GB2278020A

GB2278020A - Antenna

Info

Publication number: GB2278020A
Application number: GB9307000A
Authority: GB
Inventors: Nigel Frewin
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-04-02
Filing date: 1993-04-02
Publication date: 1994-11-16
Also published as: GB9307000D0

Abstract

An antenna comprises a laminate 17 of at least two flat layers 24 of dielectric material, and a collector or feeder device 20 in front of the laminate 17 for receiving from or feeding to the laminate 17 electromagnetic radiation. A particular application is a receiving antenna for signals from a communications satellite. Radiation falling on the laminate 17 is focussed onto the collector 20 by a series of nested bands of electrically conducting material 25 positioned on the front face of each dielectric layer 24. At the rear of the laminate 17, a continuous layer of electrically conducting material 26 presents a forwardly facing reflecting surface. The laminate 17 is rotatable about an axis perpendicular to the laminate, to vary the direction from which radiation is received, and the collector 20 can be adjusted along a predetermined path to a position at a required focus. <IMAGE>

Description

ANTENNA The present invention relates to an antenna for receiving or transmitting electromagnetic radiation, and is concerned in particular, but not exclusively, with a flat frequency selective reflector antenna for receiving electromagnetic communications signals at microwave frequencies from communication satellites, the antenna being adapted to be secured to an exterior building structure such as a wall.

Of all the antenna systems known the parabolic dish enjoys greatest popularity owing to its high gain characteristics. However, these antennas tend to be deep, bulky, heavy, are difficult to construct and expensive to manufacture, are subject to considerable wind loading and are often obtrusive to install because of bulk and directionality.

It is an object of the present invention to provide the same advantages as the parabolic dish, while overcoming most of its disadvantages, i.e. to provide high gain and a narrow flat profile, and to provide a device which is light and easy to construct.

According to the present invention there is provided an antenna for receiving or transmitting electromagnetic radiation comprising a laminate of at least two flat layers of dielectric material, and a collector or feeder device spaced from and positioned in front of the laminate for receiving from or feeding to the laminate electromagnetic radiation, each layer of dielectric material having a series of flat bands of electrically conducting material, the flat bands being curved in plan view and being arranged in a nested series of bands of increasing size of perimeter, and the rear of the laminate having a continuous layer of electrically conducting material, the configuration and spacing of the bands being such that the laminate acts as a reflector, for concentrating at the collector or feeder device radiation falling on the laminate, or for transmitting from the laminate radiation fed from the collector or feeder device.

The bands may have the shape of whole or truncated circular or ellipsoidal rings. Other shapes of bands may be used.

Preferably the configuration and spacing of the bands are such as to transform a plane wave front of electromagnetic radiation of a particular frequency or narrow band of frequencies incident on the front of the laminate from a predetermined direction relative to the plane of the laminate, into a converging spherical wave front such as to focus the energy at the collector or feeder device.

In some preferred arrangements, it is arranged that the outer perimeter of each band of a dielectric layer is positioned to be coincident in plan view with the inner perimeter of the next larger band of the next forward layer of dielectric material.

In some forms of the invention, the focusing is achieved by arranging that the path difference between a fixed reference path and a path which includes a reflection at the inner perimeter of any conducting band, is zero or equal to an integral number of whole wavelengths of an electromagnetic signal to be received or transmitted.

Each path can be considered as finishing at the collector or feeder device and starting perpendicularly at an arbitrary wave front of the incident radiation.

It is preferred that the laminate comprises more than two said dielectric layers each having a set of bands of electrically conducting material. In one convenient preferred form, the laminate consists of four said dielectric layers each having a set of bands of electrically conducting material.

Preferably each layer of dielectric material has its series of bands of electrically conducting material on the front surface of the layer of dielectric material.

The electrically conducting bands can be formed by attaching metallic foils or other conductive materials to the dielectric material. However it is particularly preferred that the said electrically conducting bands are produced by silk screening metallic conductive ink onto the dielectric material.

The rear of the laminate may be made electrically conductive for example by depositing an appropriate material directly onto the rear surface or by attaching to the rear surface of the laminate a further lamina treated to produce the electrically conductive layer.

