GB2314523A - Antenna dielectric - Google Patents

Abstract

An antenna dielectric element is formed from expanded polystyrene 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 and (d) the substrate is removed from the mould and heated in dry air. The element forms part of a flat plate, or layered, antenna in which it is located between two apertured metallic ground planes 12,30, a further reflecting ground plane 32 being spaced from ground plane 12 to provide a degree of directionality. The dielectric supports on one side a metallisation pattern 22 which provides a feed network for radiating probes 18,20 and the ground planes 12,30 have rectangular apertures 14,16 positioned such that the probes can radiate in a primary radiating direction.

Description

AN ANTENNA DIELECTRIC FIELD OF THE INVENTION This invention relates to antennas employing dielectrics such as flat plate antennas (otherwise known as layered antennas).

BACKGROUND ART One form of antenna that is in widespread use is the triplate antenna which, in one form, comprises a radiating element including a pair of closely spaced correspondingly apertured ground planes with an interposed printed film circuit, electrically isolated from the ground planes, the film circuit providing excitation elements or probes within the areas of the apertures, to form dipoles, and a feed network for the dipoles. 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.

Another type of layered antenna array comprises a single aperture per radiating element. 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.

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

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.

The antennas described above have spacers to separate the various dielectric film or ground plane layers. Whilst some types of antennas have employed rigid dielectric boards, made from for instance, FR4 or PTFE, these require solid supports to maintain their position, are expensive, both by way of raw material and in the manufacture of an antenna. Polystyrene is cheap, has a near unity dielectric constant, a very low dielectric loss, but presently cannot normally be produced at a thickness of less than 4 mm by normal manufacturing techniques with acceptable tolerances. The dimensions of a fabrication mould allowing an even distribution of polystyrene to be expanded is one of reasons why this is so. In summary, the above mentioned ideal attributes are not attainable with current practice.

In the design of such antennas there is a requirement to provide a dielectric which is of low permittivity and low loss coefficient and which is, moreover, relatively thin. With these attributes, feed line and radiative element losses are thereby minimised, and overmoding in the feeder lines can be minimised whilst providing an acceptable range of transmission line characteristic impedances for design selection. (Overmoding occurs in transmission line elements which can support undesired propagation modes which in turn can corrupt radiation patterns ). Commonly such an ideal arrangement would have a dielectric much like air and which would be some few mm in thickness and be produced cheaply and repeatably to a tolerance of 5-10 %.

An object of the invention is to provide an antenna dielectric having a thickness of less than 3 mm, having a dielectric performance approaching that of air whereby circuit and transmission line losses are minimised.

SUMMARY OF THE INVENTION In accordance with the invention, there is provided an antenna dielectric substrate comprising expanded polystyrene formed by a two stage process.

In accordance with another aspect of the invention, there is provided a method of producing an 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; and removing the substrate from the mould and heating the substrate at a temperature above ambient in dry air.

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.

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. Spacing between the network and the ground planes can be determined by foamed dielectric sheets, dielectric studs interposed between the various layers or 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 dielectric substrate of reduced thickness from expanded polystyrene. By applying a special two stage moulding technique, an expanded polystyrene dielectric substrate can be produced which has a thickness approximately half that which was previously attainable.

It further provides a particularly smooth surface to which a conductive coating may subsequently be applied.

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 good tolerance and 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 dielectric substrates have a uniform consistency whereby the microwave parameters required to meet the overall antenna performance are not compromised.

Claims (4)

1. An antenna dielectric element formed from expanded polystyrene produced by a two stage process.

2. A method of producing an 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; and removing the substrate from the mould and heating the substrate at a temperature above ambient in dry air.

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

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