GB2253743A - Reflectors for electromagnetic radiation - Google Patents

Abstract

This invention relates to a reflector for electro-magnetic radiation and in particular to such a reflector forming part of an antenna. The reflector comprises a plurality of superconductive elements 9 on a dielectric substrate 8, which elements each may be selectively electrically switched from a superconductive to a nonsuperconducting state to produce a derived pattern of reflecting areas. In this manner it is possible to construct a flat Fresnel aerial for focusing a beam of radiation which may be employed with different frequency bands, or in a second embodiment the elements may be arranged parallel and selective elements switched to provide a means of beam steering by constructive and destructive interference. In a third embodiment, the reflector includes separate sets of respectively vertical and horizontal strips in order to reflect vertically or horizontally polarised waves according to which net is arranged to be superconductive and which non- superconductive. <IMAGE>

Description

Reflectors for Electromagnetic Radiation This invention relates to reflectors for electromagnetic radiation, especially to such reflectors forming part of an antenna.

In antenna systems, particularly microwave systems, it is often necessary to focus incident radiation on to a receiver. This is often achieved by employing a parabolic reflecting dish which has as its focal point a feed horn which is connected to the transmitter/receiver by means of a waveguide. When the signal is transmitted via the feed horn, the parabolic reflector shapes the divergent output from the feed horn into the desired beam pattern.

It is also known to focus radiation onto a flat surface to a given point by means of a Fresnel antenna which is illustrated with reference to Figures 1 and 2 of the drawings. In its simplest form the Fresnel antenna comprises a feed horn 1 and reflector 2. The reflector comprises a substrate 3 of a lossy material, on to which annular elements 4 of reflective material are concentrically placed.

Figure 2 a cross section of Figure 1, illustrates the principle of operation of the Fresnel antenna. The path lengths 5 from the feed horn 1 to each adjacent annular member 4 starting at the central reflective element 6, increases by 1 wavelength of the frequency which it is desired to transmit. Because of this path difference between adjacent paths from the feed horn to respective elements, wavefronts strike the elements 4 in phase, and therefore due to constructive interference reflected wave 7 has wave fronts parallel to 3 providing a collimated beam in direction X. This is a known technique employing constructive inference and may also be applied to a signal arriving from a distant source to focus that signal at the feed horn 1.

The problem with the antenna described above is that it may only optimally be used to transmit or receive one frequency, for, as will be appreciated from Figure 2 if the wavelength is to be increased the separation of the annular elements 4 will also have to be increased otherwise the path lengths 5 would no longer differ by one wavelength from each adjacent one and therefore destructive interference would occur.

The invention provides a reflector for electromagnetic radiation including a plurality of superconductive elements and means for changing the state of at least one of the elements from superconducting to non-superconducting, in order to change the reflection characteristics.

Such a reflector, when used in an antenna, enables different transmission and/or reception characteristics to be produced. For example, the superconductive element may be in the form of concentric circles, and different annular regions may be activated to enable the antenna to be used with different wave lengths. Alternatively, the strips may be parallel if it is required to use the antenna for beam steering.

Superconductivity is the property exhibited by some materials when their temperature is reduced below a critical transition temperature Tc Below this temperature, the electrical resistance of the material becomes effectively zero and the material behaves as a perfect conductor. Thus, when the element is in its superconductive state, there is virtually no electrical resistance at its surface and it reflects incident electromagnetic radiation. Superconductivity occurs in certain metals when cooled to a temperature near to absolute zero but has also been discovered to exist in a number of compounds, for example various ceramic oxide compounds having much higher transition temperatures, the highest known at present being of the order of 140 Kelvin.

Once an element is in a superconductive state it is possible to switch it back to non-superconducting state by inducing a magnetic field which is dependent on temperature. This magnetic field may conveniently be generated by passing an electric current through the element greater than a critical current which induces a magnetic field greater than the critical field.

In the above manner once all the elements have been changed to their superconducting state the elements which it is desired to be non-superconducting ie non-reflective may, by passing an electric current through them, be converted back to their non-superconducting state.

The invention will now be described by way of example only with reference to Figures 3 to 7 of the accompanying drawings of which; Figure 3 illustrates apparatus in accordance with a first embodiment of the invention for producing a Fresnel aerial for use with different frequency bands; Figure 4 illustrates apparatus in accordance with a second embodiment of the invention for producing a steerable beam antenna; Figure 5 illustrates a possible steered beam antenna; Figure 6 illustrates principle of operation of the steered antenna of Figure 5; Figure 7 illustrates a reflector of electromagnetic waves embodying the invention; and Figure 7A is a side elevation of the apparatus of Figure 7.

With reference to Figure 3 there is shown a plan view of reflector in accordance with the invention for use with a feed horn 1 as illustrated in Figure 1. The reflector comprises a dielectric substrate 8 on which are placed concentric annular element 9 of a high temperature superconductor ceramic oxide material. The pattern may be placed on the substrate by thin film lithographic techniques enabling a very fine ring pattern to be achieved if this is required. Each annular element is discontinuous in the region C with one end of each element being connected to earth 10. The other end of each respective element being connected to a lead connecting it to control means 11.

