further embodiment of a geometrical transition absorber according to an embodiment; Figure 20 depicts another embodiment of a geometrical transition absorber;

Figure 21 illustrates an embodiment of a geometrical transition absorber having a ferrite substrate;

Figure 22 shows a further embodiment of a geometrical transition absorber according to an embodiment;

Figure 23 schematically illustrates variation of reflectivity with frequency for embodiments of the present invention;

Figure 24 illustrates an embodiment of still further geometrical transition absorber;

Figure 25 shows a still another embodiment of a geometrical transition absorber bearing 3 dimensional frequency selective elements;

Figure 26 illustrates another embodiment of a geometrical transition absorber; and

Figure 27 depicts a substantially planar absorber according to an embodiment.

Description of Preferred Embodiments

Embodiments of the present invention aim to increase the electrical thickness of absorbers and absorbing structures as a whole at relatively low frequencies such as, for example, pyramidal absorbers or pyramidal structures by incorporating one or more very thin and lightweight layers into the base or substrate of the absorber. Any such layer, preferably, comprises an electrically insulating or dielectric substrate, such as, for example, a mylar substrate or substrate of a similar material, onto which is deposited by printing, sputtering, etching or otherwise, a pattern of conductive elements. The conductive or resistive elements could, however, be printed onto the foam surface itself using a direct write process. The element pattern might comprise an array of conductive or resistive squares, circles, hexagons, triangles, loops, lines (straight or otherwise), spirals, combinations thereof or any other shapes to form a Frequency Selective Surface, Meta- Material or an Electronic Bandgap Material.

The elements of a particular layer may have the same dimensions or different dimensions or may be a combination thereof. The elements of one FSS layer might be the same as or be different to the elements of another layer, if present. The elements of a given layer might all be the same or be different or be a combination thereof. It is possible that the FSS layers might be active (so as to enable dynamic electromagnetic reconfigurability) as well as passive. One skilled in the art would understand how to dynamically change the electrical loading presented by an FSS layer and, therefore, it will not be described in detail.

Figure 1 shows a cross-section of a pyramidal absorber 100 according to the prior art. The absorber has a base or substrate 102 having a thickness, W. The substrate 102 bears a plurality of geometric transition absorbers in the form of pyramidal absorbers 104 to 112 having heights, H, and widths, Z. Typically, the pyramidal absorbers have square bases. The actual dimensions of a geometric transition absorber be varied to suit the needs of a particular application, that is, according to the frequency or range of frequencies of interest.

Referring to figure 2, there is shown a cross-section a pyramidal absorber 200 according to an embodiment. The absorber 200 also bears a plurality of repeating geometric transition absorbers 202 to 210, which, in preferred embodiments, are often pyramidal cones, but could be other shapes as described hereinafter. The absorber 200 comprises a single frequency selective surface (FSS) layer 212 comprising a dielectric substrate 214 bearing a plurality of conductive elements 216. It will be appreciated that the elements are embodiments of a plurality of repeating structures and additionally, that although they are shown in this example as being laid upon a single substrate, each element could have a separate substrate or indeed be directly written onto the foam. The elements are shaped and distributed to relate to a particular frequency range, range of wave incident angles and polarisations or to improve the frequency response when different types and arrangements of absorber structures are required. The FSS layer 212 is embedded within the base or substrate 218 of the absorber 200. The FSS layer 212 is formed such that it is at a predetermined distance, X, from the rear surface 220 of the base 218 and a predetermined distance, Y, from the base of the repeating structures 202 to 210 of the absorber 200. The depth, Z, of the base or substrate 218 of the absorber is also predetermined. The values of X, Y and Z can be established according to the frequency or frequencies of interest.

Figure 3 shows a cross section of an absorber 300 according to a second embodiment. The absorber 300 comprises a plurality of geometrical transition absorbers 302 to 310. The plurality of geometrical transition structures 302 to 310 are pyramidal cones, but could be other shapes as described hereinafter. The third embodiment also comprises a plurality of FSS layers 312 to 314. Each FSS layer 312 to 314 comprises a dielectric substrate 316 to 318 bearing respective pluralities of elements 320 to 322. The FSS layers 312 to 314 are separated by a distance of W. The depth, X, of the substrate or base 324 of the absorber 300 is predetermined. The uppermost FSS layer 314 is positioned at a predetermined distance, Y₅ from the base of the geometrical transition absorbers. The lowest FSS layer 312 is positioned at a predetermined distance, Z, from the rearward face 326 of the base 324 of the absorber 300. The elements used within the FSS layers 312 to 314 can be the same or different, that is, at least one of the intra-layer elements and the inter-layer elements can be different or the same. Again, it will be appreciated that the elements of the FSS layers can be produced in a numbers of ways and that the dielectric substrate is optional.

