A layered frequency selective surface assembly and method of modulating the power or frequency characteristics thereof

Abstract: The power or frequency characteristics of a layered frequency selective surface assembly (1) having a dielectric substrate layer (2) and, superimposed therewith at a spacing (X) therefrom, at least one layer (3) of electrically conductive elements (4) disposed in an array, is modulated by moving at least one layer (2, 3) relative to the or each other layer (2, 3). The layers (2, 3) maybe substantially planar or substantially non-planar and may be disposed substantially parallel to one another. Relative movement between the layers (2, 3) may retain the substantially parallel orientation or may not retain the substantially parallel orientation. The relative movement may have the effect of changing the spacing between the layers (2, 3) for example to tilt, skew or rotate one layer relative to the or each other layer.

Description: This invention relates to a layered frequency selective surface assembly and to a method of modulating the power or frequency characteristics thereof, particularly, but not exclusively, suitable for use with radar applications. Passive planar frequency selective surface assemblies are known in the form of an array of identical electrically conductive elements printed on a flat dielectric substrate. Such known assemblies have the useful property, by virtue of resonances within and between the conductive elements, that the transmission and reflection of microwave energy by the surface of the assembly is strongly frequency dependant. This allows the assembly to be used in the construction of, for example multi-band reflector antennae, in which the routing and possible transmission of microwave signals is strongly frequency dependent. Thus the assembly can be used to reflect signals in one frequency band and to pass signals in another frequency band. Additional to such two layer assemblies, assemblies are also known with a plurality of layers of electrically conductive elements. US-A-4307404 discloses a dichroic antenna scanner having two or more dielectric dichroic layers spaced apart from one another at a fixed tapered spacing and with the dichroic layers comprising a plurality of conductive elements such as metallic film elements. The tapered spacing provides a fixed variation in cut-off frequency across the layers and selective scanning of the different frequencies may be accomplished by rotating the scanner. Recent research has proposed that the characteristics of a frequency selective surface assembly can be electronically altered on a very short time scale by switching or otherwise controlling active electronic devices embedded into the individual electrically conductive elements. Such an active frequency selective surface assembly could have many potential applications but embedding electronic devices in individual electrically conductive elements is difficult and would make such assemblies expensive and difficult to make and use. There is thus a need for an improved layered frequency selective surface assembly with which the power or frequency characteristics thereof can relatively easily be modulated without great expense and for a method of modulating the power or frequency characteristics of such a layered frequency selective surface assembly. Preferably such layered frequency selective surface assemblies should be capable of relatively rapid modulation control. According to one aspect of the present invention there is provided a method of modulating the power or frequency characteristics of a layered frequency selective surface assembly having a dielectric substrate layer and, superimposed therewith at a spacing therefran, at least one layer of electrically conductive elements disposed in an array, characterised in that at least one layer is moveable relative to the or each other layer to modulate such characteristics. Preferably said layers are substantially planar or substantially non-planar and disposed substantially in parallel to one another and in which said relative movement is effective in a manner such as to retain such substantially parallel disposition, so as to modulate the frequency characteristics. Conveniently said layers are substantially planar or substantially non-planar and disposed substantially in parallel to one another and in which said relative movement is effected in a manner such as not to retain such substantially parallel disposition. Advantageously for a substantially fixed frequency, the direction of power transmission from the layered frequency selective surface assembly is modulated by moving one said layer relative to the or each other layer in a substantially symmetric substantially non-parallel manner. Preferably said relative movement is effected by pivoting or rotating one said layer relative to the or each other layer about an axis normal to said layers so as to change the relationship to the array of electrically conductive elements relative to the dielectric substrate layer. Conveniently the characteristics are modulated at least in part by varying the spacing between said layers. Advantageously the relative movement between said layers is brought about mechanically, electro-mechanically, hydraulically or pneumatically. The relative movement between said layers may be brought about by distortion of one layer. According to a further aspect of the present invention there is provided a layered frequency selective surface assembly having a dielectric substrate layer, and at least one layer of electrically conductive elements disposed in an array, which at least one conductive element layer is superimposed with and spaced from the substrate layer, characterised by including means for moving at least one layer relative to the or each other layer to modulate the power or frequency characteristics of the assembly. Preferably said layers are substantially planar or substantially non-planar in form and disposed substantially parallel to one another. Conveniently said moving means are operable to retain said substantially parallel disposition. Alternatively said moving means are operable so as not to retain said substantially parallel disposition. Conveniently said moving means are operable to move one said layer relative to or each other layer in a substantially symmetric substantially non-parallel manner. Advantageously said moving means are operable to pivot or rotate one said layer relative to the or each other layer about an axis normal to said layers. Preferably said moving means are operable at least in part to vary the spacing between said layers. Conveniently said moving means are operable to bring about said relative movement mechanically, electromechanically, hydraulically or pneumatically. The moving means may be operable to distort the shape of one said layer relative to the or each other layer. