AN ANTENNA ARRANGEMENT

Abstract: An antenna arrangement is disclosed, suitable for use in a fixed radio access environment. A first embodiment of the invention provides a layered antenna arrangement comprising a ground plane (460) and a dielectric film layer (440) having a radiating array of radiating elements over a feed network therefor printed thereon, wherein the ground plane has depressions formed therein corresponding in position to the patch elements. The depressions have radiussed edges and preferably are filled with a dielectric substance (450) to maintain the printed circuit elements in a desired plane and to maintain continuity of the dielectric permitivity between the dielectric film and the ground plane. The depressions correspond in position to the radiating elements whereby the bandwidth is increased. The ground plane can comprise a formed plastics moulding, having a metal coating applied thereon operable to provide a conductive ground plane.

Description: An Antenna Arrangement Field of the Invention This invention relates to an antenna arrangement and in particular to a low cost planar array antenna. More particularly, the present invention relates to an antenna arrangement suitable for use in a wireless local loop or fixed radio access environment. Background of the invention Radio communication systems are currently employed for receiving various types of communications, from television broadcasts via satellites to local telecommunication networks. Known systems comprise an antenna - for example a satellite dish - and decoding means. The antenna receives the signal and provides a further signal by wire to a decoding means. In the case of wireless local loop or fixed radio access telecommunications, subscribers are connected to a telecommunications network by a radio link in place of the more traditional method of copper cable. The radio transceivers at the subscribers premises communicate with a base station, which provides cellular coverage over, typically, a 5km radius in urban environments. Each base station is connected to the standard PSTN switch via a conventional transmission link/network. The decoder for each fixed radio access subscriber system will decode the received signal and encode signals to be transmitted, whilst in the case of a satellite broadcast receiving arrangement, the decoder will provide demodulated signals for a television receiver. The distance between the antenna and the decoder can sometimes be many metres apart: this can lead to a degradation of the received signal and either require a larger receiving antenna; a higher power decoder; or a higher quality connector between the antenna and decoder. In many instances the solutions can be overly expensive and/or result in large apparatus being employed.* Layered (triplate) antennas - whether having probe feeds or having radiating patches - operate in conjunction with a ground plane in order to attain a certain level of directivity. Hitherto, this ground plane has been formed from a metallic substrate, since the ground plane must be electrically conductive. The disadvantages of having a metallic substrate are readily apparent; in any antenna arrangement metallic components make a significant contribution to the weight of the structure, are prone to corrosion and are expensive to manufacture. Attempts to produce antennas from plastics materials have not been successful; the need for a ground plane means that there is always going to be a certain amount of metallization and, in the case of directive antennas such as layered antennas, the ground plane needs to have smooth surface definition - bulk plastics such as expanded polystyrene have rough surfaces which does not enable a flat ground plane to be achieved, whilst extruded or other types of plastics mouldings cannot always provide the required rigidity. The normal arrangement of an antenna featuring a distribution track layout connected to various patches on a single film supported above a ground plane has several disadvantages:- The tracks radiate causing unwanted antenna polar diagram perturbations, for example they produce high side lobe levels or beam squint and also contribute to an asymmetrical beam pattern. Further, a very narrow bandwidth results, due to the radiating elements being too close to the ground plane. There is no particular preferred solution since either the match of the antenna is broad but the antenna performance is poor due to the tracks radiating, or the antenna performance is adequate but the antenna is very narrow band and inefficient. These problems are mainly due to the patches being too close to the ground plane. One solution to this problem is to introduce another dielectric film layer above the patches which layer provides parasitic patches to improve the antenna bandwidth, but this increases the cost of both materials and the manufacture of the antenna. It is an object of the invention to provide a broadband directional antenna. It is another object of the invention to provide an antenna structure which is compact and of light weight. It is a further object of the invention to provide an easily fabricated antenna ground plane. It