GB2406718A - Antenna beam steering using a Fresnel zone plate with controllable conductivity - Google Patents

Abstract

A method or means of steering an antenna beam comprises controlling the conductivity of selected portions of a plate member 40 to provide a Fresnel pattern 42, 44 on the said plate 40 which is adjustable to steer a radiation beam from an antenna 46. The conductivity portions 42 may be formed by a coating layer of photoconductive material or an array of photo-sensitive elements such as photo-diodes, photo-transistors or photo-cells. The photo-sensitive element or elements may be controlled by an optical projector 48 arranged to project the required Fresnel pattern on to the plate 40. Alternatively the said portions may be a grid array of transistors or p-i-n diodes which can be controlled via electrical signals to create the desired Fresnel pattern. The plate 40 may be flat, semi-spherical, conical or pyramidal. The arrangement is intended to provide a cheap, reliable and rapid antenna beam steering system.

Description

METHOD ANT) APPARATUS FOR PROVIDING BEAM SrfEERIN(J IN AN ANTENNA [quell

The present invention relates to methods and apparatus for beam steering an antenna. More particularly, it provides methods and apparatus lor providing reliable, rapid control of beam direction at relatively low cost.

10002J It is often required to provide beam steering in an antenna, that is, to be able to controllably direct the direction of peak power transmission, or peak reception sensitivity, to a required direction. Examples of known methods for doing this include mechanically displacing the antenna, or phase control ol arrayed antennas (for example, as discussed in 1() our UK patent application GB 2372378). Both of these known methods have drawbacks.

10003] The mechanical method suffers from reliability problems. Maintenance of the moving mechanism is required, and the mechanism may be susceptible to jamming due to the presence of foreign objects such as ice, birds' nests, or twigs from nearby trees. A mechanical alignment device also requires a relatively long time to move from one orientation to another.

100041 Phase control in a phased array antenna allows almost instantaneous switching from one bcan1 direction to another, since all direction control is electronic. rlhere are no moving parts, and so the drawbacks of the mechanical steering methods are avoided.

5] However, a phased array antenna requires a large number ol antenna elements, each having a specific phase delay applied. Each element must accordingly be provided with its own receiver and phase shifter, leading to a relatively high cost of the antetma assembly taken as a whole.

10006] The use of I7resnel zone plates for controlling the directional sensitivity of an antenna to radio signals such as signals Irons communications satellites is well known in the art. 1 resnel zone plates may be used as an inexpensive and convenient substitute for parabolic dishes. In "I,argc Reflecting Cone Plates lor Satellite Signal Reception", by if. M. B. Wright, published in IRE Colloquium on Mechanical Aspect of Antenna L)csign, Digest No. 1989/67, 24 April 1989, the characteristics of such l;resnel zone plates and some design aspects were discussed. A copy ol this paper is filed with the present application, and this paper is incorporated herein by refcrcnce.

10007J l;ig. 1A illustrates a simple Fresncl zone plate 10. Rings 12 ot reflecting or absorbing material are mounted on a substrate 14, which is transparent at the wavelength of interest. In the illustrated example, the material ol the rings 12 reflects incident radio signals. The gaps between the rings accordingly act as ring-shaped emitters in the forward direction, and the rings themselves act as ring-shaped emitters in the reverse (reflected) direction. l'he signals emitted by the rings interfere with each other, causing constructive and destructive interference at respective locations.

l 8l As is known in the art, and illustrated in Fig. 1B, the Fresnel zone plate l0 is designed to have a selected local point 1-', displaced along a focal axis A by a focal length /: The rings 12 are designed such that corresponding locations on the inner and outer radii of the rings are distanced loom the focal point F by a distance greater or less than the focal length f by an integral number of half wavelengths. In the illustrated example, the focal axis A is perpendicular to the Fresnel zone plate 10. The rings 12 are concentric circles. In such an example, the radius xp of the pith ring may be expressed as xp = sort.'+ (p..12)2), where f is the focal length and is the wavelength of interest.

