EP1647070B1

EP1647070B1 - An antenna

Info

Publication number: EP1647070B1
Application number: EP04743481A
Authority: EP
Inventors: David Hayes; Richard Brooke Keeton
Original assignee: Plasma Antennas Ltd
Current assignee: Plasma Antennas Ltd
Priority date: 2003-07-22
Filing date: 2004-07-19
Publication date: 2008-03-19
Anticipated expiration: 2024-07-19
Also published as: WO2005013416A1; ATE389959T1; GB0317121D0; DE602004012565T2; DE602004012565D1; EP1647070A1

Abstract

An antenna comprising: (a) a pair of spaced apart conducting plates which are separated by a cavity containing a dielectric medium; (b) radio frequency feed means for coupling radio frequency energy into, or from, the cavity formed by the conducting plates; (c) generating means for forming a pattern of conducting elements between surfaces of the conducting plates and thereby to focus an electromagnetic wave front towards an edge of the cavity; (d) control means for influencing the pattern of conducting elements and thereby to direct or otherwise influence the electromagnetic wave front; and (e) redirecting means for redirecting the electromagnetic wave front to illuminate an associated radiating means.

Description

Field of the Invention

This invention relates to an antenna.

Description of the Prior Art

WO-02/089250-A1 discloses how the incorporation of distributed switches along a propagation path of an electromagnetic microstrip structure is able to be used to control the time delay of a signal through the structure. The switches are of a semiconductor medium which is subject to electrical carrier stimulation through optical illumination, or through electrical carrier injection by electrical means.
WO-02/01671 A1 discloses that electromagnetic energy may be reflected or absorbed by the control of conducting elements within a guiding medium. A microwave antenna is disclosed comprising essentially parallel conducting plates that enclose an intrinsic semiconductor medium. Conducting elements may be generated between wave guiding conductors. The conducting elements are used to simulate reflective or absorptive geometries. Alternatively, the conducting elements may be of a mechanical nature in the form of metallic filaments positioned in the semiconductor medium. Different patterns of the conducting elements are able to influence the directivity of the antenna. Generally, the conducting elements are produced by optical or electrical stimulation in selected regions of the semiconductor medium.

Brief Description of the Invention

In accordance with claim 1, there is provided an antenna comprising:

(a) a pair of spaced apart conducting plates which are separated by a cavity containing a dielectric medium;
(b) radio frequency feed means coupling radio frequency energy into, or from, the cavity formed by the conducting plates;
(c) generating means forming selectable patterns of conducting elements to either transparent, absorbing or reflecting states, the patterns of conducting elements being positioned between surfaces of the conducting plates;
(d) control means, controlling said patterns of conducting elements and thereby to direct or otherwise influence an electromagnetic wave front; and
(e) redirecting means redirecting the electromagnetic wave front to illuminate an associated radiating means characterized in that said conducting elements are either each a plasma generating PIN diode device or each a micro-actuator device.

The antenna of the present invention may be a compact, high efficiency, directable monolithic antenna which is appropriate for use throughout and beyond the microwave and millimetric radio spectrum. The antenna may be produced as a rugged, low cost, adaptive antenna with high coverage in elevation and steerable in azimuth. The antenna has widespread applications, including telecommunications, radar and tracking.
The antenna of the present invention may have the following advantageous characteristics.

• low power requirement
• high electromagnetic frequency limitation
• low added electromagnetic noise
• low temperature sensitivity
• low electromagnetic attenuation
• enhanced precision of control
• reduced spatial sidelobes
• broad elevation coverage
• steerable azimuthal coverage
• wide bandwidth performance
• compact monolithic design