Whereas conventional parabolic dishes are made directional by moving the entire dish to the appropriate direction, it is a particularly preferred feature of the present invention that there may be provided means for rotating the laminate about an axis perpendicular to the laminate to vary the direction from which or to which radiation is received or transmitted. Also preferably there is provided means for varying the position of the collector or feeder device relative to the laminate along a predetermined path so as to position the device at a focus of radiation to be received or transmitted.

The invention finds particular application where the antenna comprises an antenna for receiving an electromagnetic communications signal transmitted by a communications satellite, the laminate being adapted to be secured to a flat exterior building structure. In such an arrangement there may be provided means for rotating the laminate about an axis perpendicular to the laminate to vary the direction from which radiation is received, and means for adjusting the position of the collector device relative to the laminate along a predetermined path so as to position the collector device at a focus of radiation to be received.

Thus in a preferred form of the invention there may be provided a flat satellite antenna designed to present a minimal profile. The antenna may be mounted flat onto any surface facing within 45" of the direction of a satellite of suitable signal strength. The antenna can achieve direction by rotation on the surface it is fixed to.

Suitable camouflaging materials can be used to coat the front of the antenna, minimalising its visual impact.

Alternatively, under certain circumstances, the flat surface could be used for a sign, picture, or other visual display or even mirrored to reflect immediate surrounding environment. Architectural cladding could be used or replicated as suitable disguising covers, e.g. vacuum formed brick replication.

In such a preferred form, the laminate acts as a reflector which concentrates electromagnetic wave energy falling on its upper, forward, surface at a point in front of the device where it may be collected and processed. The antenna is designed in its preferred form for high frequencies, e.g. 1 gigahertz and beyond, and is frequency selective.

Other applications of an antenna embodying the invention include use as a frequency selective reflective telescope such as for radio astronomy.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross section through a known flat reflecting element achieving a focusing action based on a Fresnel zone plate; Figure 2 is a cross section through another known zoned reflector antenna using a plurality of paraboloid reflecting surfaces; Figure 3 is a diagrammatic side view of an antenna embodying the invention; Figure 4 is a front view of the antenna shown in Figure 3; and Figure 5 is a diagrammatic representation of the layers of a laminate antenna embodying the invention; Figure 6 is a diagrammatic cross section through the centre of a laminate antenna embodying the invention, showing the relationship between succeeding conducting rings on layers of the laminate; Figure 7 is an explanatory cross section through part of a reflecting laminate antenna embodying the invention, showing the relationship between path lengths travelled by radiation signals at different parts of the laminate; Figures 8, 9 and 10 show examples of patterns of rings on a laminate antenna embodying the invention; and Figure 11 shows a modification of the embodiment of Figure 3, including a sub reflector.

Figure 1 shows a diagrammatic cross section through a known device for reflecting radiation from a flat plate 11 to a focal point 12. The plate 11 carries a layer of electrically conductive rings 13 positioned around an axis of circular symmetry 14. The flat reflecting elements 13 achieve a focusing action based on the well known Fresnel zone plate. The single layer of nested conducting rings 13 serves to reflect only the even or odd (depending on the design) half period Fresnel zones. More usually though, this single layer is combined with a flat continuous conducting sheet indicated at 15, spaced a half wave length behind the rings 13, to achieve an improved performance.

However, the efficiency of this device is relatively poor for its size, although it is fairly simple to prototype.

Figure 2 shows a diagrammatic cross-section through another zoned reflector antenna which is known and which consists of a plurality of paraboloid reflecting surfaces 16 lying within each other, and formed on a reflecting plate 11. This design enables a relatively flat, low profile antenna to be achieved which is also of high performance. The radiation is focussed at a focal point 12 lying on a axis of symmetry 14.

Figures 3 and 4 show a diagrammatic side view, and front view, respectively, of an embodiment of the present invention, showing the main layout of the components. A circular laminate structure 17 which will be described in more detail hereinafter, is mounted on a frame 18 which provides a main arm 19 extending forwardly of the laminate 17. Mounted on the arm 19 is a collector device 20 connected by a sliding arm 21 and clamps 22 to the main arm 19. The clamps 22 and 23 and the sliding arm 21, allow adjustment of the collector 20 for movement along a predetermined path during setting up of the device. The antenna shown comprises a reflecting laminate 17 which reflects radiation falling on the front face thereof and transforms a plane wave front of electromagnetic radiation of a particular frequency or narrow band of frequencies, into a converging spherical wave front which is focussed at the collector 20.