At high frequencies, the repeating structures of the absorbers 100 to 300 alone are of sufficient length to absorb the incident radio wave energy. As the frequency decreases (wavelength increases), more and more of the absorber length is required to absorb the incident energy. Thus, it is only at the lower frequencies (longer wavelengths) that both the thickness of the absorber repeating structures and the FSS layer or layers become effective in energy absorption. A conventional solution to improve low frequency performance would be to increase the thickness of the absorber (or to add a ferrite tile layer). However, as indicated above, such a solution adds weight and cost. Embodiments of the present invention can be used in addition to these conventional techniques to reduce the thickness of foam or ferrite tiles required. The arrays of conducting patterns act as distributed impedances and make the base foam layer appear to be much thicker electrically than its physical thickness implies. The embodiments add negligible weight and thickness to the base layers and should be very inexpensive to implement in volume production.

Figure 4 shows a first arrangement 400 of a plurality of elements used to form an FSS layer according to an embodiment. The plurality of elements comprises a number of loops 402 to 418. The loops 402 to 418 are conductive or resistive and can be fabricated from a good conductor such as, for example, copper, silver, silver epoxy or a conductive ink. In some embodiments, the conductivity of the ink may be made so low that the loops are resistive rather than conductive. The dimensions of the loops and the spacing between the loops can be set according to desired performance or impedance characteristics for a given frequency or range of frequencies of operation. Embodiments can be realised in which one or more solid squares are used as the frequency selective elements rather the square loops depicted in figure 4. The periodicity of the arrays of loops etc need not be identical to that of the repeating geometric transition absorbers in either the X or Y planes across the face of the absorber, with the Z dimension corresponding to that of the absorber thickness.

Figure 5 illustrates a second embodiment 500 of the repeating structures of an FSS layers. The repeating structures comprise a plurality of strips 502 to 506 forming a grating. Embodiments can be realised in which the grating is vertically or horizontally arranged. It will be appreciated that the arrangement of the grating will determine the plane of polarisation in which the grating is effective.

Referring to figure 6, there is shown a repeating FSS layer structure in the form of an inductive mesh filter 600 comprising a plurality of mutually orthogonal tracks 602 and 604.

Figure 7 shows a further embodiment 700 of a repeating FSS layer structure in the form of an inductive cross-mesh filter comprising a plurality of squares such as, for example, squares 702 to 708, interconnected via a plurality of tracks such as, for example, tracks 712 to 716. In effect, the crosses of figure 7 are "holes" in a conductive plane defined by the edge of the overall square of figure 7.

Figure 8 shows a further embodiment 800 of a repeating FSS layer structure in the form of a capacitive cross-mesh filter comprising a plurality of crosses 802 to 818.

Figure 9 illustrates, in greater detail, an embodiment of an FSS layer element 900 in the form of a square-shaped loop used in the FSS layer 400 described above with reference to figure 4. The element 900 would be repeated to form an FSS layer of a plurality of such elements as shown in figure 4. An alternative embodiment can be realised that is the inverse of the element shown in figure 9 such that the FSS layer element comprises a solid conductive or resistive square.

Figure 10 shows yet another embodiment of an FSS layer element 1000 that is based on the embodiment shown in figure 8 but with the crosses being interspersed with solid squares and rectangles of various sizes to demonstrate that complex FSS element arrangements can be realised according to the impedance characteristics the FSS element is desired to exhibit. Like the embodiment shown in figure 8, the FSS element 1000 of figure 10 comprises a capacitive cross- mesh filter having a plurality of crosses 1002 to 1018 having squares of a first size 1020 to 1026 interspersed therebetween. Rectangles 1028 to 1042 are disposed at the edge of the cell defining the FSS element in between adjacent crosses. Further squares 1044 to 1050, of a second size that is smaller than the first size, are disposed at the corners of the cell of the FSS element 1000.

Preferably, the resistivity of the FSS layer elements in the embodiments of the present invention is less than 1 Ohm per square. It will be appreciated that other embodiments can be realised using some other value of Ohms per square. Preferred embodiments realise the metallisations or conductive/resistive portions of the FSS elements using, for example, copper, or silver or silver epoxy in the case of direct writing or any suitable conductive fluid or powder such as, for example, conductive ink. However, other elements can be used subject to performance requirements, that is, appropriate levels of conductivity/resistivity. For example, embodiments can be realised that have a resistivity of 1 Ohm per square or much more than 1 Ohm per square.