The antenna may be one in which the conducting plates extend parallel to each other.
The dielectric medium may be a semiconductor dielectric medium, a gaseous dielectric medium, or a vacuum dielectric medium.
The antenna may be one in which the conducting elements are each a plasma generating PIN diode device, capable of changing its states from transparent to absorptive to reflective by increasing the current flow through the device and hence the level of carrier concentration from <10^13 carriers per cm^3, which is a mostly transparent state, to < 3 x 10^15 carriers per cm^3, which is a mostly absorptive state, and to >10^15 carriers per cm^3, which is a mostly reflective state.
The antenna may be one in which the conducting elements are each a micro-actuator device capable of introducing transparent, absorbing and reflective materials between the conducting plates.
The redirecting means may be an electromagnetic redirecting means. The redirecting means may be a reflective redirecting means or a refractive redirecting means.
The associated radiating means may be a horn, a lens, or a slotted waveguide reflector. Other types of associated radiating means may be employed.
The antenna may include electromagnetic means for producing a desired illumination pattern. The electromagnetic- means for producing the desired illumination pattern may be a reflector, a lens, or a horn.
The antenna may be one in which the control means enables the disposition of the conducting elements to be such as selectively to steer the direction of the reflected energy within the antenna.
The antenna may be one in which the pattern of the conducting elements is formed in electrical conductors or in electrical resistors.
The antenna may include a shaped dielectric medium at an external surface of the antenna, whereby electromagnetic coupling between the antenna and an external medium is enhanced.
The antenna may be one in which the radio frequency energy is of wavelengths characterised by electro-optical dimension rather than millimetric. The antenna may also be one in which the radio frequency energy is received rather than emitted by the antenna.
A plurality of the antennas of the present invention may be excited cooperatively in order to effect control of the direction of the emitted, or received, electromagnetic energy. A plurality of the antennas may be configured to enable sectoral or omni-directional operation.
The antenna may be such that it enables the means to illuminate, or receive from, a wide angle in elevation and with a controllable azimuthal angle. The antenna may afford a high efficiency, low sidelobe performance from a compact design without moving parts. The compact size of the antenna may be used to advantage in that the antenna may readily be incorporated into mobile and low maintenance applications. The antenna may be especially applicable for millimetric or terahertz technologies within the fields of telecommunications, radar, sensing and dynamic tracking.
In one embodiment, the antenna comprises a parallel assembly of two narrowly spaced conducting plates, across which conducting and absorbing elements may be generated and influenced. The elements may be in the form of locally injected or generated charge carriers, or alternatively mechanical elements. The disposition of the elements may be such as to reflect illuminating electromagnetic energy from a focal point source to a prismatic deflector. Energy refracted by the prismatic deflector radiates orthogonally to the guidance plane and illuminates a sector of a mechanical reflector. This enables an illuminating beam to be generated. By reciprocity, the antenna is able to receive electromagnetic energy and focus it upon the focal point.
The antenna may generate an electromagnetic wavefront that is characterised by a designated elevational coverage, and that may be electronically steered in azimuthal angle by a monolithic antenna having no moving mechanical parts. The antenna may operate both in a receiving mode and in a transmitting mode.
A plurality of the antennas of the present invention may be used together to enable azimuthal coverage extending through 360°.

Brief Description of the Drawings

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a basic antenna block;
Figure 2 is a side view of the antenna block shown in Figure 1;
Figure 3 is a plan view of the antenna block shown in Figure 1;
Figure 4 shows a polygonal prismatic structure with a single central feed;
Figure 5 shows a polygonal prismatic structure with multiple feeds;
Figure 6 shows a beam steering effect;
Figure 7 shows the equivalence between a generated parabolic reflector rotation and the angle of a reflected beam upon a front surface of a dielectric medium;
Figure 8 shows a point of total internal reflection and additionally energy lost to a main beam;
Figure 9 shows alternative configurations to achieve beam steering;
Figure 10 shows the interception of parabolic reflector surfaces as a function of rotation;
Figure 11 shows the use of a planar active reflectors;
Figure 12 shows the disposition of a plurality of active reflectors;
Figure 13 shows the separate or collective operation of multiple reflectors;
Figure 14 shows a design for coupling to a fixed reflector;
Figure 15 shows a design for directing output energy;
Figure 16 shows a design which couples energy directly from a parallel plate waveguide to freespace;
Figure 17 shows a design for enhanced coverage by use of multiple emitters; and
Figure 18 shows a means of providing enhanced uniformity of coverage.

