GB2215137A - Antenna screen - Google Patents

Abstract

A shield for masking an antenna comprises a plurality of panels (P1, P2) positioned, in use, side-by-side so as to surround the antenna. Each panel (P1, P2) has a sheet (3, 8) fixed to, and tensioned over, a frame constituted by at least three frame members (1 or 4, 5, 6). Each pair of adjacent frame members (1 or 4, 5, 6) of each frame is connected together by a respective corner joint (2, 7). Each sheet (3, 8) is made of material which is substantially transparent to radio waves and/or microwaves. <IMAGE>

Description

ANTENNA SCREEN This invention relates to a shield or screen for masking an antenna or antennae.

Radio, and in particular microwave, communications have proliferated in recent years. This has led to a proliferation of antennae, masts and towers in urban areas, particularly on the roofs of tall buildings in towns. These installations are disfiguring and unsightly, and pressure is mounting among planning authorities to demand masking of such equipment. In order to do this, a screen needs to be made of a material which permits free transmission of the waves concerned, whilst being capable of standing up to the high wind forces that occur at the roof top level of high buildings.

The present invention provides a shield for masking an antenna, the shield comprising a plurality of panels positioned, in use, side-by-side so as to surround the antenna, each panel having a sheet fixed to, and tensioned over, a frame constituted by at least three frame members, each pair of adjacent frame members of each frame being connected together by a respective corner joint, wherein each sheet is made of material which is substantially transparent to radio waves and/or microwaves.

Advantageously, the shield is constituted by four panels which, in plan, define a generally rectangular shape. Preferably, each of the panels is a quadrilateral, a first pair of opposed panels are generally rectangular, and the other pair of opposed panels are trapezoidal.

In a preferred embodiment, each sheet is made of polyvinylchloride-coated woven polyester. Preferably, this material is pierced by a regular array of holes having a diameter of, say, 18 mm. Alternatively, each sheet is made of polytetrafluoroethylene-coated woven glass fibre material or a woven polytetrafluoroethylene material. Each of these materials has the advantage of being light, strong and substantially transparent to radio waves and microwaves. The pierced material has the additional advantage of helping to prevent raindrop build-up, and of permitting wind to pass therethrough.

Advantageously, the frame members are made of extruded aluminium or pultruded glass reinforced plastic.

Preferably, each frame member is hollow, and of a generally hexagonal cross-section. Conveniently, each frame member is provided with at least one longitudinally-extending groove. In this case, each edge portion of each sheet is beaded. Preferably, each beaded edge portion is formed by wrapping the respective sheet edge portion round a nylon rope, and sealing the wrapped-over sheet portion to the adjacent main portion of the sheet.

In a preferred embodiment, each corner joint has first and second spigots which, in use, are fixed to the associated frame members. The first and second spigots of each corner joint may be fixed to respective first and second corner members which are connected together in such a manner as to permit relative rotation.

Advantageously, the first and second corner members of each corner joint have interlocking fingers, each interlocking finger having a generally circular aperture provided with internal splines, whereby the apertures define a generally cylindrical bore in the corner members. Each corner joint can, therefore, be locked in any one of a plurality of positions by means of an externally-splined pin which engages with the aligned splines of the fingers.

Preferably, the spigots of each corner joint are a telescopic sliding fit within the associated hollow frame members.

The invention also provides a method of making a panel constituted by a sheet fixed to a frame constituted by at least three frame members, the method comprising the steps of fixing the edges of the sheet to the frame members, connecting the ends of the frame members together loosely by corner joints, forcing the corner joints apart to tension the sheet, and fixing the frame members to the corner joints with the sheet member under tension.

A flat wall antenna screen constructed in accordance with the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a side elevation of one panel of the screen; Fig. 2 is a side elevation of another screen panel; Fig. 3 is a side elevation, on a larger scale, of one corner region of the panel of Fig. 2; Fig. 4 is a schematic transverse cross-section through a frame member of a screen panel; and Fig. 5 is a partial plan view of the material used for covering the panels.

A screen for masking an antenna or other radio or microwave transmitter is constituted by an open-topped structure having four adjoining side walls. A first pair of opposed side walls is constituted by a pair of identical panels P1 (see Fig. 1), the other pair of opposed side walls being identical panels P2 (see Fig.

2).