Referring to Figure 5, the laminate 17 is shown diagrammatically as consisting of four layers of dielectric material 24. Each layer 24 carries on its front surface a series of flat bands 25 of electrically conducting material in the shape of flat rings or truncated rings 25 the rings being of increasing size with each ring positioned symmetrically within the next larger ring. The rear of the laminate 17 has a continuous layer 26 of electrically conducting material which presents a forwardly facing reflecting surface for reflecting microwave energy.

The rings shown in Figure 5 are purely diagrammatic.

Figure 6 shows a further diagrammatic representation of a cross section through the laminate 17, and shows the four layers 24 of dielectric material, each bearing a nested set of bands 25 of electrically conducting material, and the rear of the laminate 17 having a back surface layer 26 of electrically conducting material. Preferably in Figure 6, the outer perimeter of each band 25 on a dielectric layer 24 is positioned to be coincident with the inner perimeter of the next larger band 25 on the next forward layer 24 of dielectric material.

Figure 7 shows an enlarged detail portion of the cross section of Figure 6, and is an explanatory diagram illustrating the reflection of an equiphase surface S of an electromagnetic wave signal incident on the laminate 17.

Figure 7 illustrates the equiphase surfaces of the wave form striking the laminate 17 at an angle a to an axis of symmetry 14 which extends perpendicular to the laminate 17 through the focal point 12 of the system, at which is positioned the collector 20.

The action of the aforementioned electrically conducting bands is to transform the plane wavefront of electromagnetic radiation of a particular frequency, or narrow band of frequencies, incident on the surface into a spherically converging one so as to focus the energy at a particular point 12 in front of the laminate 17. This effect is achieved by arranging for the conducting bands 25 to be so located within the volume of the laminate 17 that the radiation falling on all parts of the surface will, following its reflection, combine at the focal point 12 with no more than an acceptable maximum phase error, which reduces with the separation of the layers.

The operating principle of the antenna will be apparent following an inspection of Figure 7. This shows a part cross-section through the side of the laminate with the flat profile of the conductive bands drawn. Similar stepped band configurations are repeated beyond the truncations made for the purpose of the illustration. The following notation is used in Figure 7.

D = overall thickness of the assembled laminate F = "focal" distance, the length of the perpendicular drawn from the plane of the laminate to the focal point a = angle the source of plane electromagnetic radiation makes with the perpendicular to laminate N = number of layers in laminate d = D = depth of each layer N x = DL, where L is layer number (0 to N) n = refractive index of dielectric material at frequency of interest (n 2 1).

The value of D is chosen using the following criteria: a) D should be large enough to use effectively all available surface area, b) D should not be so large as to result in some conducting strips shadowing others (D The value of F is chosen to suit the feed appropriate for the application and the largest dimension of the reflector.

The value of a is determined from the knowledge of the direction of the incident wavefront relative to the chosen plane for fixing the reflector upon.

The value of N is chosen to be greater than or equal to two. Antenna efficiency increases with the value of N.

The value of n is dependent on the layer material and its structure. n will be 1 for free space and approximately 1 for low density, tenuous layer structure.

The value of d is fixed by that of D and N.

In Figure 7 path d1 is chosen as the reference and d2 is used as the generator of the smaller or inner perimeter of the conducting bands. The outer perimeter of the band is set equal to that of the inner band directly above it.

What is sought is that the difference between the reference path length and that which includes the inner perimeter of each band should be nothing or equal to an integral number of whole wavelengths, viz: d2-d d1 misaninteger.

Now putting

gives a= C B2- sin2α b = B V = Asina 82'sin a where

a = semi-major axis b = semi-minor axis |~ inner v = offset of band from I perimeter focal perpendicular The shape and position of the inner perimeter of each conducting band is specified by the values of a, b and v defined above, the outer perimeter being made the same as the inner perimeter of the band directly above.