Description of preferred embodiments

Referring to the drawings, the underlying principle of the present invention is illustrated in Figures 1 - 3. Figures 1 - 3 show how electromagnetic energy from a feed point 1 is radiated within the volume of a radiated beam 7. This volume will usually be a dielectric medium, although it may also be a gaseous medium. Selectable reflectors 2b are illustrated. These selectable reflectors 2b may comprise filamentary volumes of electrical carriers, or they may comprise mechanical reflectors such as MEMS Mechanical Electromagnetic Switches. The selectable reflectors 2b are configured to direct electromagnetic energy toward the illustrated forward edge of a parallel plate waveguide 2. At the termination of the waveguide 2, a directed feed 1 is employed to illuminate a metallic fixed reflector 6, thereby enabling selective volumetric illumination by the electromagnetic energy in the radiated beam 7. It will be appreciated that generally the efficiency of coupling energy between a dielectric medium and something else, such as free space, will be enhanced by the incorporation of an appropriate intermediary impedance matching medium.
Also shown in Figures 1 - 3 are element generation means in the form of a set to absorber 2a, element generating means in the form of a set to reflector 2b, and element control means 3. The element generating means can be either a plasma generating PIN diode or a micro-actuator device. The element generating means has transparent, absorbing and a reflective settings. In the case of the PIN diode, the change in state is effected simply by changing the current flow through the device to change the level of carrier concentration. That is below 10^13 carriers per cm^3, PIN device is transparent to microwaves. Between 10^13 carriers per cm^ 3 to 10^15 carriers per cm^3 the PIN device is absorbing. Above 10^15 the PIN device is reflective. For the micro-actuator case, transparent, absorbing the reflective materials are introduced between the plates electromechanically.
Extended angular coverage may be achieved by the use of multiple emitters. Figure 4 shows how a single feed point 8 may be directed by the selectable reflector 9 in such a configuration that a plurality of fixed reflectors 10 may be separately illuminated to enable broad coverage. In an alternative design, shown in Figure 5, a plurality of selectable reflectors 11 are used to illuminate fixed reflectors 12. By these means, the antenna provides simultaneous wide angle coverage. The plurality of fixed reflectors 12 may be four or eight in number, or any other suitable and appropriate number.
The controllable position of the selectable reflector facilitates steering of the electromagnetic energy. Figure 6 shows that as the curvature of the selectable reflector 13 is rotated about its axis the reflected energy 14 is able to be directed. The relationship between the angular rotation of the selectable reflector and the resultant beam steering at the transition to free-space may be readily anticipated through the application of classical physics as exemplified by Snell's laws of refraction.
In Figure 7, the steering angle relative to the front face is determined by Snell's law: $θ_{steer} = {Sin}^{- 1} (η . {Sinθ}_{rotate}) ≅ η . θ_{rotate} {for small θ}_{rotate}$

where η is the refractive index of the dielectric material between the parallel plates, and θ_steer and θ_rotate are the beam pointing angle (15) and the rotation angle of the parabola (16) respectively.
Figure 8 illustrates that the limitation in steering that may be achieved by the means of Figure 7, is reached at a so-called critical angle at which total internal reflection 17 occurs. A silicon medium would limit the steering by this means to approximately 17° from axis.
Advantageously, the boundaries of the dielectric medium may include absorptive regions to reduce the otherwise harmful effects of reflections in undesirable directions.
Figure 9 shows selective antenna patterns that may be employed to effect beam steering. In one illustrated case 18, reflectors are disposed with angular offsets 19 in such a manner that each locus of reflecting points is independent. The angle and focal length of each selectable pattern is different. Alternatively, Figure 9 also illustrates a design in which the selectable reflector patterns 20 have a common focal length and are rotated about a common point 21. This design has the advantage of being compact and efficient but-does require that some areas of reflection are common to all of the plurality of reflectors, as is illustrated in Figure 10 in the region 22.
The present invention may be advantageously constructed in the form shown in Figure 11. In this embodiment of the invention, a fixed reflector 23 illuminates a secondary selectable planar reflector 24. The secondary selectable planar reflector 24 is controlled such as to determine the direction of the emitted beam.
Multiple implementations of the present invention may advantageously be configured in the form of a polygon as illustrated generally in Figure 12. Emitting areas 25 may be excited independently or, as shown in Figure 13, in conjunction with neighbouring areas 26. Contiguous areas may be selected to operate in union, thereby effectively increasing the emitter effective area and thereby producing a reduced beamwidth. Individual phasing of the illuminations of the contiguous areas may be employed to enhance performance. Alternatively the adjacent area may be operated independently 27.
Figure 14 shows an example of a method of transferring energy from a planar waveguiding medium to an illuminating fixed reflector 28. A metallic transition is used to reflect energy orthogonally in either one direction 29 toward an offset parabola, or alternatively in both orthogonal directions 30 to illuminate a symmetrical centre fed reflector.
Figure 15 shows a dual plate waveguide with offset reflectors (31) providing independent time delays at a feed point 33. The resultant wavefront may by these means be electrically steered, as illustrated at 32. The illustrated concept may have application to so-called monopulse radar systems.
Figure 16 illustrates schematically a method by which a mechanical means may be employed to directly transfer electromagnetic energy from a parallel plate waveguide 34 to freespace 35.
A plurality of the antennas of the present invention may be operated in unison to enable broad spatial coverage and enhanced signal strength. Figure 17 shows by way of example three adjacent emitters 36. Improved uniformity of cover may be obtained by stacking overlapped modules as illustrated in Figure 18.
It will be appreciated from the description of the invention with reference to the accompanying drawings that an electromagnetic antenna may be designed and implemented for the transmission and reception of millimetric radio waves. The present invention is especially directed at miniaturised monolithics steerable antennas applicable throughout the presently exploited microwave spectrum and extendable to frequencies well in excess of 100 GHz. The antenna may be used in telephony, data links, mobile telecommunications and radar systems.
It is furthermore to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Otherwise known techniques of implementation have not generally been included in the description. Thus, for example, various components of the antenna as shown in the drawings need not be in the illustrated shapes or in the illustrated configurations. Other shapes and configurations may be employed. The mechanical means to induce and control the active media as described in the drawings have been given by way of example and not as a limitation of methods that may be used. Descriptions and details of well known components and techniques have not been described where these are not required.