is also an object of the invention to provide an antenna which can be easily and cheaply produced on a mass production basis. Disclosure of the Invention In accordance with one aspect of the invention, there is provided a layered antenna arrangement comprising a ground plane and a dielectric film layer having a radiating array of radiating elements over a feed network therefor printed thereon, wherein the ground plane has depressions formed therein corresponding in position to the patch elements. The depressions have radiussed edges and preferably are filled with a dielectric substance to maintain the printed circuit elements in a desired plane and to maintain continuity of the dielectric permitivity between the dielectric film and the ground plane. The signal feed arrangement is supported such that the spacing between the feed tracks and the ground plane are low whereby radiation from the feed tracks is minimised. This feature also allows the feed tracks to be of reasonable dimensions in that they may be reproduced easily. In accordance with another aspect of the invention, the ground plane comprises a formed plastics moulding, having a metal coating applied thereon operable to provide a conductive ground plane.. In accordance with a further aspect of the invention, there is provided a method of producing a metallic coated polystyrene substrate suitable for use as a ground plane for an antenna and an enclosure for shielding against electromagnetic interference or for use as a ground plane for an antenna and electrical feed network and radiating elements for the same, the method comprising steps of: inserting untreated polystyrene pellets into a mould comprising at least two relatively moveable portions, heating the mould whereby to cause expansion of the polystyrene pellets; moving one of the mould portions towards the other of the mould portions, whereby the thermo-expanded polystyrene is compressed to form a product having a smooth pore-free finish, and; coating the polystyrene with a conductive layer. By employing a water- based solvent in a conductive paint to be applied for the conductive layer, then the integrity of the plastics materials will not be compromised. Since the polystyrene formed in this fashion has a high degree of smoothness then an electromagnetic interference enclosure for electronics components can easily be formed if this plastics material is treated with a metallic coating. Similar components or metallic components having a smooth finish can abut the metallised plastics to fully close the electro-magnetic interference shielded enclosure. Description of preferred embodiments 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: Figure 1 is a diagrammatic perspective view of a mounted integrated antenna assembly according to one aspect of the present invention; Figure 2 and 2a show an interior view and a side view of the radome of the assembly as shown in Figure 1 ; Figures 3 and 3a show a rear plan view and a cross-sectional view of the body casting of the assembly shown in Figure 1 ; Figures 4 shows a cross-sectional view through the radiating elements of the antenna assembly; Figures 4 a-e show, in plan, the radiating elements of the antenna assembly as shown in Figure 4; Figure 5 shows, in cross-section, the first dielectric spacer as shown in Figure 4b; Figure 6 shows, in cross section, the second dielectric spacer as shown in Figure 4d; Figures 7a, b show, in cross-section, the ground plane as shown in Figure 4e corresponding to lines X-X and Y-Y respectively; Figures 8 and 8a show, in perspective connected to a mounted antenna assembly and in cross-section respectively, a trunking assembly in accordance with a further aspect of the invention; and Figure 9 shows a portion of one face of an integrated antenna ground plane and electronics enclosure. The integrated antenna assembly 10 shown in Figure 1 comprises one part of a residential service system and is mounted on the side wall 12 of a house. The antenna assembly is mounted for use in a fixed radio access telecommunications environment and is connected via the cable 14 to a junction unit for connection with standard telephone and/or facsimile equipment and a power unit. The power unit is driven from local a.c. mains supply and provides a d.c. supply for all the transceiver equipment and indoor apparatus. It is preferred that the power unit houses rechargeable batteries in case of mains failure. Optionally an interface unit is provided, which is an externally mounted junction box for connection of all external cables and provides an externally accessible serial data connector for remote communication with the roof/mast/wall mounted transceiver unit by service personnel. Referring now to Figures 1 to 3a, the antenna assembly according to the invention is shown having an octagonal configuration, with a front radome 210 and a rear baseplate 310. The radome 210 is preferably moulded from an amorphous plastics such as an acrylnitryl-styrol-acrylester polymer and the rear baseplate 310 can be fabricated as a casting of an aluminium alloy. The fabrication techniques are well known and need not be discussed here. Aluminium alloys are