Figure 11 shows a cell 1100 of a frequency selective element 1102. The element 1102 is substantially similar to that shown in and described with reference to figure 6. However, the embodiment is not limited to the element shown is figure 6 and is equally applicable to any other element. The cell 1100 has a horizontal cell pitch, A, which determines, in part, the overall area of a cell and, hence, the horizontal spacing between elements of adjacent cells. The cell 1100 has a vertical cell pitch, B, which determines, in part, the overall area of a cell and, hence, the vertical spacing between elements of adjacent cells. The cell pitch can be set to any arbitrary value according to the performance requirements of an FSS of which the cell forms a part. Although the cell 1100 has substantially equal horizontal and vertical pitches, embodiments are not limited thereto. Embodiments can be realised in which the cell horizontal and vertical cell pitches are different. Furthermore, it can be appreciated that the cell is square. One skilled in the art will appreciate that other cell shapes can be realised such as, for example, hexagonal cells or other cell shapes that are not Cartesian based. Preferably, all of the sides of a cell are capable of abutting at least one corresponding side of an adjacent cell. It can be appreciated that the cell 1100 is arranged such that the metallisation 1102 is positioned at predetermined distances C to F from the edges of the cell 1100. The distances will vary according to the performance requirements of the FSS layer cell. It will be appreciated that the metallisation 1102 is shown as being substantially centrally disposed within the cell. However, embodiments are not limited thereto. Embodiments can be realised in which the metallisation 1102 is other than substantially disposed. For example, the values C to D might be different, the same or any combination thereof.

The thickness of the metallisations, that is, depth of the metallisations or conductive/resistive portions of the FSS elements of embodiments of the present invention can be varied according to requirements. However, preferred embodiments have a thickness of between one and five skin depths.

In other prior art, FSS layers have been used to reduce the electrical thickness of radio wave absorbers such as a Salisbury screen and a Dallenbach layer. In these cases, the FSS has been incorporated into the front (rather than the back) of the structure. However, one skilled in the art appreciates that such prior art operates using a different principle, that is, using wave interference effects. Accordingly, the absorber can only be effective over a small, that is, narrow, range of operating frequencies. Furthermore, to make the thickness of the resulting structure as small as possible, the FSS has to be at the front of the structure otherwise its loading effect becomes reduced and the absorber thickness has to be increased to compensate. In contrast, embodiments of the present invention have increased operating bandwidths. This follows for two reasons. Embodiments of the present invention do not rely on destructive interference between two waves to achieve low reflection. Destructive interference is an inherently narrowband phenomenon and any energy loss only takes place in a narrow region of the absorber, that is, within a lossy FSS. Instead, according to embodiments of the present invention energy is absorbed over a wide range of frequencies by the whole of the structure. At high frequencies only a portion of the absorber such as, for example, a portion of the tip region, is influential but as the frequency reduces, and, consequently, wavelength increases, more and more of the overall structure contributes to the absorbing process. Hence, the geometrical profiling results in a wideband, well behaved absorption process. One skilled in the art will appreciate that the size of any such influential portion varies with frequency for a given level of attenuation, reflection or absorption.

Although the above embodiments have been described with reference to the FSS layer or layers being embedded with the base or substrate of an absorber, embodiments are not limited to such arrangements. Embodiments can be realised in which an FSS layer forms, in effect, the substrate of a geometric transition absorber, that is, the FSS layer is adjacent to the base of the geometric transition absorber.

Embodiments of the present invention have been described with reference to the FSS structures being deposited onto a dielectric substrate. However, the embodiments are not limited to such an arrangement. Embodiments can be realised in which the FSS structures are deposited onto some other form of substrate. Still further embodiments can be realised in which the FSS structures are deposited directly onto the material such as, for example, foam and, preferably, loaded foam, from which an absorber is formed. Embodiments of FSS layer cells can be realised that are the inverse of the embodiments described herein in a substantially similar manner to the FSS element shown in and described with reference to figure 9 and its inverse.

The embodiments described herein have been depicted, as can be appreciated, for example, from figures 2 and 3, with the FSS layer elements having substantially the same shape and/or spacing.

However, embodiments can be realised in which the element shapes and/or spacing can vary or be different. The embodiment described with reference to figure 3 discloses an FSS layer in which the FSS elements of the different layers are substantially aligned with one another.

However, embodiments can be realised in which the FSS elements are so not aligned with one another. Furthermore, although figure 3 shows the FSS elements as being populated with similar

FSS elements, embodiments can be realised in which the FSS layer bear different FSS elements inter se. Still further, the intra-FSS layer elements can also be the same or different.