As shown in Fig. 1, the panel P1 is square, and has four identical frame members 1 joined together by corner joints 2. The edges of a tensioned fabric sheet 3 are fixed, in a manner described below, to the frame members 1 to complete the panel P1. Similarly, the panel P2 (see Fig. 2) has four frame members 4, 5 and 6 joined together by corner joints 7, and the edges of a tensioned fabric sheet 8 are fixed to these frame members. The panel P2 is, however, trapezoidal, so that it has two identical side frame members 4, a 'short' end frame member 5 and a 'long' end frame member 6. The sheets 3 and 8 are made of a polyvinylchloride-coated woven polyester material which is pierced by a regular array of 18 mm diameter holes (see Fig. 5). The advantage of this material is that it is light, strong, and substantially transparent to radio waves and microwaves.This transparency is enhanced by the piercings, which also help to ensure that the material does not suffer from rain drop build-up (which is a serious problem with all so-called 'radome' materials). The pierced material has the added advantage of permitting the wind to pass therethrough, so that panels P1 and P2 made using this sheet material can stand up to very high wind forces.

Alternatively, the sheets 3 and 8 are made of polyvinylchloride-coated woven polyester or polytetrafluoroethylene-coated woven glass fibres. The first of these alternative materials has the advantage of being cheap, light and strong, as well as being substantially transparent to all but higher frequencies.

The second of these alternative materials has all the advantages of the other materials, as well as being able to handle transmission at higher frequencies. It is, however, expensive. The choice of sheet material will, of course, depend upon the frequency of the waves transmitted by the antenna being shielded, and it is expected that the preferred (pierced) material will be satisfactory for most purposes. Where an antenna transmitting very high frequencies is to be shielded, the sheets 3 and 8 could be made of an all-polytetrafluoroethylene fabric, though this material is extremely expensive.

Each of the frame members 1 and 4 to 6 is made of extruded aluminium or pultruded glass reinforced plastic. A glass reinforced plastic frame is, however, preferable, as it has the considerable advantage over aluminium in that it is less disruptive to radio and microwave beams. Consequently, the location of glass reinforced plastic frame members relative to a shielded antenna is less crucial than for an aluminium frame. The frame members 1 and 4 to 6 have the hexagonal cross-section shown in Fig. 4, and each frame member has grooves 9 formed in a pair of opposed sides. Each of the frame members 1 and 4 to 6 is provided with one or more Y-shaped connecting lugs 10 (see Fig. 5). These lugs 10 are fixed to the frame members 1 and 4 to 6 by rivetting (or by any other suitable means), so that the stems of the Y-shaped lugs lie at right-angles to the common plane of the grooves 9.

Each sheet 3, 8 has the fabric of each edge portion wrapped round a nylon rope 11 (see Fig. 4). The nylon ropes 11 are held in position to define the peripheries of the sheets 3 and 8 by seam-welding the 'wrapped-over' sheet portions to the adjacent sheet portions.

Fig. 3 shows a pair of frame members 4 and 5 joined together by a corner joint 7. This joint 7 is typical of all the joints 2 and 7 used to connect adjacent frame members together, and is made of cast aluminium or cast glass reinforced plastic. The joint 7 has a pair of spigots 12a and 12b which each have a complementary cross-section to that of the hollow interiors of the respective frame members 4 and 5 so as to be a sliding fit therein. Each spigot 12a, 12b is fixed to a corner member 13a, 13b respectively. The corner members 13a, 13b are relatively rotatable about an axis which extends at right-angles to the axes of the spigots 12a and 12b. The corner members 13a and 13b have interlocking fingers (not shown), each of which has a generally circular central aperture which is internally splined.The splines of these central apertures are spaced apart by 100, and the corner joint 7 can be locked in any of 36 different angular positions by a hollow, externally-splined pin 14. Obviously, the external splines of the pin 14 are also spaced apart by 100. In order to lock the corner joint 7, with the spigots 12a and 12b at a given angle, the corner members 13a, 13b are rotated until their splines are aligned, and the pin 14 is then driven into position.

In order to construct a panel, say one of the panels P1, the sheet 3 is slotted into three of the frame members 1 by engagement of the rope-containing edge portions of the sheet within appropriate grooves 9 of the frame members. The corner joints 2 associated with these three frame members are then positioned by inserting their spigots 12a, 12b into the respective frame members, and by locking the joints with their spigots at right-angles by inserting the pins 14. The fourth frame member 1 is then slotted over the sheet 3, and the second pair of corner joints 2 positioned. Because of the restraint now imposed by the fabric of the sheet 3, the spigots 12a, 12b of these corner joints 2 can only be inserted into the hollow interiors of the associated frame members 1 via cut-outs (not shown) in the end portions of these frame members.The panel P1 is then placed on a flat jig (not shown) for tensioning. The frame of the panel P1 is gripped at each of the corner joints 2 by means of pins (not shown) passing through the hollow pins 14. The four corners of the jig are then screwed apart simultaneously, thus drawing the spigots 12a, 12b of the corner joints 2 out of the frame members 1, and thereby tensioning the sheet 3. This process is continued until the sheet 3 is tensioned to a predetermined degree. The spigots 12a, 12b of the corner joints 2 are then drilled through the frame members 1 and rivetted, thereby fixing the frame members to the spigots with the sheet 3 pre-tensioned. Four small plates (not shown) are then rivetted over the cut-out portions at the ends of the frame members 1 associated with the second pair of corner joints 2.The panel P1 can then be removed from the jig.

