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Title:
FREQUENCY SELECTIVE TRANSMISSIVE PANEL
Document Type and Number:
WIPO Patent Application WO/2019/105929
Kind Code:
A1
Abstract:
A frequency selective transmittance panel has first and second conductive layers separated by dielectric spacing layer, wherein the first conductive layer comprises a plurality of unit cells, each cell comprising a conductive region at least partially surrounded by a non-conductive region, and the second conductive layer comprises a plurality of unit cells each comprising a non-conductive region at least partially surrounded by a conductive region, wherein the unit cells from the both the first and second conductive layers are of the same dimensions. The dielectric may comprise a solid dielectric, or may comprise a liquid dielectric having a variable dielectric constant. The relative dimensions of the conductive and non-conductive regions, and the value of the dielectric constant affect the peak transmission frequency through the panel. Some embodiments may have a third conductive layer spaced from the second conductive layer by a second dielectric material.

Inventors:
AGUILAR, Miguel Camacho (University of Exeter, Physics BuildingStocker Road, Exeter Devon EX4 4QL, EX4 4QL, GB)
SAMBLES, John Roy (University of Exeter, Physics BuildingStocker Road, Exeter Devon EX4 4QL, EX4 4QL, GB)
HIBBINS, Alastair Paul (University of Exeter, Physics BuildingStocker Road, Exeter Devon EX4 4QL, EX4 4QL, GB)
LAWRENCE, Christopher Robert (QinetiQ Limited, Intellectual PropertyMalvern Technology Centre,St. Andrew's Road, Malvern Worcestershire WR14 3PS, WR14 3PS, GB)
TREMAIN, Benjamin James (University of Exeter, Physics BuildingStocker Road, Exeter Devon EX4 4QL, EX4 4QL, GB)
Application Number:
EP2018/082696
Publication Date:
June 06, 2019
Filing Date:
November 27, 2018
Export Citation:
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Assignee:
QINETIQ LIMITED (Cody Technology Park, Ively Road, Farnborough Hampshire GU14 0LX, GU14 0LX, GB)
International Classes:
H01Q15/00
Other References:
KAOUACH H ET AL: "Design and characterization of wideband high-efficiency unit cells for frequency selective surfaces in 10GHz-band", 2015 INTERNATIONAL CONFERENCE ON ELECTROMAGNETICS IN ADVANCED APPLICATIONS (ICEAA), IEEE, 7 September 2015 (2015-09-07), pages 149 - 152, XP032793010, DOI: 10.1109/ICEAA.2015.7297093
HUI ZHAO ET AL: "Study on the transmission characteristics of a double layered complementary frequency selective surface", 2016 11TH INTERNATIONAL SYMPOSIUM ON ANTENNAS, PROPAGATION AND EM THEORY (ISAPE), IEEE, 18 October 2016 (2016-10-18), pages 731 - 734, XP033053319, DOI: 10.1109/ISAPE.2016.7834062
MENG LI ET AL: "Fluidically Tunable Frequency Selective/Phase Shifting Surfaces for High-Power Microwave Applications", IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, IEEE SERVICE CENTER, PISCATAWAY, NJ, US, vol. 60, no. 6, 1 June 2012 (2012-06-01), pages 2748 - 2759, XP011445836, ISSN: 0018-926X, DOI: 10.1109/TAP.2012.2194645
ARUP RAY ET AL: "A Dual Tuned Complementary Structure Frequency Selective Surface for WLAN Applications", JOURNAL OF MICROWAVES, 20 June 2012 (2012-06-20), pages 144 - 153, XP055266427, Retrieved from the Internet [retrieved on 20160418], DOI: 10.1590/S2179-10742012000100012
SHUNLI LI ET AL: "A novel design methodology for bandpass frequency selective surfaces using complementary loading structure", MICROWAVE, ANTENNA, PROPAGATION AND EMC TECHNOLOGIES FOR WIRELESS COMMUNICATIONS, 2009 3RD IEEE INTERNATIONAL SYMPOSIUM ON, IEEE, PISCATAWAY, NJ, USA, 27 October 2009 (2009-10-27), pages 831 - 833, XP031581905, ISBN: 978-1-4244-4076-4
Attorney, Agent or Firm:
CLARKE, Alan (Malvern Technology Centre, St Andrews Road, Malvern Worcestershire WR14 3PS, WR14 3PS, GB)
Download PDF:
Claims:
Claims

1. A frequency selective transmittance panel comprising a panel having first and second conductive layers separated by dielectric spacing layer, and further wherein:

the first conductive layer comprises a plurality of unit cells, each cell comprising a conductive region at least partially surrounded by a non-conductive region, and

the second conductive layer comprises a plurality of unit cells each comprising a non-conductive region at least partially surrounded by a conductive region,

wherein the unit cells from the both the first and second conductive layers are of the same dimensions.

