GB2221351A

GB2221351A - Antenna

Info

Publication number: GB2221351A
Application number: GB8916938A
Authority: GB
Inventors: Richard Peter Incledon Scott; Ian Gordon Rose
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1988-07-27
Filing date: 1989-07-25
Publication date: 1990-01-31
Anticipated expiration: 2009-07-25
Also published as: JPH03500004U; AU4048789A; WO1990001223A1; GB2221351B; GB8817885D0; GB8916938D0; AU634485B2; DE8990076U1

Abstract

A millimetre wave antenna for television reception comprises an offset parabolic reflector (3) surrounded by an integral skirt (4), against which a feed-horn (7) may be located. The antenna may be moulded from plastics material using thermo-forming, or (if formed integrally with a radome (6)) using blow-moulding.

Description

ANTENNA This invention relates to a millimetre wave antenna, and to methods of forming a millimetre wave antenna; particularly, but not exclusively, a millimetre-wave video signal reception antenna.

It has been proposed ("Millimetre Waves applied to TV Distribution", Digest of IEE Colloquium on mm-Wave and IR Applications, Devices and Propagation, 19 May 1987, M. Pilgrim) to employ the as-yet untapped millimetre-wave region of the radio spectrum for multichannel TV distribution. A transmitter in such a distribution system would typically have a range of about 3km, and (in urban areas) be able to serve typically 5,000 to 10,000 households; ths is sufficient for many small communities.

Each household to be served will therefore require a mm-wave receiver antenna. With the advent of satellite broadcasting, it became apparent that providing accurately formed reflector antennas (accuracy is of great importance in mm-wave and microwave reflecting surfaces) at an attractive price to domestic consumers posed many major manufacturing problems, and satellite reception antennas, even after exhaustive research into manufacturing processes, still cost on the order of hundreds of pounds sterling. The problems found with producing satellite antennas apply equally to mm-wave reflector antennas.

The trend is to avoid these problems by using instead a printed array antenna, but this technology is not yet fully developed.

According to one aspect of the invention, there is provided a micro- or millimetre wave antenna comprising a reflector portion, a cylindrical skirt portion surrounding the reflector portion, the reflector portion being non-axisymmetrically positioned around the longitudinal axis of the skirt portion and having a convex curvature relative to the interior of the skirt portion, and a feed positioned adjacent the periphery of the skirt, so as to provide an offset feed to the reflector portion, the skirt and reflector portions constituting an integral body.

Other aspects of the invention are defined in the claims.

The invention will now be described, by way of example only, with reference to the drawings in which; - Figure 1 illustrates a millimetre wave transmission system; - Figure 2 is a sectional view of an antenna according to a preferred embodiment of the invention; - Figure 3 is an exploded view of an antenna according to a preferred embodiment of the invention; - Figure 4 illustrates a first method of manufacturing an antenna according to the invention; and - Figure 5 illustrates a second method of manufacturing an antenna according to the invention.

Referring XQ Figure 1, a millimetre-wave transmission system comprises the transmitter (1), sited at a local high point and associated with the head-end (la) which controls a programme mix drawn from: - TVRO/DBS satellites at several orbit positions; - off-air UHF channels; - cable TV programmes, extending nearby cable system; - taped programmes injected at the head-end; - high definition TV (HDTV) when available; - local-interest programmes.

Using mm-wave transmission, eg at 29 GHz or 39 GHz, the transmitter would serve an area approximately 3km in diameter. Typically, this will encompass between 5,000 and 10,000 households. Coverage will be determined by considerations of transmitter power, antenna gain, receiver sensitivity, grade of performance required, and line-of-sight limitations due to trees, buildings etc. A customer at the edge of the coverage area would receive a picture quality better than CCIR Grade 4 for 99.9% of all time (compare the DBS target of better than Grade 4 for 99% of the worst month). Under normal conditions, a Grade 4.5 picture (or better) would be achieved. If neither HDTV nor extended definition TV (EDTV) were required, then the same performance could be achieved over about 3.5 km diameter.

The receiver antenna (2a, 2b, 2c) is a small unobtrusive reflector of about 15 cm diameter or a printed array of similar area. The signal passes via a downlead to the indoor receiver, which is of the type already in volume production for satellite reception.

Between 10 and 30 channels can be carried, selected to meet market requirements whilst remaining compatible with channel spacing, spectrum availability, and frequency re-use objectives. For example, 20 channels spaced at m'ltiples of 19.18 XHz, as used for direct broadcast satellite (DBS) service, could be chosen. The system can be configured for either PAL or MAC formats, and is transparent to conditional access. The mm-wave spectrum offers sufficient bandwidth potential to handle EDTV and HDTV transmissions when these become available.