not mandatory, but such alloys provide good corrosion resistance combined with high strength and low weight. Since the antenna assembly is capable of being mounted upon a mast, the light weight of alloys is of particular significance. However it is possible to form the rear base plate from a plastics material, as will be discussed below. The antenna comprises a planar array, which array is housed in the assembly having a radome and a backplate, wherein the assembly houses electronic circuits capable of converting microwave signals into electrical signals, which electrical signals are fed to receiving equipment such as a telephone receiver. By combining electronic circuits within an antenna assembly, high frequency feeder losses are minimised, enabling more efficient use of input power, both for uplink and downlink communications. Figures 3 and 3a detail, on the rear body casting, mounting lugs 312 and cooling fins 314. Apertures 316 for connection with the radome are arranged about the flange 318 of the octagon and a further aperture 320 is provided for feeder cables. Apertures 316 may allow clip fastening means or screw fastening means to be employed. Figure 3a shows in greater detail the flange on this casting, which is shaped to correspond with flange 212 of radome 210 as seen in figure 2a. Provision is made for a gasket to be fitted. In a further embodiment (not shown), the radome may be attached by a snap-fit construction. External lugs 312 allow the casting to be securely attached to an external wall or the like, preferably with the aid of a mounting bracket or similar. Internal lugs 314 allow for the fixture and/or placement of printed circuit boards or other radio frequency or electrical devices. Referring now to Figures 4 through to 7, and more especially Figure 4, there is shown an antenna assembly comprising an outer radome 210 - as previously discussed - a first dielectric spacer 430, a dielectric film supporting a microstrip antenna network 440, a second dielectric spacer 450 and a ground plane reflector 460. The first dielectric spacer 430 maintains the microstrip array in a plane at a fixed distance from the radome 210. It has been found that this dielectric also tunes the circuit and trials have shown that 7mm is a preferred distance for a radome having a diameter of 0.3m - but this figure will vary from one application to another. The film must be reliably and uniformly supported to maintain flatness. The second dielectric spacer 450 maintains the microstrip array at a fixed distance tightly toleranced from the ground plane or backplane 460. The ground plane has a number of apertures 462 which enable the ground plane to be secured to the radome by means of plastics studs 412. Connector 414 provides a microwave link from the feeder cables to the antenna array. The ground plane if manufactured from a metal such as steel with an aluminium coating can be 0.5mm thick. Alternatively, the ground plane can be manufactured from a plastics material, which plastics material after forming by a moulding process is coated with a metal film. This type of ground plane exhibits cost and weight saving features since such a film need only be ten microns or so thick. The dielectric spacers can be produced from expanded polystyrene foam having a dielectric constant as close as possible to that of air. The Shell Company Ltd. produce polystyrene particles suitable for making such spacers and is known as SHELL STYROSHELL. The second dielectric is preferably contoured to correspond with the depressions in the ground plane 460, but additional spacers may be separately provided to fill the voids left by these depressions. The ground plane 460 can be fabricated from a variety of materials such as copper, steel, aluminium, aluminium coated steel or metallised plastics. When formed of a plastics material, such as STYROSHELL, the polystyrene needs to be processed in a particular fashion: Since the spacing between the ground plane and the radiating elements and distribution tracks needs to be tightly toleranced, it is not sufficient that a plating, foil or otherwise be applied to the surface of the polystyrene since the particle size of the component polystyrene balls represents too large a variation in the ground plane spacing. Accordingly a two-stage moulding technique is employed whereby a smooth surface finish may be attained. If a 2mm ground plane is required, then a first stage of the moulding process will entail the forming of a 4mm ground plane - this enables the constituent polystyrene particles to be easily inserted within the mould cavity. The polystyrene balls or pellets are then heated by steam or otherwise in order that they expand. As the balls expand, the mould closes up - for instance, by the application of hydraulic rams to the side of the mould- whereby the dimensions are reduced to 2mm. The use of two-stage moulding process enables a product to be produced which can have a range of thicknesses, from the same moulding, for example 2-7mm. This is of benefit with respect to the production of antenna substrates where an antenna ground plane substrate can be produced which can be tuned to different operating