2. A panel as claimed in claim 1 wherein the spacing layer comprises a solid dielectric material.

3. A panel as claimed in claim 1 wherein a variable dielectric material occupies at least part of the spacing layer, and where a peak transmission frequency through the panel is dependent upon the value of the dielectric constant of the variable dielectric material.

4. A transmittance panel as claimed in claim 3 wherein the variable dielectric material is a liquid crystal and wherein the dielectric constant of the liquid crystal is controlled by a voltage applicable to the first and second conductive layers.

5. A panel according to any of claims 3 or 4 wherein the spacing layer further incorporates electrically insulating separators positioned throughout the layer that act to separate the first and second conductive layers.

6. A panel according to claim 5 wherein the electrically insulating elements comprise glass beads and/or polymer beads.

7. A panel as claimed in any of claims 3 to 6 wherein the first and second conductive layers are connectable to a signal generator.

8. A panel according to any preceding claim wherein the conductive regions of each of the unit cells on the first layer, and the conductive regions of each of the unit cells on the second layer are electrically connected together, such that each conductive layer comprises an electrode.

9. A panel according to any preceding claim wherein the planform overlap of the first and second conductive layers within a unit cell is between 5% and 95% of the unit cell area.

10. A panel according to any preceding claim wherein the conductive regions of each unit cell of the first conductive layer, and the non-conductive region of each unit cell of the second conductive layer are located centrally within each unit cell.

1 1. A panel as claimed in any of the above claims wherein a second dielectric spacing layer is located on a side of the second conductive layer opposed to the first dielectric spacing layer, and a third conductive layer is positioned on the second dielectric spacing layer, with the second conductive layer sandwiched between the first and third conductive layers, and wherein the third conductive layer comprises a plurality of unit cells, each cell comprising of a conductive region at least partially surrounded by a non-conductive region. 12. A panel as claimed in claim 1 1 wherein unit cells on the third conductive layer match in form those on the first conductive layer.

13. A panel as claimed in claim 1 1 or claim 12 wherein the non-conducting region in each unit cell on the second conductive layer occupies between 0.1 % and 99% of the area of the unit cell.

14. A panel as claimed in any of claims 11 to 13 wherein a variable dielectric material forms at least a part of the second dielectric layer.

15. A panel as claimed in any preceding claim wherein the area of the conductive region on each unit cell on the first conductive layer is greater than the area of the non-conductive region on each unit cell on the second conductive layer. 16. A panel as according to any preceding claim wherein the conductive region of the first conductive layer and the non-conductive region of the second conductive layer within each unit cell are both square.

17. A panel according to any preceding claim wherein, within each unit cell there is an overlap, when seen in planform, of the conductive region from the first layer, with the conductive region of the second layer.

18. A panel according to any preceding claim wherein a central point of the conductive region on each unit cell on the first conductive layer is aligned with a central point on the non-conductive region on each unit cell on the second conductive layer.

19. A panel as claimed in any of claims 1 to 17 wherein a central point of the conductive region on each unit cell on the first conductive layer is offset from a central point on the non-conductive region on each unit cell on the second conductive layer.

20. A panel as claimed in any of the above claims wherein the conductive region in a unit cell on the first layer comprises of a regular polygon or a circle.

21. A panel according to any preceding claim wherein each conductive layer sits on a substrate.

22. A panel according to claim 21 wherein the substrate is chosen from: FR4, Mylar, and polyester.

23. A panel as claimed in any of claims 12 to 22, when dependent upon claim 1 1 , wherein the third conductive layer has the same form as, and is aligned with, the first conductive layer.

24. A switchable transmittance system comprising a panel according to any of claims 3 to 10 and a signal generator arranged to provide a switching voltage between the first and second conductive layers. 25. A switchable transmittance system comprising a panel according to claim

14 wherein the third conductive layer comprises an electrode, and a signal generator arranged to provide a switching voltage between any of the first, second and third conductive layers.