100091 As illustrated in Figs. 2A-2B, a Fresnel zone plate 20 need not be perpendicular to the focal axis A. In this case, the rings 22 are ellipses, displaced along a mutual major axis.

The elliptical rings still obey the design rule that corresponding locations on the rings are distanced from the local point F by a distance differing from the focal length f by an integral number of half wavelengths.

l0010l Fig. 3 shows a reflective antenna using a Fresnel zone plate. The zone plate 30 may be as described with reference to Figs. I and 2, with reflective or absorbing rings 32.

Again, the signals re-emitted or reflected froin the rings will perform constructive and destructive interference, and will focus a signil'icant proportion of the incoming energy at the wavelength of interest at a focal point I;. By placing a receiver such as a horn antenna 36, or a dipole antenna, at the focal point F. the signal ol interest may be received. The portion ol'the incident signal that passes through the transparent portions 34 of the Fresnel zone plate 30 meets a reflector 40 This reflector is placed at a distance of one quarter of the wavelength of interest (/4) behind the I;resnel zone plate 30. Most of the signal reaching the reflector is accordingly reflected back through the transparent rings 34 of the Fresnel zone plate with a one half wavelength additional path delay, bringing it into phase with the reflected or reetnitted signal which was incident onto the reflecting or absorbing 3() rings 32. The reflected signal accordingly participates in the constructive and destructive interf'crence to reinforce the signal concentrated at the focal point F. [00111 Known l;resnel zone plates provide only a fixed directional beam, the direction being fixed by the shape of the rings and the orientation of the l;'resnel zone plates. - 3

Applications of the Fresnel zone plate have to date served in replacing a parabolic dish antenna with a lower-cost lresnel zone plate, while control of the beam direction has relied on mechanical orientation of the Fresnel zone plate.

l0012l '['here is accordingly a need lor a relatively low cost beam steering method and apparatus for an antenna.

100131 The present invention addresses this need, and accordingly provides methods and apparatus as recited in the appended claims.

100141 In particular, the invention provides apparatus for antenna beam steering comprising a Fresnel zone plate, itself comprising absorbing or reflective rings alternating with rings which are transparent at the wavelength of' interest, placed at a selected focal distance (6 Prom an antenna. The rings are embodied as conductive regions on a plate of controllable conductivity. 'lithe apparatus further comprises means for controlling the conductivity of selected regions of the plate of controllable conductivity.

l0015l The plate ol'controllable conductivity may comprise a grid array of photodetectors.

100161 The photodetectors may comprise at least one of: photodiodes, phototransistors and photoconduetive cells.

100171 'l'he plate of controllable conductivity may comprise a coating of a photoconduetive material.

10018] The means for controlling the conductivity of selected regions of the plate of controllable conductivity may comprise an optical projector arranged to project an image of the required sequence of rings onto the plate of controllable conductivity.

lOOI9l The plate of controllable conductivity may comprise a plurality of elements of electrically controllable eondtietivity. The elements may be transistors arranged to receive a gate bias signal thereby to control their eondtctivity. Alternatively, the elements may be p-i-n diodes arranged to receive a bias current thereby to control their conductivity.

100201'1'he elements may be serially connected in mutually perpendicular rows and columns. 'I'he rows and columns may be not connected together.

100211 The elements may be serially connected in mutually perpendicular lines. The lines may be not connected together.

100221 The present invention also provides a method of beam steering an antenna, comprising the steps of placing the antenna at a selected focal distance (f) from a Fresnel zone plate comprising absorbing or reflective rings alternating with rings which are transparent at the wavelength of interest; and receiving signals focused by the Fresnel zone - 4 plate in a known manner. The method further comprises: constructing the Fresnel zone plate of a plate ol' controllable conductivity; controlling the conductivity of selected portions of tile plate ol' controllable conductivity to form the rings; and controlling the conductivity of selected portions of the plate of controllable conductivity to vary the location and form of the rings, thereby to steer the beam of the antenna.