frequencies. By the use of the two stage manufacturing process, a high degree of surface smoothness and cell closure can be obtained as is necessary in order that a correct spacing between successive substrates may be maintained and also for subsequent surface treatment. This two stage process provides a surface finish which has been found, when coated by a metallic coating, to give a satisfactory microwave performance as a ground plane for an antenna. The plastics moulding material is preferably expanded polystyrene since it has a dielectric constant close to that of air. Obviously any surface metallization needs to be uniform both in terms of smoothness and in terms of ohms per square where the substrate is employed as an antenna ground plane. A silver copper water-based spray such as ELECTRO DAG 7050 as sold by Acheson Limited may be used to provide a metallic surface on the plastics substrate. Such sprays can give low resistance coatings, 0.3 - 0.5 Ω per square, combined with a degree of scratch and mar resistance. The binder for such sprays is typically thermo-plastics resin and high volume low pressure spray guns can be employed. A sufficient coating can be provided by a 35-15 micron spray film coating. Sputter coating is also possible. The use of a water-based spray ensures that the mechanical integrity of the plastics material is not compromised. Furthermore, the use of coatings which cure at either room temperature or lightly elevated temperatures obviates problems which may arise through the use of coatings which cure at high temperatures or carbon compounds solvents which can dissolve certain plastics. Electrical contact with the ground plane is preferably effected by physical compression contact or by conductive adhesive. The application of solder is possible if the thermoplastics employed can exist at elevated temperatures and/or the solder has a low temperature melting point. To increase the bandwidth of the radiating patch elements a depression is placed underneath the patches to increase the ground plane spacing. This lowers the Q factor for the antenna elements and will yield an appreciable bandwidth improvement, for example, 200 MHz bandwidth at 3.5 GHz centre frequency. Since the depressions do not exist in the region of the distribution track layout, the ground plane spacing to the dielectric supporting the tracks and patches can be maintained in these areas, so the tracks will not radiate in an interfering fashion with the antenna radiation pattern. This technique yields reasonable track widths that enable a designer to realise a wide range of impedances so that he can design his patch feed distribution network over wide ranges so that different distributions can be achieved to the various patches so that antenna polar diagrams of either narrow beam with high side-lobe levels or broad beam with low side-lobe levels can be realised. As can be seen from Figures 4a and 4e, the depressions 462 in the ground plane correspond in position with the patch antenna elements 442. Obviously, if it is desired to increase further the bandwidth of the antenna, it would be possible to employ parasitic patch antennas spaced in front of the radiating patch, but as discussed previously would increase the cost. Previously experimental antennas have been made with ground plane depressions formed by milling, stamping or other similar techniques. The use techniques have resulted in the formation of right angle contours which are costly to produce and also result in formation of spurious radiation characteristics. As a result ground plane depressions have not easily been employed in a commercial environment. By providing radiussed corners 464 the ground plane can be produced from metallic sheetin a variety of fashions such as by stamping. The radiussed corners can be seen in more detail in Figures 7a and 7b and the corresponding features 454 in the second dielectric can be seen in Figures 6 and 6a. It has been found that the bandwidth has also improved. The ground plane shown also exhibits several radiussed cut-outs on its periphery which are used for placement to ensure accurate fitment. Alternatively, positive engagement may be achieved solely by stud fastening means. Referring now to figure 9 an embodiment of the invention is shown wherein the reverse side of the antenna ground plane is integrated with electronic circuit means, the metallised plastics component 900 providing a shielding effect for the integrated group of circuits. For simplicity, only a portion of the component is shown - to satisfactorily cooperate with the octagonal antenna design, obviously the whole ground plane component would also need to be of an octagonal configuration. Sealing of each compartment can be easily effected by the attachment of a metal sheet (not shown) which abuts the smooth edges 910 of each radiation shielding compartment 912 Such electronic circuit means can include the radio transceiver and control processor. The antenna can thus be provided in a compact design. The radio transceiver communicates with the antenna via microstrip connections: the input/output is switched by means of a diplexer. The diplexer is preferably