Description:
Frequency Selective Transmissive Panel

The present invention relates to the field of devices and materials used to control the passage of electromagnetic energy. More specifically, it relates to structures, such as panels and the like, that allow some frequencies to pass through, whilst stopping others, generally providing a passband of frequencies.

It is sometimes required to shield an antenna from electromagnetic (EM) radiation of particular frequencies. For example, a receiver arranged to receive EM radiation having a particular frequency may also be sensitive to nearby frequencies, reception of which may cause distortion or non-linearities affecting its wanted received signals. It may also be required to temporarily shield an antenna from unwanted signals that share the same band as those of its own intended signals.

It may also be desirable to shield certain rooms from EM radiation, to prevent certain types of signal (such as e.g. WIFI) from penetrating the room, whilst allowing other types (such as mobile 3G or 4G phone signals) to pass through. Absorbers, such as those disclosed in international patent application

W02005/013663 may be used where appropriate in some circumstances.

However, the nature of their frequency absorption profile can be inconvenient when this may cause undesired partial absorption of the frequencies of interest (e.g. for the operation of an underlying antenna). It is also the case that many such absorbers are unsuited to the obscuration of an underlying antenna as they prevent transmission of any RF frequencies due to their reliance upon an electrically conductive back-plane (e.g. the Salisbury screen described in RADAR CROSS- SECTION, 2 nd Edn - EF Knott, JF Shaeffer, MT Tuley, Artech House, ISBN 0- 89006-618-3 (1993)

According to the present invention there is provided a frequency selective transmittance panel comprising a panel having first and second conductive layers separated by dielectric spacing layer, and further wherein: the first conductive layer comprises a plurality of unit cells, each cell comprising a conductive region at least partially surrounded by a non-conductive region, and

the second conductive layer comprises a plurality of unit cells each comprising a non-conductive region at least partially surrounded by a conductive region,

wherein the unit cells from the both the first and second conductive layers are of the same dimensions. The invention provides a panel, that may take the form of a sheet, such as a flexible sheet, or other thin, generally planar form, that provides a bandpass filtering function to radiation having a free-space wavelength that is typically at least 5, 10, 20, 40 or 100 times greater than the thickness of the panel. The first conductive layer may have in some embodiments a conductive region that is entirely surrounded by a non-conductive region, and the second conductive layer may have in some embodiments a non-conductive region that is entirely surrounded by a conductive region. Advantageously, each unit cell of the first conductive layer may have a non- conductive region that is a regular polygon or a circle, and more advantageously comprises a square or a hexagon.

Where the non-conductive region on the first conductive layer is square, then advantageously the unit cells on both conductive layers may be arranged in a square array.

Where the non-conductive region on the first conductive layer is hexagonal, then advantageously the unit cells on both conductive layers may be arranged in a triangular lattice.

It will be appreciated that the non-conductive regions on the conductive layers may comprise insulating material, or may merely comprise an absence of conductive material in such regions, effectively acting as holes, gaps or“windows” in the conductive layer. Thus, each of the conductive layers have gaps therein where there is no conductivity. The term“conductive layer” will thus be understood as being a layer that has conductive parts thereon, but which, over some of its extent, is non-conductive.

Conveniently, the spacing layer may comprise a dielectric material. This may be a solid dielectric, such as Mylar, FR4, or some other dielectric that provides sufficiently low loss to transmission passing through the material at a transmission frequency of interest.

Advantageously, the spacing layer may comprise in some embodiments, at least partially, a variable dielectric material. Thus, with such embodiments, a

transmission frequency of interest may be varied by varying the dielectric properties, such as the dielectric constant, of the dielectric material.

Advantageously, where the spacing layer comprises at least partially a variable dielectric material, the variable dielectric material may be a liquid crystal, wherein the dielectric constant of the liquid crystal is controlled by a voltage applicable to the first and second conductive layers.

Advantageously, electrically insulating separators may be positioned throughout the variable dielectric material regions of the spacing layer that act to separate the first and second conductive layers. The separators may comprise glass beads, or other such materials. Alternatively the separators may comprise raised or embossed areas on one or both conductive layers, where the conductive material is not present, or on additional layers between the conductive layers.