10023]'1'he step of controlling the conductivity of selected regions of the plate ol' controllable conductivity may comprise projecting an optical image of the required sequence of rings onto the plate of controllable conductivity [00241 The step of controlling the conductivity of selected regions of the plate of controllable conductivity may comprise electrically controlling elements of controllable conductivity, in a pattern corresponding to the required pattern ol'rings. The elements of controllable conductivity may be transistors, and the step of controlling their conductivity may comprise providing respective gate bias signals. Alternatively, the elements of controllable conductivity are p-i-n diodes, and the step of controlling their conductivity may comprise providing a bias current.

10025] The above, and further, objects, advantages and characteristics of the present invention are described in further detail in respect of certain particular embodiments, given by way of example only, in the following description and in the accompanying drawings, wherein Figs. 1-3 show Fresnel zone plate antennas of the prior art; Figs. 4A-4B show apparatus for antenna beam steering according to an embodiment of the present invention.

Fig. 5 shows detail of elements of controllable conductivity, used in a Fresnel zone plate according to an embodiment of the invention; Figs. 6A6B show models of a controllable liresnel zone plate according to the present invention; Fig. 7 shows a polar diagram of the directional gain of an antenna comprising the Fresnel zone plate of Figs 6A-6B, using a dipole antenna; Fig. shows a polar diagram of the directional gain of an antenna comprising the E'resnel zone plate of Figs 6A-6B, using 3-element Yagi antenna; Fig. 9 shows an embodiment of tile present itlventioil, wherein a Fresnel zone plate is formed on a surface of a sphere; and Fig. 10 shows a polar diagram of the directional gain of an antenna comprising the Fresnel zone plate of Fig 9.

100261 Fig. 4A illustrates apparatus for antenna beam steering according to an embodiment of the present invention. Fresnel zone plate 40 comprises conductive rings 42 alternating with rings 44 which are non- conductive and transparent at the wavelength (a) of interest.

An antenna, such as horn antenna 46, is placed at a selected focal distance f' from the Fresne] zone plate 40. According to an aspect of the present invention, the rings 42, 44 are embodied as respectively conductive and non-conductive regions on a plate of controllable conductivity. The apparatus further comprises means 48 for controlling the conductivity of selected regions of the plate of controllable conductivity.

100271 In the embodiment illustrated in Fig. 4A, the plate of controllable conductivity is photosensitive. An optical projector 48 is provided, and projects a pattern of rings onto the plate of controllable conductivity. As shown in Fig. 4A, the projector is preferably not aligned with the focal axis of the Fresnel zone plate 40. The image projected by the projector 4X will need to be distorted such that the required pattern of rings 42, 44 is produced on the plate of' controllable conductivity, which is inclined to the axis of projection.

100281 The plate of controllable conductivity may comprise a grid array of photodetectors, such as photodiodes, phototransistors or pht'toconductive cells. Alternatively, the plate may comprise a relatively uniform coating of a photoconductive material. In each case, the material or arrangement used should be substantially transparent to the wavelength of interest in either high or low light levels, and electrically conductive in the other condition.

An alternative, if less efficient, embodiment has electrically conductive rings formed on a plate of material which is an absorber at the wavelength of interest. 'I'he resulting zone plate would need to be used in a reflective arrangement.

100291 For example, the plate of controllable conductivity may comprise a sheet of material which is transparent at the wavelength of interest, overlain with a matrix of photodiodes connected as shown in Fig 4B. Photodiodes have a relatively low resistance at RF frequencies when they are exposed to light, but a relatively high dark resistance.

Accordingly, zones of photodiodes in illuminated areas will respond as conductive zones.