Claims

An antenna comprising:
(a) a pair of spaced apart conducting plates (2) which are separated by a cavity containing a dielectric medium;

(b) radio frequency feed means (1) coupling radio frequency energy into, or from, the cavity formed by the conducting plates;

(c) generating means (2a, 2b) forming selectable patterns of conducting elements to either transparent, absorbing or reflecting states, the patterns of conducting elements being positioned between surfaces of the conducting plates and thereby to focus an electromagnetic wave front towards an edge of the cavity;

(d) control means (3) controlling said patterns of conducting elements and thereby to direct or otherwise influence an electromagnetic wave front; and

(e) redirecting means (5) redirecting the electromagnetic wave front to illuminate an associated radiating means (6) characterized in that said conducting elements are either each a plasma generating PIN diode device or each a micro-actuator device.
An antenna according to claim 1 in which the conducting plates extend parallel to each other.
An antenna according to claim 1 or claim 2 in which the dielectric medium is a semiconductor dielectric medium, a gaseous dielectric medium, or a vacuum dielectric medium.
An antenna according to any one of the preceding claims in which the conducting elements are each a plasma generating PIN diode device, capable of changing its states from transparent to absorptive to reflective by increasing the current flow through the device and hence the level of carrier concentration from <10^13 carriers per cm^3, which is a mostly transparent state, to <3 x 10^15 carriers per cm^3, which is a mostly absorptive state, and to >10^15 carriers per cm^3, which is a mostly reflective state.
An antenna according to any one of claims 1 to 3 in which the conducting elements are each a micro-actuator device capable of introducing transparent, absorbing and reflective materials between the conducting plates.
An antenna according to any one of the preceding claims in which the redirecting means is a reflective redirecting means or a refractive redirecting means.
An antenna according to any one of the preceding claims in which the associated radiating means is a horn, a lens, or a slotted waveguide reflector.
An antenna according to any one of the preceding claims and including electromagnetic means for producing a desired illumination pattern.
An antenna according to claim 8 in which the electromagnetic means for producing the desired illumination pattern is a reflector, a lens, or a horn.
An antenna according to any one of the preceding claims in which the control means enables the disposition of the conducting elements to be such as selectively to steer the direction of the reflected energy within the antenna.
An antenna according to any one of the preceding claims in which the pattern of the conducting elements is formed in electrical conductors or in electrical resistors.
An antenna according to any one of the preceding claims and including a shaped dielectric medium at an external surface of the antenna, whereby electromagnetic coupling between the antenna and an external medium is enhanced.
An antenna according to any one of the preceding claims in which the radio frequency energy is of wavelengths characterised by electro-optical dimension rather than millimetric.
An antenna according to any one of the preceding claims in which the radio frequency energy is received rather than emitted by the antenna.