GB2314524A - Antenna ground plane substrate - Google Patents

Abstract

An antenna conductive element, e.g. a ground plane, is formed from expanded polystyrene and coated with a conductive compound by a process in which (a) unexpanded polystyrene beads are expanded by steam treatment, cooled and freed from stabilising agents, expansion agents and moisture, (b) the expanded beads are introduced into a mould where passage of steam causes further expansion, (c) the pressure within the mould is reduced whereby the polystyrene is cooled to a solid state and moisture removed, (d) the substrate is removed from the mould and heated in dry air and (e) a conductive coating is applied, e.g. as a metal pigment in an aqueous based thermoplastic resin binder, the metal being, inter alia , silver, silver plated copper, nickel and gold. The element forms part of a flat plate, or layered, antenna comprising a dielectric substrate 10, two apertured ground planes 12,30 and a reflecting ground plane 32 which provides a degree of directionality. The dielectric substrate has, on one side, a metallisation pattern 22 which provides a feed for radiating probes 18,20 and the apertures 14,16 of ground planes 12,30 are positioned such that the probes are able to radiate in a primary radiating direction.

Description

AN ANTENNA GROUND PLANE SUBSTRATE FIELD OF THE INVENTION This invention relates to antennas employing ground planes such as flat plate antennas (otherwise known as layered antennas) and in particular relates to a method of coating a dielectric substrate to provide a surface ground plane.

BACKGROUND ART One form of antenna that is in widespread use is the triplate antenna which, in one form, comprises a radiating element including an apertured ground plane with an interposed printed film circuit, electrically isolated from the ground plane, the film circuit providing excitation elements or probes within the areas of the apertures and a feed network for the excitation elements. In an array antenna a plurality of such aperture/element configurations are spaced at regular intervals collinearly in the overall triplate structure. This antenna construction lends itself to a cheap yet effective construction for a linear array antenna such as may be utilised for a cellular telephone base station. Such an antenna is disclosed in our copending patent application No. EP-A-91 24291.7.

A further type of antenna comprises a primary aperture with two secondary apertures placed on opposite sides of the primary aperture. Such arrays may extend in a single direction (a linear array) or in two directions (a planar array). Alternatively, a number of linear arrays may be spaced apart to form a multi-antenna planar array.

Another type of layered antenna array comprises apertures in both ground planes of each radiating element. An important factor in the design of an antenna is the gain of the antenna. In order to increase the gain from the antenna in a primary radiating direction, the antenna may have a further, continuous (non-apertured) ground plane placed parallel to and spaced from one of the apertured ground planes to form a rear reflector for the antenna. Signals transmitted by the antenna towards the back plane are re-radiated in a forward direction. Provision of this reflector can increase the forward gain of the antenna whilst reducing the reverse gain.

A further type of antenna with a single ground plane is the patch antenna which comprises a reflective ground plane and a dielectric sheet which is supported from the ground plane by a dielectric spacer and supports a microstrip pattern comprising printed patch radiating elements.

A still further form of antenna is the dipole antenna in which a pair of collinear quarter wavelength radiators are fed in anti-phase to produce a substantially omni-directional radiation pattem in a plane normal to the axis of the radiators. If the radiators are placed parallel to and a quarter of a wavelength from a reflecting ground plane the radiation pattern similarly becomes substantially directional, see e.g. EP-A-355898 (Rammos).

For modern telecommunications application at high frequencies, e.g. above 100 MHz, apart from the electrical performance of the antenna other factors need to be taken into account, such as size, weight, cost and ease of construction of the antenna. Depending on the requirements, an antenna can be either a single radiating element (e.g. one dipole) or an array of like radiating elements.

With the increasing deployment of cellular radio, an increasing number of base stations which communicate with mobile handsets are required. Similarly an increasing number of antennas are required for the deployment of fixed radio access systems, both at the subscribers premises and base stations. Such antennas are required to be both inexpensive and easy to produce. A further requirement is that the antenna structures be of light weight yet of sufficient strength to be placed on the top of support poles, rooftops and similar places and maintain long term performance over environmental extremes.

One part of an antenna which is particularly heavy is the grounding.

The grounding can comprise one or more ground planes and must have an electrically conductive surface with a low resistivity per square whereby the microwave surface currents may propagate.

Typically ground planes have been formed from a metal casting or sheet, typically from an aluminium alloy for reasons of weight or from plated steel for reasons of cost. Nevertheless, the weight of the ground planes, whether formed from an alloy or not, comprise a considerable amount of the overall weight of an antenna structure.