For example, refed ing to L'ig. 4A, a highly illuminated ring 42 will be relatively conductive. This will act as the absorbing and re-emitting material discussed above. 'I'he - 6 dark rings 44 will not be conductive, and Will be transparent to the incident signal. The signal incident on these transparent regions will not be focused. At RF frequencies, photodiocies may be conductive in both directions, relying on their junction capacitance to supply short-term reverse current. The type of' photodiode may be selected to optimise this performance. For example, p-in diocies have a larger junction charge storage capacity than most other photodiodes. Selection of p-i-n diodes may ensure substantially symmetrical AC resistance. Alternatively, or in addition, adjacent rows and columns of photodiodes could be arranged in opposite directions, as shown, to provide improved symmetry of operation throughout each cycle of the incoming signal waveform. Alternatively, and t'or similar reasons, adjacent photodiodes in each row and column may be arranged in opposite directions. Although Fig. 4B shows the matrix arranged in perpendicular rows and columns, other arrangements may he used, such as triangular or hexagonal cells.

[00301 In an alternative embodiment, the plate of controllable conductivity may comprise an array of electrically controllable elements. For example, transistors such as field et't'ect or bipolar transistors. These may be controlled by supplying a gate bias voltage or base current to transistors in regions of the plate of controllable conductivity where high conductivity is desired. In the remainder of the plate, no gate or base bias will be provided, and those regions will remain non-eonduetive. Alternatively, other electrically controlled devices may be provided. For example, the p-i-n diodes referred to above may be controlled electrically. If a constant DC current, for example I MA, is passed through the p- i-n diode, it becomes conductive to RF signals.

[00311 Fig. 5 shows an example of such an electrically controlled plate of controllable conductivity, in schematic form. In this embociiment, the rows and columns of series connected diodes are not corrected together. The locus of a required conductive ring 42 is shown. According to this embodiment of the invention, regions of the plate of controllable conductivity lying within the locus 42 should be rendered conductive, while the other regions shown in Fig. 5 should remain non-conductive. An electric current is injected through row addressing line 61, at one edge of a required conductive zone. An equivalent current is drawn from row adciressing line 62, at the opposite edge of the required conductive zone. Similarly, An electric current is injected through row addressing line 63, at one edge of a required conductive zone An equivalent current Is drawn from row addressing line 64, at the opposite edge of the required eontluctive zone. An electric current may also be injected through column line 71, at one edge of a required conductive zone.

An equivalent current is drawn from row addressing line 72, at the opposite edge of the required conductive zone. By repeating this for each row and column covered by the required conductive zone 42, the conductive zone may el'fectively be formed.

100321 In the illustration shown in Fig. 5, it may be appreciated that the limits of conductivity are defined by the location of non-conductive diodes. It may accordingly not be possible for the conductive regions ol'the p-i-n diode matrix to align exactly with the required locus of the conductive ring. However, in practice, it will be preferable to have the rows and columns of diode spaced at a distance ol'no more than one quarter of the wavelength of interest; about 2.5em for a 3GHz signal. It would be yet more preferable to have the diodes spaced at an even closer spacing. In that way, the conductivity of the ring - ' 43 may be increased, and the defined conductive region may be made to more accurately -r align with the desired shape t:,fthe conductive rings.

100331 In an alternative to the embodiments illustrated with reference to Fig 4A, a reflector may be placed at a distance of one quarter the wavelength of interest behind the I7resnel zone antenna, as shown in Fig. 3. '['his may be applied to either photosensitive l4'resnel zone plates, by placing the projector 48 on the 'active' side of the zone plate. A reflector may also be employed in conjunction with the electrically controlled plate such as shown in Fig. 5. In this ease, the row and column addressing lines 61-64; 71, 72 may be led through the reflector, in a direction substantially perpendicular to both the Fresnel zone plate and the reflcetor. Corresponding small through holes would need to be provided in the relleetor, but these should not cause a significant degradation in the effectiveness of the reflector, as their diameter should be significantly less than one quarter of the wavelength of interest.

[00341 In certain embodiments, it may be necessary to reflect, or absorb and re-emit only radiation of a certain polarization. In this case, the diode matrix of Figs. 4B and 5 may be simplified to include only rows or columns, aligned with the direction of the polarisation of interest.