of a special configuration such that it may be conveniently installed within the antenna assembly and can be generally of a Υ' configuration. The provision of the electronic circuits within the antenna assembly enable a minimum of connecting wires to be attached to the assembly, necessitating only d.c. input voltage source and a wire enabling connection with telephone and facsimile equipment. Since track widths are directly proportional to track to ground plane spacing, the track to ground plane spacing in an antenna can cause many problems in the realisation of an antenna. In an antenna made in accordance with the present invention, the track impedances are not necessarily 50Ω; preferably only those at the input should be of this impedance, since this is a common standard interface impedance. By using varying track impedances, varying track widths and/or track to ground plane spacing can be utilised. A further advantage of having a low track to ground plane spacing is that a wide range of track impedances of 30Ω to 200Ω are easily realisable whilst a high track to ground plane spacing e.g. 4.5mm would result in track impedances typically in the range of 100-300 Ω. In order to further reduce fabrication costs it is preferred that expensive coaxial cable is not employed for the cable between the antenna assembly and equipment within a building served by the antenna assembly. Figures 8 and 8a show a protected cable as could be employed with an antenna made in accordance with the present invention, whilst Figure 8a shows a cross-section therethrough. In certain environments this requirement is particularly pertinent since the distance between the antenna and an internal junction box can be several tens of metres. It is to be understood that the cable needs to be rodent proof, strong enough to resist damage when weights or other damage inducing forces have been placed upon it. Accordingly, in order to further reduce costs, there is provided an improved form of cable trunking comprising standard telephony wiring inserted through metallic tubing. Preferably, the metal tubing is a galvanised steel for cheapness of fabrication and its resistance to corrosion. Fixing means are provided along the channel section to enable the trunking to be positioned against a fixture such as an external wall. A metallic construction for the trunking provides isolation from spurious electro-magnetic interference effects - as provided by coaxial cable but at a greater cost.

Claims: CLAIMS 1. A layered antenna assembly comprising a dielectric layer which supports antenna radiating elements and a feed network therefor and a ground plane, wherein the ground plane is spaced from the dielectric layer and comprises a metallization layer deposited on a formed plastics support. 2. An antenna assembly according to claim 1 , wherein the application of the metallization of the plastics support is performed by spray. 3. An antenna assembly according to claim 2, wherein the metallization spray is a water based spray comprising metallic particles suspended in a thermoplastic resin binder. 4. An antenna assembly according to any one of claims 1 to 3 wherein the plastics support for the ground plane comprises a polystyrene plastics compound. 5. An antenna assembly according to claim 4 wherein the polystyrene is produced in a two stage moulding process, the process comprising the steps of : placing non-expanded polystyrene granules in a two part mould, the mould parts being relatively movable with respect to each other; heating the polystyrene granules whereby the granules start to expand; compressing the two mould parts together whereby the therm o-expanded polystyrene is compressed to form a product having a smooth pore free finish. 6. An antenna assembly according to any one of claims 3 to 5 wherein the metallised plastics substrate forming the ground plane comprises at least two faces, one of which provides the ground plane and the other provides a partial enclosure for an electro-magnetic interference shield, whereby an enclosure of an electrical circuit board susceptible of creating electromagnetic interference can be effected. 7. An antenna assembly according to any one of claims 3 to 5 wherein the metallised plastics substrate forming the ground plane comprises two faces spaced apart in a parallel fashion, wherein one face is metallised fully to form a ground plane and the other face has a patterned metallization which provides both the feed network and the radiating elements for the antenna. 8. A layered antenna comprising a ground plane having depressions formed thereon, a dielectric layer and a dielectric film having an antenna array printed thereon, wherein the depressions correspond in position to the antenna elements; and wherein the depressions have radiussed edges. 9. A layered antenna according to claim 8, wherein the depressions have dielectric inserts positioned thereon.

Inventors: MCKINNON JOHN PETER BRUCE, MCKINNON, JOHN, PETER, BRUCE
Applicants: MCKINNON JOHN PETER BRUCE, NORTHERN TELECOM LTD, MCKINNON, JOHN, PETER, BRUCE, NORTHERN TELECOM LIMITED
Application year: 1995
Main classification: H01Q1
IPC codes: H01Q1, H01Q13, H01Q21