As an alternative to the pyramidal structures described above, embodiments can be realised in which an absorber is wedge-shaped or substantially flat like a sheet. In such embodiments the FSS elements might comprise elements as described above or, more preferably, might take the form of an interrupted wire or other narrow, planar, structure or a wire or a number of wires that are arranged in parallel or otherwise.

Embodiments can be realised in which the FSS layer, rather than being substantially normal to a line drawn from the apex of a structure to its base, is oriented in some other manner. For example, embodiments can be realised in which the FSS layer or at least the individual FSS layer elements are themselves normal to the base of the structure or at any other angle.

It will be appreciated that an absorber can accommodate less than one, one, or more than one FSS element in either the area defined by the base, or any other area of the structure. For example, embodiments of the present invention might use one, two, three, four or any other number of FSS elements per geometric transition absorber base. Furthermore, the number of FSS elements per geometric transition absorber can be the same across a number of geometric transition absorbers or can be different as between different geometric transition absorbers or any combination thereof. For example, a given geometric transition absorber might have four associated FSS elements whereas an adjacent geometric transition absorber might have one FSS element. Still further, the four FSS elements can be the same as each other or different to one another and the same as or different to the FSS element of the adjacent geometric transition absorber. Referring to figures 12a and b, there is shown still further embodiments of the present invention. Figure 12s shows an absorber 1200 in the form of a slab bearing an FSS layer 1202. The FSS layer. 1202 has been depicted as being embedded within the slab. However, embodiments can be realised in which the slab has an FSS element bearing surface. The surface has one or more FSS layer elements and is disposed on one of the surfaces of the slab as can be appreciated from figure 12b. The slabs can comprise multiple FSS layers comprising respective numbers of FSS elements.

The embodiments of the geometrical transition absorbers described herein can be constructed using the embodiments of the slabs shown in figures 12a and 12b.

It will be appreciated that the embodiments shown in figures 12a and 12b are multi-part structures or assemblies that are formed by disposing an FSS layer or element between two slabs of a lossy material such as, for example, a loaded foam or any other type of material from which the geometrical transition absorbers are constructed. Those base portions of embodiments of the present invention that use more than one FSS element or more than one FSS layer should preferably be fabricated via a number of portions accordingly.

The embodiments shown in figures 12a and 12b are, preferably, fabricated from a material having a loss tangent of at least 0.05 or greater and, preferably, of 0.25 or greater.

The slabs depicted in figures 12a and 12b can be used to improve the performance of conventional existing geometrical transition absorbers by placing such a slab at the base of such a conventional geometrical transition absorber. Accordingly, embodiments of the present invention are directed to a method of fabricating a geometrical transition absorber comprising the steps of combining a slab of lossy material bearing at least one FSS element, having a respective performance characteristic or frequency response, with an existing geometrical transition absorber, having a respective performance characteristic or frequency response, to produce an assembly having a desired overall frequency response. The method of embodiments of the present invention might be employed to retrofit such slabs to an anechoic chamber to improve the performance of the chamber.

Figure 13 shows a graph 1300 of theoretical data showing variation of reflectivity with frequency and width of an FSS element positioned at a distance of 9 mm from the base of a geometrical transition absorber. The thickest line 1302 illustrates the performance for a pyramidal structure without the benefit of an FSS layer. The second line 1304 depicts the performance of a pyramidal structure having 2 mm wide metallisations or conductive portions for the FSS element. The third line 1306 shows the performance of a pyramidal structure having a 4mm wide metallisation or conductive portions of an FSS element. The fourth line 1308 shows the performance of a pyramidal structure having an FSS layer comprising a 6 mm metallisations or conductive portions of the FSS element. The fifth line 1310 depicts the performance of a pyramidal structure having an FSS element with 8 mm wide metallisations or conductive portions. The sixth line 1312 depicts the performance of a pyramidal structure having an FSS layer having an FSS element with 10 mm wide metallisations or conductive portions. It can be appreciated that the performance at 2mm, 4 mm and 6 mm widths is dramatically improved over the performance of the absorber without the benefit of an FSS layer, especially at frequencies of between 3 and 6 GHz.

Figure 14 shows a graph 1400 of theoretical data showing variation of reflectivity with frequency and distance of an FSS element from a base of a geometrical transition absorber in which the metallisations or conductive portions of the FSS element are fixed at 2 mm in width. The thickest line 1402 illustrates the performance for a pyramidal structure without the benefit of an FSS layer. The second line 1404 depicts the performance of a pyramidal structure having an FSS layer positioned at a distance of 2 mm from the base. The third line 1406 shows the performance of a pyramidal structure having an FSS layer positioned at a distance of 4 mm from the base. The fourth line 1408 shows the performance of a pyramidal structure having an FSS positioned at a distance of 6 mm from the base. The fifth line 1410 depicts the performance of a pyramidal structure having an FSS layer positioned at a distance of 8 mm. The sixth line 1412 depicts the performance of a pyramidal structure having an FSS layer with metallisation or conductive portions positioned at 10 mm from the base.