100351 Accordingly, one aspect of the invention provides an antenna having a controllable Fresnel zone plate for beam steeritlg. As may be appreciated, the electrical control method described above allows the present invention to provide a plate of controllable conductivity of which the conductive and notl-conductive zones may be adjusted very rapidly.

Similarly, the optical control method illustrated in Fig. 4A may include a computer controlled image generator providing an hnage to the projector 48. The computer - 8 controlled image generator may be controlled to rapidly change the image projected onto the plate of controllable conductivity, thereby allowing very rapid antenna beam steering, with no moving parts. Furthermore, the beam steering antenna of the present invention requires only a single antenna receiver such as a commercially available horn antenna 46, or a dipole antenna. No phase shifting elements are required, so the financial cost of the antenna is reduced, and control of the beam steering is simplified. A computer may be programmed to calculate the required shape of the conductive and non- conductive rings ol' the plate of controllable conductivity. Such computer Nay then provide a corresponding image to projector 48, or may send corresponding signals to an electrically controllable plate of controllable conductivity, such as that illustrated in Fig. 5, to almost instantaneously change the beam steering direction. With computer control of a projector, an image such as that shown in Fig. 4A may be caused to flicker in a certain way, thereby allowing serial data to be provided to an array of more sophisticated control elements.

10036] lithe following description relates to modelling results for examples of the present 1 5 invention.

10037] Fig. 6A illustrates a modelled Fresnel zone plate, according to an embodiment of the present invention. 'I'he model defines the conductive regions 82 as a series of parallel conductive lines. As the conductive regions 82 are conductive in one direction only, this modelled Fresnel zone plate will only focus incident signals of the corresponding polarization. The Fresnel zone plate of Fig. 6A is arranged for use in a plane perpendicular to the focal axis if the antenna.

10()381 1 ig. 6B illustrates the same Fresnel zone plate, controlled according to an embodiment of the present invention, to steer the beam of maximum sensitivity (gain) to a direction offset from the focal axis of the antenna by l 5 .

9] Fig. 7 shows a polar diagran1 of the directional gain (sensitivity) of an antenna comprising the E4'resnel zone plate illustrated in Figs. 6A-6B together with a dipole antenna feed element, at a focal point 2.5 wavelengths from the zone plate. As can be clearly seen, the direction of maximum gain 88 has been effectively steered through 15 from its nominal orientation 86 by control of the Fresnel zone plate Prom the situation shown in Idg.

6A to the situation shown in Fig. 6B, without significant loss in gain. Tile sidelobes in the direction -60 to -90 have increased, but these are still about lOdB down on the peak antenna gain in direction 15 .

100401 Fig. 8 illustrates corresponding modellcd results for the l;rcsnel zone plate of Figs. 6A-6F\, used with a 3-element Yagi iced antenna. Tile peak gain using the Fresnel zone plate as illustrated in Fig. 6A was 21.8dRi' while the peak gain when steered to 15 was 22dFli. Once again, the sidelobes have increased, but are still about 15dB down on the peak gainatl5 .

l()()41l The modelling referred to above included parameter values, such as sheet or line resistance similar to those that could be expected with the embodiments of the invention described earlier.

2] The focal axis is typically and preferably, aligned with the peak gain ol' the chosen antenna clement.

[00431 While the preceding description has referred to the Fresnel zone plate as being formed on a planar plate of controllable conductivity, the present invention may also be applied to non-planar plates of controllable conductivity. As an example, Fig. 9 shows a single-zone Fresnel zone plate according to an embodiment of the present invention, formed on a spherical surl'ace. In the illustrated example, a spherical, single zone Fresnel zone plate 122 according to an embodiment of the invention was provided with a dipole antenna element 124. 'I'hc gain of this arrangement was found to be 12dBi. By forming a similar spherical Fresnel zone antenna with two rings, a gain of 14dBi could be formed. An advantage of forming the zone plate on a spherical surface is that the pattern of rings need not change when the beam is steered, but need only be moved across the surface of the sphere, assuming that the antenna element 124 is located at the centre of the sphere.