Advantageously, the first and second conductive layers may be connectable to a signal generator. Thus the signal generator may be arranged to control the voltage supplied to the conductive layers to change the dielectric constant of the variable dielectric layer.

In some embodiments, one or more of the conductive layers may comprise a highly conductive material. In other embodiments, one or more of the conductive layers may comprise of material having a predetermined resistivity, that is arranged to be more resistive than that of a highly conductive layer.

Conveniently, in some embodiments of the invention the conductive regions of each of the unit cells on each of first and the second layers are electrically connected together, such that each conductive layer comprises an electrode. Advantageously, the conductive regions of each cell may be connected together over a small part of their boundary, such as just at their corners or some other part of the boundary, with the connection occupying less than 10%, and preferably less than 5%, and more preferably less than 3% of the perimeter of the conductive region.

Advantageously, the planform overlap of the first and second conductive layers within a unit cell may lie between 5% and 95% of the unit cell area.

Advantageously, the conductive regions of each unit cell of the first conductive layer, and the insulating region of each unit cell of the second conductive layer may be located centrally within each unit cell. Advantageously, the area of the conductive region on each unit cell on the first conductive layer is greater than the area of the non-conductive region on each unit cell on the second conductive layer. This is particularly beneficial where a variable dielectric material is used as part or all of the spacer layer, as it guarantees a region in which the variable dielectric material is subject to an electric field from a voltage source applied between the conductive layers.

In some embodiments of the invention a second dielectric layer is located on a side of the second conductive layer opposed to the first dielectric spacing layer, and a third conductive layer is positioned on the second dielectric spacing layer, with the second conductive layer sandwiched between the first and third conductive layers, and wherein the third conductive layer comprises a plurality of unit cells, each cell comprising a conductive region at least partially surrounded by a non-conductive region. Where a third conductive layer is present, then each unit cell on the third layer is advantageously arranged to match, in form, such as in pattern, orientation and alignment, each unit cell on the first conductive layer.

Where a third conductive layer is present then there may be a variable dielectric material at least partly occupying the spacer layer between the first and second conductive layers, or between the second and third conductive layers, or between both the first and second, and second and third conductive layers. Where such a variable dielectric material comprises a liquid crystal, and where it occupies a spacing layer between the second and third layers, then the third conductive layer may be used as an electrode.

In those embodiments incorporating a liquid crystal, then additional layers, sitting in contact with the liquid crystal, may be used to provide an alignment of the molecules making up the liquid crystal. Such layers are known from the field of liquid crystal displays, and typically comprise thin (compared to the LC layer itself) layers of material that are brushed or rubbed in a particular direction to orient LC molecules in contact therewith. Alternatively, the alignment may be provided by treating the conductive layers themselves, such as by brushing, to provide an orientation to the layer.

Advantageously, each of the first and second layers are arranged with their alignment directions running parallel to each other. Some embodiments may however have other alignment arrangements, such as an anti-parallel alignment.

Advantageously, the conductive region of the first conductive layer and the non- conductive region of the second conductive layer within each unit cell are both square. Other shapes may be used, including hexagons, other regular polygons and circles. Square regions have particular benefits, as they provide equal performance with both horizontal and vertical polarisations, and it will be appreciated that other shaped regions may have similar benefits. In some embodiments of the invention, within each unit cell there is an overlap, when seen in planform, of the conductive region from the first layer, with the conductive region of the second layer. A greater degree of overlap of the conductive regions generally leads to a reduced transmittance through the panel. However, in those embodiments having a variable dielectric, the effect of switching the dielectric between two extremes of its range will create a greater change in the resonant frequency (and hence the peak passband) of the panel where the overlap is greater, due to an increased switchable area. The degree of overlap therefore may be chosen based upon desired system performance. In some embodiments the non-conductive region on the second conductive layer may occupy between 0.1 % and 99%, or more preferably between 0.1% and 50%, or more preferably between 0.1% and 20% of the unit cell area.

Likewise, on the first (and, where present the third) conductive layers, the non- conducting regions within a unit cell may occupy between 1 % and 99% of the unit cell area.