Referring to Figure 2, an embodiment of an antenna according to the invention and suitable for use in the above system comprises a single plastics moulding having a rear dish portion 3, and a cylindrical surrounding skirt portion 4, the dish portion 3 being disposed non-axisymmetrically about the long axis of the cylindrical skirt portion 4. The degree of non-symmetry is sufficient that radiation along, or close to, the longitudinal axis of the skirt portion is focussed at a point proximate the surface of skirt portion, so that an offset feed may be mounted on the skirt (or a component affixed to the skirt) without blocking the radiation path. The shape of the dish portion follows the formula for an offset parabola, as follows: z= 2, 3 z=x + y 4f where f is the focal length (which may be, for example, 77.5 mms).It is not, of course, possible to produce non-axisymmetrical structures of this kind by spinning (the normal process used to fabricate antennas).

At the other end of the skirt portion 4 a mating flange 5 may be provided, to which a radome 6 is sealed.

A feed horn 7 is mounted, offset relative to the dish portion 3 through a hole in the skirt portion 4, so as to illuminate a considerable proportion of the dish. In one embodiment, the exterior surface cf the reflector dish may be metallised, so as to act as the reflecting surface.

From a technical viewpoint, the effectiveness of this arrangement is somewhat surprising since a signal has to pass through plastics layers three times (once through the radome, and twice through the reflector), each time at a different angle - and the dielectric constants of plastics are significantly non-unity. It is therefore necessary, if using this structure, to slightly redesign the shape of the reflector (or the radome) to correct the distortions introduced by the plastics layer. Referring to Figure 3, the long side of skirt portion 4 is mounted on a conformal mounting block 8 by screws 9a, 9b etc passing through slots in skirt portion 4; the mounting is slideable to allow the antenna to be aligned during assembly on the block 8 using an alignment key, so as to accurately position the antenna relative to the feed horn 7.The feed horn 7 is rigidly fixed to an integrated down-converter module 10, to which mounting block 8 is also fixed, and the whole assembly is unobtrusively affixed at a suitable location using suitable supporting brackets.

It will be seen that the skirt portion 4 performs several useful functions. Firstly, it is accurately formed with the reflector, and provides a mounting for the reflector which obviates the need to apply mechanical stresses to the reflector (either in fixing the reflector or as a result of in-use applied stresses). Secondly, if metallised, it reduces the unwanted radiation impinging upon the reflector and hence increases its directivity.

Thirdly, it mechanically stiffens the reflector dish portion against elastic deformation due to, for example, wind loading. The flange 5 also stiffens the skirt portion 4.

The antenna as a whole has a much-simplified structure, with a small number of parts to be assembled together, thus reducing misalignment problems. The feed horn 7 may conveniently be fabricated as a single die-casting integral with its waveguide, giving manufacturing advantages over prior feeds which are formed as separate parts and then assembled together.

The plastic radome 6 may be attached to the flange 5, as shown, using a sealing strip. Alternatively, the antenna may be formed as a single unitary whole, including the radome, thus further reducing the assembly time and providing a weathertight seal.

Preferably, in either case the face of the radome 6 is inclined so as to have an overhang in use, to prevent the accumulation of rain, snow or ice in the radiation path.

This is particularly important in the mm waveband, as water can severely attenuate such radiation.

The invention therefore provides an antenna which can be accurately and cheaply formed, with sufficient rigidity and robustness for long term outdoors mounting at residential premises and a method of manufacturing such an antenna.

It will be appreciated that the features of the antenna also lend themselves to the provision of attractive features of appearance (such as a streamlined appearance in the farings between the portions) which enhance the attractiveness of the product for the domestic consumer.

Referring now to Figure 4, a method cf manufacturing an antenna according to the invention will now be described.

A mould 11 including a cavity having a circular horizontal cross-section truncated by an off-set paraboloid vertical cross-section, is positioned in a vacuum-forming machine. The dimensions of the mould are slightly bigger than those required in the antennas so as to allow for the degree of shrinkage which is common with thermo-plastic materials; with a 155 mm diameter antenna, for example, a half millimetre shrinkage is not uncommon.

The easiest method of ascertaining the required shrinkage is by trial and error, as is well known to those skilled in the art; this will depend on the material, the thickness of the material, and the process temperature, but is constant for a fixed set of these parameters.

A sheet of plastics material 12, for example, Acrylonitrile-Butadiene Styrene (ABS), having a thickness of, for example, 2.5 mm, is clamped over the mould in known fashion and heated to its softening point. A pressure differential is created across the sheet, for example by connecting a vacuum pump to an orifice or orifices 13 in the mould, so that the softened sheet is drawn down into close contact with the mould. Both heat and vacuum are then removed, and the sheet is allowed to cool on the mould; in so doing, it shrinks to the required dimensions. It will be understood that other suitable thermo-forming techniques, such as drape moulding or billow moulding, may equally be applied to manufacturing antennas according to the invention.