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, in which: Figure 1 is a diagrammatic perspective view of a layered frequency selective surface assembly according to one embodiment of the present invention, and Figure 2 is a diagrammatic perspective view of a layered frequency selective surface assembly according to a further embodiment of the present invention. A layered frequency selective surface assembly of the present invention, generally indicated at 1 in Figures 1 and 2 of the present application includes a dielectric substrate layer 2 and at least one layer 3 of electrically conductive elements 4 disposed in an array which may conveniently be of the grid form illustrated in Figures 1 and 2. The conductive element layer 3 is superimposed with and spaced from the substrate layer 2 at a spacing X. The assembly also includes means generally indicated at 5 for moving at least one layer 2, 3 relative to the or each other layer 2, 3 to modulate the power or frequency characteristics of the assembly. In the illustrated embodiments the means 5 are arranged to move the electrically conductive element layer 3 relative to the substrate layer 2 although the reverse could apply if desired. Additionally, for convenience, the assembly 1 of the invention has been shown with only two layers 2 and 3 but it is to be understood, however, that a plurality of layers 3 can be provided each separated from one another, superimposed over one another and moveable either singularly or together relative to one another. The power or frequency characteristics of a two layer assembly 1 are strongly dependent on the spacing between the two layers 2 and 3. Hence the properties of an assembly of two layers 2 and 3 arranged substantially parallel to one another in overlying relationship and separated from one another by a variable spacing X of the order of a quarter of a wavelength can be switched or controlled rapidly, on a timescale significantly shorter than a second, by moving one layer relative to the other, while keeping the two layers substantially parallel. This can be used for example to alter the frequency characteristics of a wave filter in a reflector antennae. Thus in the embodiment of Figure 1 the relative movement of the surfaces 2 and 3 takes place in the direction of the arrows 6. In the embodiment of the invention illustrated in Figures 1 and 2 the layers 2 and 3 are shown as being substantially planar and sheet like in form, disposed substantially parallel to one another. Movement of the layers 2 and 3 may not be towards and away from one another to vary the size of the spacing X but could be carried out by the means 5 to retain the substantially parallel disposition of the layers 2 and 3. In this event movement may be in the direction of the arrow 7 so as to move the layer 3 backwards and forwards across the layer 2 to vary the relative position of the elements 4 with respect to the layer 2 or maybe in the transverse direction as shown by the arrows 8. Alternatively the means 5 may be operable so as not to retain the substantially parallel disposition of the layers 2 and 3. In this event movement of the layer 3 with respect to the layer 2 maybe rotational about an axis parallel to the plane of the layer 2 in the direction of the arrow 9 or maybe a pivotal movement about the means 5 in the direction of the arrow 10 so as to make the spacing X variable between the two layers 2 and 3 between the layer ends nearest to the means 5 and the layer ends furthest away from the means 5. In other words the spacing between the two layers 2 and 3 can be constant along their length or vary from such that the spacing at the ends furthest away from the means 5 can be less than or greater than the spacing between the ends nearest to the means 5. In the latter instant the means 5 are operable to move the layer 3 relative to the layer 2 in a substantially symmetric substantially non-parallel manner which could when used with an antennae provide some degree of beam steering. In the embodiment of Figure 2 of the accompanying drawings the means 5 are operable to pivot or rotate the layer 3 relative to the layer 2 about an axis or axle 11 normal to the planes containing the layers 2 and 3. This has the effect of changing the registration of the elements 4 with the substrate layer 2. As an alternative, not illustrated, the layers 2 and 3 need not be substantially planar but can be substantially non-planar, for example curved. All the features outlined about in respect of the embodiment of Figures 1 and 2 can be utilised with non-planar layers 2 and 3. In the foregoing the means 5 can take any convenient form for example mechanical, electromechanical, hydraulic or pneumatic. Conveniently only one layer 2 or 3 could be given a substantially planar form and the other given a substantially non-planar form. Indeed movement of one layer relative to the or each other layer may be by means of distorting said one layer relative to the other layer. Such distortion could be used to provide a microwave lens structure. The method and assembly of the present invention although primarily intended for radar uses could also be used for optical and infra-red filters. The power or frequency characteristics of the assembly are modulated by relative movement between the layers 2 and 3 in the manner described to vary the amount of power transmitted and/or direction of the power transmission for a fixed frequency or when used for a filter, to vary or modulate the cut-off frequency of the filter. The degree of relative movement can be very small and effectively varied rapidly to provide an easily controllable high speed switching action in a relatively cheap and simple manner.