The size of the conductive region on the first conductive layer may be chosen such that it has a major dimension (e.g. a diameter, or length of side) approximating to l/2, where l is the wavelength of the EM radiation within the dielectric required to pass through the panel. Various adjustments to this dimension may be made, as would be commonly understood by a person of ordinary skill in the art, to achieve a more specific transmission centre frequency. The non-conducting gap between adjacent cells is typically small in relation to the cell size. Thus, it would typically be less than a fifth, such as less than a tenth of the length of a unit cell. It has been found that smaller gaps tend to reduce the resonant frequency of a panel, due to the increased capacitance between conductive regions on adjacent cells.

In some embodiments of the invention a central point of the conductive region on each unit cell on the first conductive layer is aligned with a central point on the non- conductive region on each unit cell on the second conductive layer. When so aligned, the conductive region of the first conductive layer sits centrally over the non-conductive region of the second layer. Thus such embodiments provide at least partial, and in many embodiments total blocking of light therethrough, even if the dielectric layer itself (and/or the substrate upon which each conductive layer is mounted upon) is transparent.

Alternatively, in some embodiments a central point of the conductive region on each unit cell on the first conductive layer is offset from a central point on the non- conductive region on each unit cell on the second conductive layer. In such embodiments the non-conductive regions may be at least partially aligned, and so allow (where the dielectric spacer layer and substrates are transparent or translucent) light to pass through.

Advantageously, each conductive layer may sit on a substrate. The substrate may comprise the dielectric material of the spacer layer in some embodiments. The substrate may advantageously be a transmissive dielectric having low loss at the frequencies of interest. Examples include polymers and epoxy-based dielectrics, such as FR4, polyester and Mylar, and glass, or any other suitable material where the loss is acceptable for a given application at the frequencies of interest.

Advantageously, in some embodiments the dielectric material may be chosen to have other useful properties, such as flexibility, or a desired transmittance to visible light.

The panel as described above may be incorporated into a system, the system comprising the panel and a signal generator arranged to provide a switching voltage between the first and second conductive layers.

Where an embodiment comprising three conductive layers is incorporated into such a system, the third conductive layer may advantageously comprise an electrode, with the signal generator being arranged to provide a switching voltage between any of the first, second and third conductive layers. The voltage is preferably an AC voltage, and preferably has a switching frequency of between 50 Hz and 50 KHz. The amplitude of the voltage is chosen based upon the switching properties of the variable dielectric material used. Where this is a liquid crystal, the switching voltage is typically between 5V and 20V. It is common for manufacturers of LC materials to provide guidelines for a suitable choice of voltage. The spacing between adjacent conductive layers may typically be between 30pm and 500pm, and more preferably be between 50pm and 100pm for panels operative at around 10 to 20 GHz. However, embodiments outside of this range may still be effective, depending upon the application and frequency of interest.

Note that the unit cell comprises the smallest symmetric area of the pattern on the conductive layer that is repeated, substantially identically, across the whole or a significant part of the structure.

The invention will now be described in more detail, by way of example only, with reference to the following Figures, of which :

Figure 1 diagrammatically illustrates a profile of an embodiment of the present invention comprising two conductive layers with a liquid crystal material present between them;

Figure 2 diagrammatically illustrates, in plan view, a) a first conductive layer, and b) a second conductive layer, showing multiple copies of the unit cell in an

embodiment of the present invention; Figure 3 diagrammatically illustrates an embodiment of the invention being driven in a) a lesser transmissive mode, and b) a more transmissive mode;

Figure 4 shows a graph of transmissivity of an embodiment of the invention in two different switching states;

Figure 5 diagrammatically illustrates an embodiment of the invention having a fixed dielectric material in the spacing layer between a pair of conductive layers, where the conductive layers are each similar to those shown in Figure 2; Figure 6 diagrammatically illustrates an embodiment having three conductive layers; and

Figure 7 shows a graph of transmittance with frequency through variations of the embodiment of Figure 6.

As shown in Figures 1 and 2, an embodiment of the invention comprises a layered structure, comprising a first substrate 10, that supports a first conductive layer 12 made from copper, that is patterned as shown in Figure 2. A second substrate 14 supports a second conductive layer 16, again made from copper, and is again patterned. The first and second substrates are made from FR4. The first and second conductive layers are spaced apart, and are held separated by a plurality of glass beads e.g. 18 that are scattered throughout the spacing, and thus define a spacing layer 20. The beads 18 are electrically insulating in nature, and ensure that the first and second substrates maintain a relatively constant separation across the structure, and so do not touch and cause an electrical short. The glass beads have a diameter of 50pm in this embodiment.