Figure 15 illustrates an embodiment of an absorber 1500 that was used in the theoretical modelling used to produce the results of figures 13 and 14. The absorber comprises a geometrical transition absorber 1502, in the form of a square-based pyramid, having a height of 76 mm, a base or substrate 1504 having a thickness of 25 mm, a width of 38 mm and an FSS element 1506.

Figure 16 depicts a cell 1600 of a single FSS element 1602 that was used within the absorber 1500 to create the theoretical data of figures 13 and 14. The single FSS element 1602 is a square- loop having a line width of 1.5m and being disposed substantially centrally within the cell 1600 at a distance, on all sides, of 0.5 mm from the edges of the cell 1600. The width of the cell corresponds to that of the base of the geometric transition absorber, that is, it had a width of 38 mm. The square-loop was modelled as a perfect conductor and the absorber was repeated infinitely in both directions of a plane.

Referring to figure 17, there is shown an embodiment of a hollow absorber 1700. The hollow absorber 1700 comprises a geometric transition absorber 1702 defining a hollow space 1704 disposed on top of a base 1706 bearing at least one FSS element 1708 or FSS layer. Preferably, the geometric transition absorber 1702 is fabricated from a number of layers. Embodiments can be realised in which the layers present different impedances for a given frequency. Preferably, the impedances are progressively increasing. This embodiment has the advantage that performance is improved for obliquely incident waves as compared to conventional geometric transition absorbers.

Figure 18 illustrates an absorber 1800 according to an embodiment bearing at least one FSS element or layer 1802 within a base portion 1804. The geometric transition absorber or region 1806 of the absorber is arranged to have a substantially curved profile rather than a substantially pyramidal shape. The geometric transition absorber could be defined by the volume of revolution of a given curve about an axis.

Figure 19 illustrates an absorber 1900 according to an embodiment bearing at least one FSS element or layer 1902 within a base portion 1904. The geometric transition absorber or region 1906 of the absorber is also arranged to have a substantially curved profile rather than a substantially pyramidal shape. The geometric transition absorber could be defined by the volume of revolution of a given curve about an axis. In the illustrated embodiment, the curved profile is defined by revolution of a trigonometric function such as, for example, a sine or cosine wave.

Referring to figure 20, there is shown an embodiment of an absorber 2000 in which the geometric transition absorber 2002 comprises at least one FSS layer 2004 bearing at least one FSS element. The geometric transition absorber 2002 might comprise a number of such layers. The at least one FSS layer 2004 or layers might comprises one or more FSS elements. Preferably, the geometric transition absorber 2002 is constructed from appropriately shaped sections such as, for example, appropriately sized frusto-conical sections of a pyramid or other overall shape intended to form the geometric transition absorber 2002. Optionally, the absorber 2000 might also comprise a base portion 2006 bearing one or more FSS elements or FSS layers 2008 having one or more such FSS elements. Figure 21 shows an absorber 2100 having, or at least in conjunction with, a ferrite tile 2102. Optionally, the ferrite tile 2102 is arranged to have at least one FSS element or at least one FSS layer 2104 bearing one or more FSS elements. Optionally, the geometric transition portion 2106 of the absorber 2100 comprises one or more FSS elements or one or more FSS layers comprising at least one FSS element each 2108. Although the embodiment has been described with reference to using a single ferrite tile, some other number of ferrite tiles, without or without respective FSS elements or FSS layers, could also be used.

Figure 22 shows an absorber 2200 according to an embodiment of the present invention that has a geometric transition absorber or region 2202 having a substantially constant cross-section, a so- called two-dimensional absorber as opposed an absorber having a varying cross-section, which is a three-dimensional absorber, or a substantially planar absorber, which is a one-dimensional absorber. The absorber 2200 has at least one or more FSS layers 2204 bearing, respectively, one or more FSS elements. Preferably, the one or more FSS layers 2204 are disposed within a base portion 2206 of the absorber 2200, but they could be positioned elsewhere such as within the geometric transition absorber or region 2002. As with the other embodiments described herein the cross-section can take a form other than a triangle. Embodiments can be realised in which the cross-section has another shape such as, for example, one of the cross-sections associated with figures 18 and 19.