It will be appreciated that it is possible to form the antenna with either a female or a male mould. If the plastics material in question is non-reflective (and therefore has to be metallised) then the reflecting layer should then be applied to the inner surface of the reflector dish portion 3, as this is the surface which has been accurately formed against the mould 11. This is somewhat problematic, for two reasons - firstly, it is physically less accessible than the outer surface, and secondly, it is necessary to accurately control the evenness of the metallising layer since the outer surface of that layer will be the actuai reflecting surface.

Satisfactory results are obtained, however, using spray-paint or vacuum metallising - provided care is taken.

In a first method according to the invention, this process is improved by forming the sheet 12 into a female mould 11 as described above, and then metallising the outer (convex) surface of the moulding. There is thus no need for accurate control over the metallising coating since it is the inner surface of the metallising coating which acts as the reflector. This surface is also thus protected against dust and grit by the plastics reflector portion 3. The coating may be simply applied as a spray paint. It is, however, necessary (as mentioned above) to employ a slightly non-parabolic reflector profile (or a non-flat radome surface).

Referring to Figure 5, a second method of manufacturing an antenna according to the invention will now be described. In this method, blow moulding is employed to form a one-piece antenna including radome.

The mould is formed in two parts; a preferred arrangement is as shown in Figure 5, where mould part lla has the same shape as mould 11 in Figure 4, and mould part llb is essentially cylindrical. Two sheets of plastics material 12a, 12b are cut to size and introduced side by side into the moulds as shown; the moulds are then clamped together. A strong hollow needle 14 is placed between them, and connected to a source of compressed air. Heat is applied to the mould Ila, lib, so that the plastics sheets soften, as before. Compressed air is then introduced between the sheets 12a, 12b, so as to create a pressure differential across each sheet, and force them out into the mould. Subsequently, the mould is cooled (eg by cooling water) and the pressure is released; when they are cold, the moulds gila, lib are separated, and the moulding removed (the heat having welded the sheets together at their edges) to be metallised as described above. Both these processes are relatively cheap to set up.

It will be understood throughout the foregoing that the term "cylindrical" does not indicate necessarily a circular cross-section, but includes similar tubular shapes (which need not have a constant cross-section), although a circular cross-section is preferred so as to present an even radiation pattern. It will equally be understood that the invention is capable of operating either as a transmitter or a receiver (although it is primarily intended to be a domestic receiver).

Claims

1. A micro- or millimetre wave antenna comprising a reflector portion, a cylindrical skirt portion surrounding the reflector portion, the reflector portion being non-axisymmetrically positioned around the longitudinal axis of the skirt portion and having a convex curvature relative to the interior of the skirt portion, and a feed positioned adjacent the periphery of the skirt, so as to provide an offset feed to the reflector portion, the skirt and reflector portions constituting an integral body.

2. An antenna according to claim 1, in which said body is formed of plastics material.

3. An antenna according to claim 1 or claim 2, further comprising a flange ring, disposed around the other end of the skirt portion to the reflector portion, adapted for coupling to a radome.

4. An antenna according to claim 3, in which the plane of said ring is inclined to the axis of the skirt portion so as to impart, in use, an overhang to the radome.

5. An antenna according to claim 2, further comprising a radome portion unitary with the skirt portion.

6. An antenna according to claim 5, in which the radome is inclined so as to overhang in use.

7. An antenna according to any one of claims 2 to 6, in which the reflector portion is transparent to radiation, and the outer surface of the reflector portion is coated with a reflective layer, so as to act as the reflecting surface thereof.

8. An antenna according to claim 7, in which the profile of the reflector portion deviates from a parabolic profile in such a manner as to compensate for refraction through the reflector portion.

9. An antenna according to claim 7, in which the surface of the radome is curved in such a manner as to compensate for refraction through the reflector portion.

10. An offset reflector antenna formed by drawing a sheet in the plastic state over a male mould.

11. A domestic video reception antenna according to any preceding claim.

12. A method of manufacturing a micro- or millimetre wave antenna comprising the steps of; - forming a sheet of thermoplastics material into a concave parabolic mould, and; - metallising the convex surface thus formed.

13. A method according to claim 12, in which the concave mould has a cylindrical wall, so that the antenna is formed with a unitary cylindrical skirt portion.

14. A method according to claim 12 or claim 13 in which the process used to form the sheet is a vacuum-forming process.

15. A method according to claim 13, in which the process used to form the sheet is a blow-moulding process, and the mould further is shaped to receive a second sheet, so that the antenna is formed with an integral radome.

16. A method of manufacturing an antenna according to any one of claims 2 to 4, comprising thermoforming a sheet of thermoplastic material over a male mould.

17. A millimetre wave antenna substantially as herein described, with reference to the accompanying Figures 2 or 3.

18. A method of making a millimetre wave antenna substantially as herein described, with reference to the accompanying Figure 4 or Figure 5.