The laminate can have any convenient shape, square and circular being two obvious choices. Having chosen the focal point and the overall shape of the laminate, values of a, b and v are calculated by adjusting the values of x and m until sufficient rings are defined to fill up the surface area of all layers. Examples of patterns of reflecting rings and truncated rings which may be used are shown in Figures 8, 9 and 10. The collector 20 is positioned at the focal point 12 lying on the axis 14, for example at the intersection with the transverse dotted line 27.

Normal parabolic antennas are aligned to receive signals by adjusting the azimuth and elevation of the complete antenna so that the focus of the reflected radiation from the surface falls upon a fixed focal point along the perpendicular to the vertical axis of the parabola. This is achieved by mounting brackets behind the antenna which allow the azimuth and elevation to be set.

The preferred embodiment of the present invention is aligned by rotating it in the plane of the surface to which it is fixed, and adjustment of the position of the collector (feed horn) is made by means of a locking slide bracket which can track the loci of potential focal points.

The important features of the embodiment of the invention are that while it is efficient in terms of its electrical performance: a) the resultant package is flat, and remains flat to the surface to which it is fixed, b) the laminate construction is light, easy to assemble and simple to install, c) forming reflective rings that are flat is straightforward, d) the laminate can be disguised to blend with its surroundings and in some circumstances can be made transparent.

The invention provides a means of adequately receiving satellite television signals whilst minimising its visual impact on the environment by presenting a very low profile.

The flat package of the reflector enables it to be fixed flush to any roughly south facing wall to be effective.

The antenna can be installed flat against a wall not directly facing the signal source by rotating it and using the sliding clamp arrangement on the feed arm which allows the feed to track the locus of potential focal points.

Considering now details of a convenient construction of the antenna system, reference will be made again to Figure 3 and 4. A rectangular metal tube forms the back spine of the plate. This is curved through 90" at the bottom to form the supporting arm for the collector, also know as a low noise block or LNB. This section is terminated by a box folded section/clamp attached at a specific angle of 135 to receive the end section of the LNB support arm. The latter slides through the box section and can be clamped at required lengths. The end of the sliding section has a clamp to hold the LNB pointing toward the plate. This can then be rotated around a horizontal axis parallel to the plate and clamped into position. The clamp may consist of a plastic moulding with built in ratchet. The combined effect of the moveable parts is that the LNB can follow a locus of focal points the locus being a result of mounting the laminate at various angles relative to a satellite and rotating it on its mounting surface to achieve the required directionality and to receive signals from the satellite in question.

The whole device is fixed to the mounting surface by use of suitable fixings which pass through the plate and back spine. The back spine is thus clamped directly to the mounting surface. Spacers consisting of tubing through the back spine give an air tight seal against the laminate.

The spacers protrude in front of the spine by the thickness of the laminate and are then used to locate the plate into correct position on the spine. The distances from the centre of the two outer spacers are unequal to prevent any confusion as to the orientation of the round plate. The spacers allow the fixings to be made very tight without causing damage to the plate and spine. They also act as a guide for drilling, and are wide enough for a drill bit of suitable size to pass through.

By way of example, the layers of dielectric material forming the laminate 17 may comprise polypropylene sheet extrusions having a density of 350 g/m2, of thickness 3mm and conveniently colourless. Alternatively layers could be formed of 3mm thickness expanded polystyrene foam. These may require the inclusion of sheets of plastics or paper for printing surfaces.

The laminate is assembled using silk screen printable glues. The back of the electrical functioning part of the laminate is covered by a solid sheet of microwave reflecting material, for example ink or metal foil. In some alternative models, all the materials may be made to be completely transparent. An example may be P.T.E. with tin-oxide reflective coating.