The lugs 10 are used to fix the panels P1 and P2 to additional supporting steelwork (not shown) surrounding the antenna to be screened. Adjacent frame members of adjacent panels could be fixed together by any suitable means. Alternatively the integrity of a complete shield could be maintained by the attachment of the panels P1 and P2 to the supporting steelwork by the lugs 10.

Depending upon the size of the antenna, or the number of antennae, to be screened, more panels can be used. For example, a pair of panels P2 could be used as end panels, and a plurality of pairs of panels P1 could be used as side panels.

Obviously, a number of modifications could be made to the screen described above. For example, the angle between adjacent splines of the central apertures of the fingers is not critical. Thus, where finer angular adjustments are required, the angle between each pair of adjacent splines may be reduced to, say, 50.

Alternatively, where the corner members are made of glass reinforced plastic, the joints could be made infinitely adjustable by doing away with the splines, and could be locked in any desired position by an injected resin.

In a further modification, each sheet 3 or 8 is tailored so as to have more fabric at the centre of each edge portion. This helps to distribute the tension forces more evenly from the sheets to the associated frames.

Claims (19)

1. A shield for masking an antenna, the shield comprising a plurality of panels positioned, in use, side-by-side so as to surround the antenna, each panel having a sheet fixed to, and tensioned over, a frame constituted by at least three frame members, each pair of adjacent frame members of each frame being connected together by a respective corner joint, wherein each sheet is made of material which is substantially transparent to radio waves and/or microwaves.

2. A shield as claimed in claim 1, wherein the shield is constituted by four panels which, in plan, define a generally rectangular shape.

3. A shield as claimed in claim 2, wherein each of the panels is a quadrilateral, a first pair of opposed panels are generally rectangular, and the other pair of opposed panels are trapezoidal.

4. A shield as claimed in any one of claims 1 to 3, wherein each sheet is made of polyvinylchloride-coated woven polyester.

5. A shield as claimed in claim 4, wherein the material of each sheet is pierced by a regular array of holes each having a diameter of, say, 18 mm.

6. A shield as claimed in any one of claims 1 to 3, wherein each sheet is made of polytetrafluoroethylene-coated woven glass fibre material or a woven polytetrafluoroethylene material.

7. A shield as claimed in any one of claims 1 to 6, wherein the frame members are made of extruded aluminium or pultruded glass reinforced plastic.

8. A shield as claimed in any one of claims 1 to 7, wherein each frame member is hollow, and of a generally hexagonal cross-section.

9. A shield as claimed in any one of claims 1 to 8, wherein each frame member is provided with at least one longitudinally-extending groove.

10. A shield as claimed in claim 9, wherein each edge portion of each sheet is beaded, each beaded edge portion being formed by wrapping the respective sheet edge portion round a nylon rope, and sealing the wrapped-over sheet portion to the adjacent main portion of the sheet.

11. A shield as claimed in claim 10, wherein each beaded edge portion of each sheet is a longitudinal sliding fit within, but cannot be moved laterally out of, the longitudinally-extending groove of a respective frame member.

12. A shield as claimed in any one of claims 1 to 11, wherein each corner joint has first and second spigots which, in use, are fixed to the associated frame members.

13. A shield as claimed in claim 12, wherein the first and second spigots of each corner joint are fixed to respective first and second corner members which are connected together in such a manner as to permit relative rotation.

14. A shield as claimed in claim 13, wherein the first and second corner members of each corner joint have interlocking fingers, each interlocking finger having a generally circular aperture provided with internal splines, whereby the apertures define a generally cylindrical bore in the corner members.

15. A shield as claimed in claim 14, wherein each corner joint is provided with an externally-splined pin which is engageable with the aligned splines of the interlocking fingers of that corner joint to lock that corner joint in any one of a plurality of positions.

16. A shield as claimed in any one of claims 12 to 15, wherein the spigots of each corner joint are a telescopic sliding fit within the associated hollow frame members.

17. A shield for masking an antenna, the shield being substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

18. A method of making a panel constituted by a sheet fixed to a frame constituted by at least three frame members, the method comprising the steps of fixing the edges of the sheet to the frame members, connecting the ends of the frame members together loosely by corner joints, forcing the corner joints apart to tension the sheet, and fixing the frame members to the corner joints with the sheet member under tension.

19. A method of making a panel substantially as hereinbefore described with reference to the accompanying drawings.