On facing sides of both the first 12 and second 16 conductive layers is a thin polyimide alignment layer that is rubbed so as to create an alignment direction.

The direction of alignment for both conductive layers is the same - i.e. they provide parallel alignment.

Each unit cell is a square of size 4.95mm. On the first conductive layer, shown in Figure 2a, the conductive region within the unit cell comprises a copper square 22 having a spacing between opposite sides of 4.7mm, wherein the square is surrounded for most of its perimeter by a non-conductive break 24 in the conductive material. A non-conductive separation between conductive squares on adjacent unit cells of 250pm exists. Each corner of the square 22 has a copper connection 26 to the squares on neighbouring cells, the connection comprising a copper square 400pm along each side, positioned centrally on the corner of each unit cell. The copper connections therefore provide conductivity between the copper squares within each unit cell.

A unit cell for the first conductive layer is shown by dotted line 28, and Figure 2a shows four unit cells in a square arrangement. Clearly, the unit cell pattern repeats a number of times dependent upon the size of the overall structure.

On the second layer, shown in Figure 2b, the unit cell comprises a copper perimeter 30, that flows seamlessly to the surrounding unit cells, and within the copper perimeter is a non-conductive square 32, formed by removal of the copper. The non-conductive square has a side length of 3mm. A unit cell for the second conductive layer is shown by dotted line 34. Again, the unit cell 34 pattern repeats a number of times dependent upon the size of the overall structure.

When the first and second layers are laid on top of each other the unit cells are aligned. Thus, the non-conductive square of the second conductive layer sits centrally, in planform, within the conductive square of the first layer. An overlap comprising a square boundary of the conductors on each layer of 0.975mm therefore exists, within each unit cell, plus a small degree of additional overlap created by the conductive connections 26 on the first layer.

Electrical connections are made to each conductive layer. This is facilitated in this embodiment by arranging a two-unit-cell offset when the layers are placed together during manufacture, so providing a strip along opposing edges where access to each conductive layer can easily be had.

The conductive and non-conductive patterns that comprise the first and second conductive layers are formed by selective removal of copper from a copper-clad board, using standard printed circuit board manufacturing techniques. Thus, large areas may be conveniently and cheaply made. The space between the conductive layers is filled with a liquid crystal material E80A. The first and second conductive layers act as electrodes, whilst the alignment layers act on the molecules of LC material to create an alignment of the molecules, when no voltage is present on the electrodes. When a voltage is put between the electrodes an electric field is generated, particularly where the first and second electrodes overlap, with an electric field vector being directed perpendicular to the plane of the conductive layers. In those overlapping regions, the alignment of the liquid crystal molecules varies according to the strength and direction of the electric field. This is shown in Figure 3. Figures 3a and 3b both show a simplified representation of a cross section through a part of a structure as described above. A first conductor 40 sits above a second conductor 42, separated by a spacing of 50pm, this therefore being representative of an overlapping region of the two conductive layers. A liquid crystal material fills the space 44 between the conductors. In Figure 3a no voltage is applied between the conductors. The LC molecules 46 naturally assume the alignment shown, due to the polarised nature of the individual molecules and their interaction with the alignment layers present on each conductive layer. The molecules tend to sit parallel to the plane of the conductors. This alignment gives the material a particular value of permittivity at K band (18GHz - 26.5GHz). When a voltage of 20V pk-pk at 10KHz from a voltage source 48 is applied to the conductors the alignment of the LC molecules changes to a state more like that shown in Figure 3b. There, the LC molecules that are not bound by edge effects tend to line up along the axis of the applied electrical field. This gives a change in the permittivity at K band.

Note that other LC materials, such as E7 may be used in other embodiments.

Figure 4 shows a graph of RF transmission properties of RF energy through an embodiment of the invention between 15 GHz and 23GHz. The horizontal axis is transmission frequency, and the vertical axis is transmitted intensity. The embodiment under test comprised of a 15cm square, planar panel, that was positioned between a transmit and a receive horn. The panel was mounted with its plane lying normal to an incident beam. With 0V applied across the conductors the transmission characteristic is as indicated in plot 50 (shown as a darker line). With the 20V AC signal applied across the conductors the transmission characteristic is as shown in plot 52. It can be seen, that at around 17.8GHz the difference between transmission levels switches between approximately 0.45 and about 0.6, in off and on states respectively. The panel therefore switches between a lesser transmissive state and a greater transmissive state.