Examples of dimensions of a circular laminate assembly may be: outer diameter: 850mm perpendicular distance from laminate to clamp 22: 48cm angle of sliding arm 21 to main arm 19 135 thickness of rear continuous electrically conducting layer 26: 20 microns thickness of electrically conducting bands 25: 20 microns typical width of conducting band 25: 0.5 to 4 cm In Figure 11 there is shown a modification of the embodiment of Figure 3. In Figure 11 the reflector laminate 17 is shown as having a focus 12 at which radiation falling on the laminate would be focussed by the laminate. A sub reflector 17A is mounted on an arm 19A and serves to reflect the radiation from the reflector laminate 17 to a focus 12A. In such an arrangement, the collector or feeder device 20 (as shown in Figure 3) will be moved to be positioned at the focus 12A (as shown in Figure 11).

Where a sub reflector 17A is provided, the electrically conducting bands 25 shown in the preceding figures, may have a configuration different from the circular or ellipsoidal shapes shown, but still such as to focus the radiation at the translated focus 12A. In such a case the reflector laminate 17 would not of itself focus the radiation at the focus 12, but would only achieve focusing in combination with the sub-reflector 17A, at the focus 12A.

The sub-reflector serves to translate the focal point of an antenna to another position. Its application in the present invention enhances the invention by: a) reducing the length of the feed arm; b) positioning the collector/feeder close to the laminate; and c) bestowing electrical and mechanical advantages.

Claims

1. An antenna for receiving or transmitting electromagnetic radiation comprising a laminate of at least two flat layers of dielectric material, and a collector or feeder device spaced from and positioned in front of the laminate for receiving from or feeding to the laminate electromagnetic radiation, each layer of dielectric material having a series of flat bands of electrically conducting material, the flat bands being curved in plan view and being arranged in a nested series of hands of increasing size of perimeter, and the rear of the laminate having a continuous layer of electrically conducting material, the configuration and spacing of the bands being such that the laminate acts as a reflector, for concentrating at the collector or feeder device radiation falling on the laminate, or for transmitting from the laminate radiation fed from the collector or feeder device.

2. An antenna according to claim 1 in which the bands have the shape of whole or truncated circular rings.

3. An antenna according to claim 1 in which the bands have the shape of whole or truncated ellipsoidal rings.

4. An antenna according to any preceding claim in which the configuration and spacing of the bands are such as to transform a plane wave front of electromagnetic radiation of a particular frequency or narrow band of frequencies incident on the front of the laminate from a predetermined direction relative to the plane of the laminate, into a converging spherical wave front such as to focus the energy at the collector or feeder device.

5. An antenna according to any preceding claim in which the outer perimeter of each band of a dielectric layer is positioned to be coincident in plan view with the inner perimeter of the next larger band of the next forward layer of dielectric material.

6. An antenna according to any preceding claim in which it is arranged that the path difference between a fixed reference path and a path which includes a reflection at the inner perimeter of any conducting band, is zero or equal to an integral number of whole wavelengths of an electromagnetic signal to be received or transmitted.

7. An antenna according to any preceding claim in which the laminate comprises more than two said dielectric layers each having a set of bands of electrically conducting material.

8. An antenna according to claim 7 in which the laminate consists of four said dielectric layers each having a set of bands of electrically conducting material.

9. An antenna according to any preceding claim in which each layer of dielectric material has its series of bands of electrically conducting material on the front surface of the layer of dielectric material.

10. An antenna according to any preceding claim in which the said electrically conducting bands are produced by silk screening metallic conductive ink onto the dielectric material.

11. An antenna according to any preceding claim including means for rotating the laminate about an axis perpendicular to the laminate to vary the direction from which or to which radiation is received or transmitted.

12. An antenna according to any preceding claim including means for varying the position of the collector or feeder device relative to the laminate along a predetermined path so as to position the device at a focus of radiation to be received or transmitted.

13. An antenna for receiving an electromagnetic communications signal transmitted by a communications satelite, the laminate being adapted to be secured to a flat exterior building structure.

14. An antenna according to claim 13 including means for rotating the laminate about an axis perpendicular to the laminate to vary the direction from which radiation is received, and means for adjusting the position of the collector device relative to the laminate along a predetermined path so as to position the collector device at a focus of radiation to be received.

15. An antenna adapted to act as a receiver, substantially as hereinbefore described with reference to any one or any combination of the accompanying drawings.

16. An antenna adapted to act as a transmitter substantially as hereinbefore described with reference to any one or any combination of the accompanying drawings.