As is known, in the case of microwave propagation, the top surface (the air-metal boundary surface of a conductive structure) carries the microwave signals by way of the skin effect. Low frequency electrical signals flow in the bulk of a conductive material. If the surface of a ground plane is rough, then it will not sustain controlled RF propagation; at best, the effect of a rough surface would be to degrade the performance of an RF circuit. The advantage of a good ground plane surface finish, to give lowest RF losses, is apparent for the many cases of ground planes. This is true, for example, in the case of cavity backing, conventional ground plane, apertured (radiating) ground plane1 balanced (triplate) or unbalanced (microstrip).

Dielectrics such as expanded polystyrene have not previously been successfully metallised and employed as ground planes, since the surface roughness of the polystyrene would result in any surface metallisation having a rough surface due to boundary interstitial voids (spaces between aggregated polystyrene groups at a surface). The technique of sputter coating or electroless plating of plastics such as ABS plastics is known1 but this type of coating is not suitable for non-rigid plastics materials such as polystyrene, where cracking of the coating would result upon stressing or flexing.

Furthermore, dielectrics such as polystyrene as commonly produced have poor dimensional tolerances which fact does not extend their use to the creation of structures other than that as a resilient spacer.

It is an object of the invention to provide an antenna ground plane structure which is light in weight, has good dimensional tolerances and which can both be easily and cheaply produced on a mass production basis.

SUMMARY OF THE INVENTION In accordance with the invention, there is provided an antenna conductive element formed from expanded polystyrene produced by a two stage process coated with a conductive compound.

In accordance with another aspect of the invention, there is provided a method of producing a conductive expanded polystyrene antenna substrate, the method comprising the steps; passing steam over unexpanded polystyrene beads whereby the beads expand to a first expanded state; introducing the beads into a ventilated silo; allowing the beads to cool whereby stabilising agents and moisture evaporate and the voids fill with air; introducing the beads into a two-position mould cavity in a first position dimensioned to enable a uniform distribution of beads; reducing the size of the cavity passing steam through the mould whereby the polystyrene beads further expand; reducing the pressure within the mould to a partial vacuum whereby the polystyrene cools and assumes a solid state and the moisture due to the steam is removed; removing the substrate from the mould and heating the substrate at a temperature above ambient in dry air; and coating the substrate with a conductive coating.

By the use of this two stage manufacturing process, substrates having a thickness of less than 2 mm can reliably be produced, with a high degree of surface smoothness and cell closure. Such twostage polystyrene is then coated with a conductive coating to provide a conductive surface of uniform smoothness to enable the substrate to be employed in antenna conductive plane applications, for example, when the conductive coating is connected to an earth potential. Preferably the conductive coating is a metal pigment held in a thermo plastic resin with an aqueous solvent which is applied by spray.

In accordance with another aspect of the invention the electrical connection to the antenna array elements and/or ground plane can be made employing a compression contact. This dispenses with the need for a connection formed by, for example, soldering which would compromise the integrity of the structure.

One advantage of the arrangement is that the ground plane substrate can also serve the function of a dielectric spacer between the ground plane and a feed network or any further ground plane, such as an apertured ground plane, thereby decreasing the number of components of the antenna, simplifying the manufacture, further reducing unit cost. Polystyrene possesses a dielectric constant which is close to that of air - i.e. near unity dielectric constant, and is of low loss. This also minimises the effect of variations in tolerances between the separate track and ground plane, without recourse to procedures such as the bonding or clamping of a substrate to a separate ground plane.

BRIEF DESCRIPTION OF DRAWINGS In order that the present invention is more fully understood, reference will now be made to the Figures as shown in the accompanying drawing sheets, wherein: Fig. 1 an exploded perspective view of a triplate single element antenna having a dipole pair with a feed network formed in microstrip, with a back reflector; Fig. 2 shows a second type of antenna; and Figs. 3a - b show three stages in the manufacture of a polystyrene antenna ground plane.

DESCRIPTION OF THE INVENTION Figure 1 shows one type of antenna to which the present invention is applicable. The flat plate antenna element comprises a dielectric substrate 10 which is positioned between two apertured metallic ground planes 12, 30. A further reflecting ground plane 32 is spaced from ground plane 12 to provide a degree of directionality for the antenna. The dielectric substrate supports, on one side of the substrate, a metallisation pattern 22 which provides a feed network for a pair of radiating probes 18, 20. The apertured ground planes each have a pair of identical rectangular apertures 14, 16 and are positioned whereby the probes may radiate in a primary radiating direction. A feed point 24 is provided for connection to an external feed (not shown).