Claims: A method of modulating the power or frequency characteristics of a layered frequency selective surface assembly (1) having a dielectric substrate layer (2) and, superimposed therewith at a spacing (X) therefrom, at least one layer (3) of electrically conductive elements (4) disposed in an array, characterised in that at least one layer (2,3) is moveable relative to the or each other layer (3,2) to modulate said characteristics. A method according to Claim 1, in which said layers (2,3) are substantially planar or substantially non-planar and disposed substantially in parallel to one another and in which said relative movement is effected in a manner such as to retain said substantially parallel disposition, so as to modulate the frequency characteristics. A method according to Claim 1, in which said layers (2,3) are substantially planar or substantially non-planar and disposed substantially in parallel to one another and in which said relative movement is effected in a manner such as not to retain said substantially parallel disposition. A method according to Claim 3, in which for a substantially fixed frequency the direction of power transmission from the layered frequency selective surface assembly (1) is modulated by moving one said layer (2,3) relative to the or each other layer (3,2) in a substantially symmetric substantially non-parallel manner. A method according to Claim 2 or Claim 3, in which said relative movement is effected by pivoting or rotating one said layer (3) relative to the or each other layer (2) about an axis normal to said layers (2,3) so as to change the relationship of the array of electrically conductive elements (4) relative to the dielectric substrate layer (2). A method according to any one of claims 1 to 5, in which the characteristics are modulated at least in part by varying the spacing (X) between said layers (2,3). A method according to any one of claims 1 to 6, in which the relative movement between said layers (2,3) is brought about mechanically, electromechanically, hydraulically or pneumatically. A method according to Claim 7, in which the relative movement between said layers (2,3) is brought about by distortion of one layer (2,3). A layered frequency selective surface assembly having a dielectric substrate layer (2), and at least one layer (3) of electrically conductive elements (4) disposed in an array, which at least one conductive element layer (3) is superimposed with and spaced from the substrate layer (2), characterised by including means (5) for moving at least one layer (2,3) relative to the or each other layer (2,3) to modulate the power or frequency characteristics of the assembly (1). An assembly according to Claim 9, wherein said layers (2,3) are substantially planar or substantially non-planar in form and disposed substantially parallel to one another. An assembly according to Claim 10, wherein said moving means (5) are operable to retain said substantially parallel disposition. An assembly according to claim 10, wherein said moving means (5) are operable so as not to retain said substantially parallel disposition. An assembly according to claim 12, wherein said moving means (5) are operable to move one said layer (2,3) relative to the or each other layer (2,3) in a substantially symmetric substantially non-parallel manner. An assembly according to Claim 11 or Claim 12, wherein said moving means (5) are operable to pivot or rotate one said layer (3) relative to the or each other layer (2) about an axis normal to said layers (2,3). An assembly according to any one of Claims 9 to 14, wherein said moving means (5) are operable at least in part to vary the spacing (X) between said layers (2,3). An assembly according to Claim 15, wherein said moving means (5) is operable to distort the shape of one said layer (2,3) relative to the or each other layer (2,3). An assembly according to any one of Claims 9 to 16, wherein said moving means (5) are operable to bring about said relative movement mechanically, electromechanically, hydraulically or pneumatically.

Inventors: BUDD CHRISTOPHER BRITISH AEROS, BUDD, CHRISTOPHER, BRITISH AEROSPACE P.L.C.
Applicants: BRITISH AEROSPACE, BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1991
Main classification: H01Q15
IPC codes: H01Q3, G02B5, H01Q15, G02B7