100441 Fig. 10 shows modelled results tor the antenna of Fig. 9 operating with a 2.5 wavelength radius. The gain plot 132 shows the gain ol' the antenna clement 124 alone, without a Fresnel zone plate. Ciain plot 134 shows the gain of the antenna illustrated in Fig. 9. Peak forward gain is increased, and side lobes arc about 17 dB down in directions +/- 7so.

10()451 Other shapes of Fresnel zone plate could be used, such as conical or pyramidal shapes. The shapes of the conductive rings would need to be amended to provide the required functionality. -

Claims (16)

1. Apparatus for antenna beam steering comprising a T7resnel zone plate (40), comprising absorbing or reflective rings (44) alternating with rings (42) which are transparent at the wavelength ot interest, placed at a selected tonal distance (I) lrom an antenna (46), eharacterised in that the rings are embodied as conductive regions on a plate of controllable conductivity; the apparatus further comprising means (46) for controlling the conductivity of selected regions of the plate of controllable conductivity.

2. Apparatus according to claim 1 wherein the plate of controllable conductivity comprises a grid array ot photodetectors.

3. Apparatus according to claim 2 wherein the photodetectors comprise at least one of photodiodes, phototransistors and photoeonduetive cells.

4. Apparatus according to claim I wherein the plate of controllable eonduetivity comprises a coating of a photoeonduetive material.

5. Apparatus according to any preceding claim wherein the means for controlling the eonduetivity of selected regions of the plate of controllable eonduetivity comprises an optical projector (46) arranged to project an image of the required sequence of rings onto the plate of controllable eonduetivity.

6. Apparatus according to claim I wherein the plate of controllable eonduetivity comprises a plurality of elements of electrically controllable eonduetivity.

7. Apparatus according to claim 6 wherein the elements are transistors arranged to receive a gate bias signal thereby to control their eonduetivity.

8. Apparatus according to claim 6 wherein the elements are p-i-n diodes arranged to receive a bias current thereby to control their conductivity. - 1 1

9. Apparatus according to any of claims 6-8 wherein the elements are serially connected in mutually perpendicular rows and columns, but wherein the rows and columns arc not comlected together.

10. Apparatus according to any of claims 6-8 wherein the elements are serially connected in mutually perpendicular lines, but wherein the lines are not connected together.

11. A method of beam steering an antenna (46), comprising the steps of placing the antenna at a selected focal distance (1) from a Fresnel zone plate (40) comprising absorbing or reflective rings (42) alternating with rings (44) which are transparent at the wavelength of interest; and receiving signals l'ocused by the Fresnel zone plate in a known manner; characterised in that the method further comprises: - constructing the Fresnel zone plate of a plate of controllable conductivity; - controlling (48) the conductivity of selected portions of the plate al' controllable conductivity to form the rings; and - controlling the conductivity of selected portions of the plate of controllable conductivity to vary the location and l'onn of the rings, thereby to steer the beam of the antenna.

12. A method according to claini 1 I wherein the step of controlling the conductivity of selected regions of the plate of controllable conductivity comprises projecting an optical image of the required sequence of rings onto the plate of controllable conductivity.

13. A method according to claim I I wherein the step of controlling the conductivity of selected regions of the plate of controllable conductivity comprises electrically controlling elements of controllable conductivity, in a pattern corresponding to the required pattern of rings.

14. A method according to claim 13 wherein the elements of controllable conductivity are transistors' and the step of controlling their conductivity comprises providing respective gate bias signals. - 12

15. A method according to claim 13 wherein the elements of controllable conductivity are p-i-n diodes, and the step ol controlling their conductivity comprises providing a bias current.

16. An apparatus or a method substantially as described, and/or as illustrated in the accompanying drawings.