The feed network 22 is positioned so as to form a microstrip transmission line with portions of the ground plane defining the rectangular apertures. The position of the feed point 24 is chosen so that when an r.f. signal of a given frequency is fed to the network the relative lengths of the two portions 22a and 22b of the network are such as to cause the pair of probes 18 and 20 to be fed in antiphase, thereby creating a dipole antenna radiating element structure. Furthermore, the dimensions of the rectangular apertures and the bounding portions of the ground plane are chosen so that the bounding portions 26, 28 parallel with the probes 18, 20 act as parasitic antenna radiating elements, which together with the pair of radiating probes 18, 20 shape the radiation pattern of the antenna.

The ground planes are spaced from the plane of the feed network by dielectric spacing means (not shown) so that the feed network is spaced from both ground planes. In practice the feed network can be formed by conventional printed circuit techniques on a fibre glass board and the ground planes have been stamped out of aluminium sheets. The spacing between the network and the ground planes is usually determined by expanded polystyrene.

A second type of antenna, a patch antenna array, is shown in Fig 2 which comprises a dielectric film 40, a dielectric spacer 42 and a ground plane 44. The dielectric film supports a feed network 46 which leads to several patch radiating elements 48. As with the dielectric spacers of the antenna shown in Fig 1, the dielectric film is spaced from a ground plane reflector by a spacer such as expanded polystyrene.

We have found that it is possible to produce an antenna conductive plane substrate from expanded polystyrene. By applying a special two stage moulding technique, an expanded polystyrene dielectric substrate can be produced which has a particularly smooth surface to which a conductive coating may subsequently be applied. Thus, for example, a light weight ground plane substrate can be produced.

The manufacture of the dielectric will now discussed, by way of example only: In the first instance, polystyrene granules are prepared for moulding in an initial stage wherein the granules of a size 0.05 - 0.1 mm in diameter and of a weight of 1000g/l are expanded to a first expanded state by passing steam at 1100C through the granules, as they are fed into a ventilated silo.

Polystyrene is typically stored in pentane for storage. The granules or beads as perhaps they are better described after expansion are then allowed to cool and dry. At the same time the pentane gas, which is heavier than air, is allowed to dissipate. Appropriate gas exhaust equipment should be utilised: pentane is flammable. The spaces vacated by the pentane is replaced with air. The ventilated silo has a mesh like structure to allow the passage of gases.

After being left to dry and evaporate for six hours, the partly expanded beads are introduced into a two-position mould cavity in a first position dimensioned to enable a uniform distribution of beads having a gap of around 4 mm. At this stage the beads are of 3-4 mm across and the density is around 25 - 40 g/l. The size of the cavity is then reduced and steam is passed through the polystyrene. The steam is typically at a slightly raised pressure e.g.

in the region of 1.0 - 1.1 bar and at 1100C. In tests, in producing a dielectric of 28 cm diameter and of 2 mm thickness, steam was passed across from one face to the other for five seconds and then in a reverse direction for a further five seconds. This further passing of steam causes the air which has diffused into voids of the polystyrene to cause the heated beads to expand and coalesce.

No heating of the mould is normally necessary. Subsequent to this, the pressure within the mould is reduced to a partial vacuum whereby the polystyrene cools and assumes a solid state and the moisture due to the steam is removed for a period of ten minutes, although this period can, of course be varied. The polystyrene substrate is then removed and dried in a heated, dry atmosphere for a period of three hours or so. The density of the substrate is now about 70 g/l.

Referring now to figure 3 a, there is shown a simple mould comprising two main components: a first, fixed mould portion 60 and a second mould portion 62 which is movable relative to the first mould portion. The chamber 64 enclosed by the mould portions has inlets (not shown) whereby polystyrene beads and steam may be passed. The mould halves are formed of a corrosion resistant material which will withstand temperature cycling and is preferably easily machined or cast. An aluminium alloy is appropriate in many cases. As can be seen, the chamber has a thickness T1 between the major faces of the mould halves. The mould portions will be shaped as appropriate for the type of antenna desired: for instance, there may be depressions in the design to correspond in position to the patch elements of a design. As seen in Figure 3 b the second mould portion is brought towards the first mould portion to realise a mould separation thickness T2. Reference letter H denotes a hydraulic ram for moving the mould parts relative to each other.

The experimental tests were carried out using polystyrene granules marketed under the name STYROSHELL and sold by the Shell Petroleum Company.