The above results were obtained with the embodiment shown in Figure 1 , with a square panel of side length 15cm. Other embodiments may be made larger or smaller, as required. Flexible panels comprising the multi-layered structures described herein may advantageously be formed, wherein Mylar sheets (or other suitable flexible material) may be used as a substrate, which are plated with a conductive material, such as copper or aluminium, to produce the first and second conductive layers. Figure 5 shows a further embodiment of the invention, in cross-sectional profile, where the cross section shows one complete unit cell, between dotted lines 61 , along with approximately half of each adjacent unit cell shown on each side of it. The embodiment comprises a Mylar sheet 60 of thickness 50pm having first and second sides. On the first side is a conductive layer 62 taking the general form of a plurality of unit cells each of the type shown in Figure 2a. Conductive patches 62 of square form are surrounded by isolating regions 63. On the second side is a conductive layer 64 taking the general form of a plurality of unit cells each of the type shown in Figure 2b. Metallised regions 64 have square voids 65 centrally located within each unit cell. Thus, the embodiment acts as a frequency selective surface having a frequency characteristic of the general form shown by trace 50 in Figure 4, although it will be appreciated that, due to the difference in dielectric constant between the Mylar and the liquid crystal, the exact frequency of peak transmittance, and transmittance values, will differ from that shown in Figure 4. Further dielectric layers may be positioned on one or both sides of the metallised Mylar, to provide a protective coating to the conductive surface(s).

Such an embodiment provides a thin, frequency selective surface that may conveniently be produced in large sizes. Figure 6 shows a further embodiment of the invention, in a perspective view. This comprises a sheet or panel structure having three conductive layers 72, 74, 76 with a dielectric spacing layer (not shown) between each one. The three conductive layers are shown in vertically exploded form for clarity. The three conductive layers all sit parallel to each other. The central conductive layer 74 is patterned with a matrix of unit cells (e.g. as indicated by dotted line 73) each broadly similar to that shown in Figure 2b, having a conductive square border 77 (shown as hatched) around a square non-conductive region 78, while the two outer conductive layers are each patterned with a matrix of unit cells each broadly similar to that shown in Figure 2a, and having a central conductive square (shown hatched) 80 with an insulation region partially or totally surrounding it. It will be appreciated that adjacent conductive squares positioned centrally within the unit cell on each of the outer two layers need not be electrically connected to each other, as the dielectric used in the spacing layers - Mylar in this case - is not a variable dielectric type.

The dimensions are as follows: Unit cell side length is 4.95 mm. The square conductive region centrally located in each unit cell on the two outer conductive layers has side length 4.5 mm. The central conductive layer is spaced 50 pm from each of the two outer conductive layers. The square hole, or non-conductive region has been modelled over a range from 0.1 mm side length to 0.75mm side length.

It has been found that the size of the central hole in the conductive central layer, as a fraction of the total area of the unit cell, changes the transmission characteristics through the panel. Figure 7 shows a (computer modelled) graph of transmission through such a panel against frequency, for different side lengths of a central square hole in the conductive region in the central conductive layer. It shows that increasing the hole diameter can alter the transmission characteristics from a single peak transmission of approximately 25% at -22.3 GHz, through to a broader peak exhibiting around 100 % transmission. Here, the hole size is varying from a side length of 0.25mm (0.3% fractional area), rising to a peak at around 0.4mm (0.8% fractional area). As the hole diameter increases further it has been found that the transmission peak splits into two peaks, with both peaks at or near 100% transmission. At least 50% transmission is maintained up to a side length of 0.6mm. From there on, as the hole size is increased, two separate transmission bands are formed, as can be seen by the hole size at 0.75mm.

The embodiments having three conductive layers may also be made from materials, such as those discussed above, and may advantageously be made from flexible materials, allowing these, and other embodiments, to be curved, and so provide, to some degree, a conformal panel. The skilled person will appreciate that other variations, and changes may be made to elements with the embodiments described, whilst still remaining within the scope of the claims. The claims therefore should be interpreted as covering any such variants and equivalents.