Figure 23 shows schematically a graph 2300 of the variation of reflectivity with frequency. A first curve 2302 shows the performance of a conventional absorber. A second curve 2304 schematically depicts the improvement in performance that can be realised by using an FSS element or layer with the conventional absorber associated with the first curve. The third curve

2306 illustrates the performance of ferrite tile such as described with reference to figure 21, which can be used to still further improve the overall performance of an absorber at lower frequencies. In essence, absorbers according to embodiments of the present invention use two mechanisms to realise an overall improvement in absorber frequency response. The first mechanism, operable at relatively high frequencies, relies on a geometric transition absorber attenuation for absorbing such relatively high frequencies. The second mechanism, operable at lower frequencies, relies on the FSS elements presenting an appropriate impedance match with the incoming electromagnetic signal to absorb the majority of the power of that signal thereby at least reducing, and, preferably, substantially eliminating, reflections. Although the above embodiments have been described with reference to the FSS elements, in the case where they are mounted on a substrate, being directed towards the apex of the geometric transition absorber or region, that is, substantially towards the direction of an anticipated incoming wave, embodiments are not limited to such an arrangement in which the FSS elements are forward facing. Figure 24 shows an embodiment, which is equally applicable to all embodiments described herein, in which the FSS layer bears at least one rearward facing FSS element 2402. This has the advantage that mounting the substrate to a corresponding slab portion of the base such that the FSS elements are not exposed protects them from damage.

Figure 25 illustrates an embodiment of an absorber 2500 in which the base portion 2502 has one or more three-dimensional FSS elements 2504, that is, the FSS elements are no longer substantially planar. An advantage of such an embodiment is that the rest 2506 of the absorber, that is, the, for example, loaded foam portion of the absorber can be mounted onto the FSS elements, which perform the dual function of providing a mount, that is, acting as a support, and performing their intended electrical function. Figure 26 depicts a further embodiment of an absorber 2600 bearing a single 3-D FSS element 2602 that also acts as a mount or support.

Figure 27 illustrates an absorber 2700 according to an embodiment in the form of a sheet, that is, it is substantially flat to allow it, for example, to be included on the wing of an aeroplane. The absorber 2700 also bears a plurality of repeating geometric transition absorbers 2702 to 2710, which, in preferred embodiments, are triangles, but which could be any other shape as described above with reference to the other embodiments. The absorber 2700 comprises a single frequency selective surface (FSS) line 2712 comprising a dielectric 214 substrate, which is optional, bearing a plurality of frequency selective elements 2716. The elements are shaped and distributed to relate to a particular frequency range, range of wave incident angles and polarisations or to improve the frequency response when different types and arrangements of absorber structures are required. The FSS line 2712 is embedded within the base or substrate 2718 of the absorber 2700. The FSS line 2712 is formed such that it is at a predetermined distance, X, from the rear 2720 of the base 2718 and a predetermined distance, Y, from the geometrical transition absorber 2702 to 2710 of the absorber 2700. The depth, Z, of the base or substrate 2718 of the absorber is also predetermined.

All or any embodiments described herein can be realised without or without a conductive backplane that is preferably metallic. Although the embodiments described above have been described with reference to using at least one FSS element within the FSS layer of an absorber, they are not limited to such an arrangement. Embodiments can be realised in which an absorber comprises an FSS layer having one or more FSS elements.

The material from which the absorbers, slabs or geometric transition absorbers is fabricated, preferably, comprises a foam that, itself, is made from polyethylene, neoprene or polystyrene that is rendered conductive via, for example, at least one of carbon or a conducting fluid.

In preferred embodiments the FSS elements are such that their dimensions are substantially equal to or less than λ/2 of the lowest frequency of interest, that is, their corresponding cell size dimensions are substantially equal to or less than λ/2 of the lowest frequency of interest. Furthermore, the size of a cell of an FSS element is preferably comparable with the area of the base of a geometric transition region.

Preferably, the frequency range over which the FSS elements or FSS layer are effective corresponds to a predetermined number of the lowest octaves of a frequency range of interest such as, for example, the lowest 4 to 5 octaves of a frequency range of interest. For some embodiments, the FSS elements or FSS layer are effective over the frequency ranges 30 MHz and below, 60 MHz and below, 120 MHz and below, 240 MHz and below, 480 MHz and below, 960 MHz and below, 1920 MHz and below, 3840 MHz and below, 7680 MHz and below, and, more preferably, over the frequency ranges 30-60 MHz, 30-120 MHz, 30-240 MHz, 30-480 MHz, 30- 960 MHz, 30-1920 MHz, 30-3840 MHz, 30-7680 MHz and, still more preferably, over the frequency ranges 30-60 MHz, 60-120 MHz, 120-240 MHz, 240-480 MHz, 480-960 MHz, 960- 1920 MHz, 1920-3840 MHz₃ 3840-7680 MHz.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