By the use of this two stage manufacturing process, substrates having a thickness of less than 2 mm can reliably be produced, with a high degree of surface smoothness and cell closure. The use of steam as an expanding agent requires the use of no other solvents, release agents or the like. Accordingly, the removal of such agents is not required for any subsequent process. Further, no measures to encourage keying are generally required for subsequent surface treatment. Such two-stage polystyrene may therefore be easily coated to provide a surface of uniform smoothness suitable for use in antenna applications, where a correct spacing between successive substrates may be maintained. It is important that the ground plane and other substrates have a uniform consistency whereby the microwave parameters required to meet the overall antenna performance are not compromised. Polystyrene can provide a dielectric spacer having a dielectric constant which is close to that of air - i.e. near unity dielectric constant, and is of low loss.

Preferably, the conductive coating is a metal coating applied as a spray coating as from an aerosol or otherwise, such as an aqueous-based metallic paint such as the one sold by the Acheson Company under the ELECTRODAG name, which comprises silver plated copper pigment suspended in a thermoplastic resin binder employing water as the diluent. A thin (-50Lm) metallic layer can thereby be reliably applied to the moulding. A spray coating can be optimised to give the required electrical characteristics for the conductive coating: ELECTRODAG is preferably applied by spray, with a recirculation system being employed from the pot to the spray gun to ensure good uniformity. Highest efficiency can be achieved using high volume, low pressure spray guns due to the minimisation of over spray losses. Conventional propeller agitated pressure pot systems may also be used, but the transfer efficiency is less. It is preferred that metals in contact with the coating are of 304 or 316 grades of stainless steel. The metal pigment is ideally of high conductivity such as silver, which can give a sheet resistivity of less than -0.05 Q per square at 15 lim thickness , but a more economic silver plated copper is preferred which can give a sheet resistivity of -0.25 Q per square at 50 lim thickness. The use of nickel is also possible but has a sheet resistivity of -0.5 n per square at 50 ttm thickness. Because of the smooth surface of the polystyrene, other coatings can be applied to give low loss microwave finishes.

A further advantage in the use of a dielectric as a support surface is that the dielectric substrate can provide the spacing means whereby the ground plane is maintained at a uniform distance from the feed network. Accordingly, the use of a dielectric substrate which is coated with a conductive layer in one or more areas can reduce the number of components employed in the manufacture of an antenna, simplifying the manufacture and also reducing the overall costs. The polystyrene substrate may, for example, carry a ground plane on a first side and radiating elements (and associated feed network) on a second side. Further, the second side may also carry grounded areas to suppress unwanted modes.

In use, the antenna ground plane will function as any 'normal' metallic ground plane in that microwave currents may be propagated between a transmission line printed on a dielectric film and a conductor coated dielectric made in accordance with the invention. The ground plane as provided by this invention is not restricted to that of a reflective back plane; metallic apertured ground planes may be replaced with the coated dielectric. In low power applications, it is also possible to print the radiating elements and feed network therefore upon an expanded polystyrene substrate. It is therefore possible to provide an antenna employing no metallic ground plane structures as such. In some applications the ground plane could also function as an EMC shield around control electronics associated with the antenna. Contact of the conductive coating with a potential may be performed by a pressure contact arrangement.

In certain applications, such as in aerospace/avionics a very light non-metal antenna structure may be realised. Rigid plastics mouldings in glass fibre may be used to maintain shape and strength of the structure. A further advantage of the invention is that conformal structures may be very easily implemented.

Claims (6)

1. An antenna conductive element formed from expanded polystyrene produced by a two stage process coated with a conductive compound.

2. An antenna ground plane formed from expanded polystyrene produced by a two stage process coated with a conductive compound.

3. An antenna according to claim 1 or 2 wherein the conductive compound is an aqueous based thermoplastic resin binder metal pigment.

4 An antenna according to claim 3 wherein the metal pigment is selected from the group comprising silver, silver plated copper, nickel and gold.

5. A method of producing a conductive expanded polystyrene antenna substrate, the method comprising the steps; passing steam over unexpanded polystyrene beads whereby the beads expand to a first expanded state; introducing the beads into a ventilated silo; allowing the beads to cool whereby stabilising agents and moisture evaporate and the voids fill with air; introducing the beads into a two-position mould cavity in a first position dimensioned to enable a uniform distribution of beads; reducing the size of the cavity passing steam through the mould whereby the polystyrene beads further expand; reducing the pressure within the mould to a partial vacuum whereby the polystyrene cools and assumes a solid state and the moisture due to the steam is removed; removing the substrate from the mould and heating the substrate at a temperature above ambient in dry air; and coating the substrate with a conductive coating.

5. An expanded polystyrene antenna conductive plane substrate substantially as described herein with reference to any one or of the Figures as shown in the accompanying drawing sheets.

6. A method of producing a conductive expanded polystyrene antenna substrate substantially as described herein with reference to any one or of the Figures as shown in the accompanying drawing sheets.