In operation the elements are cooled below their critical temperature at which they become superconducting.

Cooling may conveniently be achieved by the use of for example liquid nitrogen applied to the substrate 8, either via channels within it, (not shown), or by a reservoir backing the dielectric (also not shown).

Once the device has been sufficiently cooled a current may be applied to selective elements 9 via control means 11 in order to induce a current in selective elements greater than the critical current such that selected elements revert to their nonsuperconducting state. The selection of elements will be dependant upon the frequency it is desired to use the reflector with, the positioning of the feed horn 1, in Figure 1, and the beam pattern desired.

In operation elements will be selected such that a series of regions will be obtained corresponding to areas 4 of Figure 1 so that the antenna functions as a Fresnel antenna previously described with reference to Figure 1.

However if it is required to alter the frequency of operation the control means 11 will select another predetermined group of elements 9 to give a reflective pattern necessary to give the required constructive interference pattern forming a Fresnel antenna for that frequency band.

Referring now to Figure 4 there is shown a plan view of a reflector in accordance with the second aspect of the invention which may be configured as illustrated in Figure 5. The reflector comprises a substrate 12 upon which are placed elements of high temperature superconductive ceramic oxide material 13 aligned parallel to each other.

One respective end of each element 13 is connected to ground 14, with the each other end respectively connected separately to control means 15.

In operation the reflector is again cooled to below its critical temperature such that all the elements 13 become superconducting. The control device 15 is then used to apply current to each element 13 which is desired to be nonsuperconducting such that a electric current is induced greater than critical current within those selected elements. By this means a pattern may be produced as illustrated in Figure 5 with the superconductive elements forming the regions 16.

These areas 16 provide means for steering a beam from a feed horn as shown. This is achieved by directing a signal from the feed horn onto the flat substrate 12, on which are positioned elements 13 which may be used to form reflective regions 16. Using the same principles as previously described with reference to Figure 2, it can be seen from Figure 6 that a set of wavefronts, A, incident on the reflective regions 16, shown in cross section, will be reflected such as to generate a wavefront 17 travelling in the direction Y. This wavefront is generated from the nodal points of the reflected waves B in the same manner as is the reflected wavefront in Figure 2.

From the above it can be seen that the selection of strips 13, which are to form reflective regions 16, influences the direction of the radiation from the surface 12, and that if it were possible to select other elements 13 to effectively move the reflective regions 16, so as to alter the spacing evenly between them then the beam can be steered. The reverse principle also applies, and incoming radiation may be focused in this manner from any selected direction on to the feed horn 11.

Referring now to Figure 9 there is illustrated an alternative reflector, a side elevation of which is shown in Figure 7A. This reflector comprises a thin film dielectric substrate 18 having thin film high temperature superconductive ceramic oxide elements 19 on a front surface arranged parallel and vertical, with similar elements 20 on its rear surface again in parallel but running horizontally. The elements 19 and 20 are all earthed at one of their respective ends and those on the front surface having their other ends connected to a power supply 21. The elements on the rear of the substrate are also connected to the power supply 21 but may be independently switched from those on the front surface.

In operation the elements 19, 20, and substrate 18, all being constructed by thin film techniques have little effect on the propagating wave unless the elements are in their superconductive/reflective state. If therefore the devices are cooled below the temperature where upon the elements become superconductive, then by applying a current through one set of elements it is possible to change the state back to a nonsuperconducting state where upon there will be effectively a set of parallel reflectors either in the vertical or horizontal depending which set of elements is selected.

Radiation will be reflected if the wave vector is parallel to the surface of a reflective element. It is therefore possible, by changing which set of elements is to be reflective, to select signals polarised in either the vertical or horizontal direction.

Claims (9)

Claims

1. A reflector for electromagnetic radiation including a plurality of superconductive elements and means for changing the state of at least one of the elements from superconducting to non-superconducting, in order to change the reflection characteristics.

2. A reflector as claimed in claim 1 wherein a plurality of substantially annular elements form concentric circles.

3. A reflector as claimed in claim 1 wherein the plurality of elements are arranged to form parallel strips.

4. A reflector as claimed in claims 2 or 3 wherein the state of the elements are arranged to be changed in groups.

5. A reflector as claimed in claims 1, 2, 3 or 4 wherein the switching means comprises means for inducing an electric current in selected elements of a sufficient magnitude to render them non-superconducting.

6. A reflector as claimed in claim 3 comprising two sets of parallel elements arranged co-planar on either side of a dielectric substrate and substantially perpendicular, to each other.

7. A reflector as claimed in any preceding claim wherein the elements comprises a high temperature ceramic oxide material.

8. An antenna including a reflector as claimed in any preceding claim.

9. A reflector as substantially hereinbefore described with reference to any of the Figures 3, 4 or 7.