APPENDIX

GEOMETRIC TRANSITION ABSORBER WITH IMPROVED LOW FEIEQUENCY PERFORMANCE

K.L. Ford and B. Chambers*

Department of Electronic and Electrical Engineering

University of Sheffield, Sl 3JD, UK

Introduction

Geometric transition (GT) absorbers, which are comprised of two-dimensional periodic arrays of lossy foam pyramids, cones or wedges, are widely used in anechoic chambers to reduce wall reflections. Because of their topology, such absorbers are inherently broadband and can provide a reflectivity of at least -50 dB when the thickness is of the order of lOλ. Furthermore, the absorber performance is maintained for arbitrary incident polarisations and over a useful range of incidence angles. Although many anechoic chambers employing GT absorbers are used for antenna and RCS measurements, some are also designed with electro-magnetic compatibility (EMC) testing in mind and these may be required to operate at frequencies below 30 MHz. At such low frequencies, even an absorber with a much reduced performance still requires a thickness of at least a metre. Clearly this is rather impractical in terms of the fraction of the chamber volume which is taken up by the absorber rather than by the experimental equipment.

Several authors have reported attempts to make thin absorbers by loading the outer surface of either a Dallenbach layer or a Salisbury screen with a frequency selective surface [l]-[3]. A simplistic attempt to apply this approach to improve the low frequency performance of a GT absorber is shown in Figure 1. Figure 2 shows that the Salisbury screen formed at low frequencies by the resistive firm and the absorber backplane is indeed effective over a narrow band of frequencies, but at high frequencies, the overall absorber reflectivity is dominated by the front-face reflection from the resistive film (approximately -9.5 dB for 377 Ω/ ). A better method of producing a wide absorption bandwidth but using only an electrically thin absorber is to adopt a hybrid approach. Hence a GT absorber may be combined with a planar ferrite layer so that low frequency incident waves are absorbed by the ferrite layer and high frequency waves by the GT structure [4]. Although moderately effective at low frequencies, such absorbers are heavy and expensive to manufacture and install.

In this paper, we discuss a variation on this technique to improve the low frequency performance of a GT absorber without sacrificing overall absorption bandwidth. The resulting impedance loading schemes are much lighter in weight and potentially much cheaper to implement than the GT-ferrite tile combination discussed above.

Theory and implementation

Figure 3 shows the basis of the new hybrid GT absorber design. At high frequencies, absorption is provided by the GT structure in the normal way and the incident waves do not "see" the absorber base region. At low frequencies, however, the incident

ιq waves penetrate into the base region and thus "see" that the electrical thickness of the absorber has been increased by the inclusion of some means of impedance loading. Examples of this might include a frequency selective surface or a layer of electronic band-gap material. This approach can be applied to 3-D GT absorbers incorporating pyramids or cones, 2-D GT absorbers such as wedges or 1-D GT absorbers such as resistive sheets configured with "dragon's teeth". The addition of impedance loading might be used to achieve either specific absorption characteristics with a (physically) thinner absorber structure or improved absorption characteristics with the same thickness. The exact nature of the impedance loading structure to be used will depend on the physical and electrical geometry of the original GT absorber and its desired absorption characteristics. For example, if the loading is positioned close to the conducting back plane of the absorber, this is a low impedance region and hence additional inductance will be required. For a pyramidal GT absorber, this might take the form of a FSS comprised of a 2-D array of conducting loops, whereas for a wedge absorber, the inductance might be formed from an array of parallel conductors.

To Dlustrate this approach, Figure 4 shows the predicted reflectivity performance of a GT absorber having 89 mm long pyramids on a square pitch of 38 mm and with a base thickness of 25mm. Pyramid permittivity data was taken from Figure 5 in [5], Also shown is the effect of loading the absorber base region with a FSS consisting of a 2-D array of square loops with a pitch of 38 mm. In this case, the loading was optimised to maximise the absorber -40 dB bandwidth. It can be seen that the impedance loading has effected a considerable improvement in the low frequency performance of the absorber.

Conclusions

Impedance loading of the base region of a GT absorber has been shown to provide a worthwhile improvement in the low frequency reflectivity performance. The loading is light in weight and potentially much cheaper to manufacture than the hybrid approach based on the use of ferrite tiles. The technique is applicable to the whole range of GT absorber geometries. Further theoretical and experimental data will be reported in due course.

References

1. Engeheta N., "Thin absorbing screens using metamaterial surfaces", IEEE URSI APS 2002

2. Kornbau T.W., "Enhanced frequency selective absorber", Proc AMTA 2002

3. Kem DJ. and Werner D.H., "A genetic algorithm approach to the design of ultra- thin electromagnetic band-gap absorbers", Microwave and Optical Tech. Lett., vol 38, pp 61-64, My 2003.

4. ET S-Lindgren FerroSorb FS series hybrid absorber data sheets.

5. Yang CF, Burnside W.D. and Rudduck R.C., "A periodic moment method solution for TM scattering from lossy dielectric bodies with application to wedge absorber", IEEE Trans Antennas Propagat, vol AP-40, pp 652-660, June 1992.

IO

Figure 1 Hybrid Salisbury screen and geometric transition absorber

D 2 t § S IB 12 14 16 18

Figure 2 Predicted performance of pyramidal absorber alone and with added

377 ΩJ resistive sheet

Zi

Figure 3 Generic pyramidal GT absorber with impedance loading

Figure 4 Predicted performance of pyramidal absorber alone and with impedance loading optimised to give maximum bandwidth at -40 dB reflectivity level

Claims

1. An absorber comprising at least one or a first plurality of geometric transition absorbers arranged to influence an incident electromagnetic wave and at least one frequency selective filter realised using at least one or a plurality of electrically conductive or resistive elements.

2. An absorber as claimed in claim 1 in which the at least one frequency selective filter is embedded within a substrate portion of the absorber.

3. An absorber as claimed in any preceding claim in which the at least one or the plurality of electrically conductive or resistive elements comprise at least one of a grating, at least one strip, at least one square, at least one rectangle, at least one circle, a mesh, an inductive mesh, a capacitive mesh, a loop., a ring, a spiral, a cross or any combination thereof.

4. An absorber as claimed in any preceding claim wherein the at least one or the plurality of electrically conductive or resistive elements is fabricated from a material having a resistance of 1 Ohm per square or less.

5. An absorber as claimed in any of claims 1 to 3 wherein the at least one or the plurality of electrically conductive or resistive elements is fabricated from a material having a resistance of 1 Ohm per square or more than 1 Ohm per square.

6. An absorber as claimed in any preceding claim in which the at least one or the plurality of electrically conductive or resistive elements is positioned at a predetermined distance from a base surface of the absorber.

7. An absorber as claimed in any preceding claim in which the at least one or the plurality of electrically conductive or resistive elements are substantially planar.

8. An absorber as claimed in any preceding claim in which the plurality of electrically conductive or resistive elements are substantially three dimensional.

9. An absorber as claimed in any preceding claim in which the at least one or the plurality of electrically conductive or resistive elements have a predetermined width.

10. An absorber as claimed in any preceding claim in which the at least one or the plurality of electrically conductive or resistive elements have a predetermined depth.

11. An absorber as claimed in claim 10 in which the predetermined depth is between 1 and 10 skin depths at a given frequency.

12. An absorber as claimed in claim 12 in which the predetermined depth is between 2 and 5 skin depths at a given frequency.

13. An absorber as claimed in any preceding claim comprising two or more such frequency selective filters.

14. An absorber as claimed in any preceding claim having at least one of a varying or fixed cross-section.

15. An absorber as claimed in any preceding claim further comprising at least one ferrite portion.

16. An absorber as claimed in claim 15 in which the at least one ferrite portion comprises at least one FSS element.

17. An absorber as claimed in any preceding claim comprising at least one of a forward facing FSS element or a rear facing FSS element.

18. An absorber as claimed in any preceding claim in which at least a portion, such as an a portion bearing the at least one or the plurality of conductive or resistive elements, of the absorber is lossy.

19. An absorber as claimed in claim 18 in which said at least a portion has a loss tangent of greater than or equal to 0.05.

20. An absorber as claimed in claim 19 in which said at least a portion of the absorber has a loss tangent of greater than or equal to 0.25.

21. An absorber as claimed in any preceding claim that is substantially planar.

22. An absorber substantially as described herein with reference to and/or as illustrated in the accompanying drawings.

23. A method of fabricating an absorber comprising at least one frequency selective element; the method comprising the steps of disposing the frequency selective element between a pair of slabs of a material wherein at least one of the slabs is lossy;

24. A method as claimed in claim 23 in which said at least one slab has a loss tangent of greater than 0.05.

25. A method as claimed in claim 24 in which the loss tangent is greater than 0.25.

26. A method as claimed in any of claims 23 to 25 further comprising the step of fabricating an absorber assembly comprising the step of appropriately disposing a geometrical transition absorber relative to said absorber comprising at least one frequency selective element or relative to an absorber as claimed in any of claims 1 to

22.

27. A method of fabricating an absorber or part thereof substantially as described herein with reference to and/or as illustrated in figures 2 to 27.