WO1990013927A1

WO1990013927A1 - Antenna system

Info

Publication number: WO1990013927A1
Application number: PCT/GB1990/000688
Authority: WO
Inventors: Max Jonathan Lazarus
Original assignee: The University Of Lancaster
Priority date: 1989-05-03
Filing date: 1990-05-03
Publication date: 1990-11-15
Also published as: AU5554690A

Abstract

An antenna system for use in transmitting and receiving beams of electromagnetic waves of frequencies up to and including the sub-millimetre region of the spectrum comprises a horn (1) and a lens arrangement (4, 5) all of which are of a dielectric material. The feeder (1) may be solid or hollow and may be moulded integrally with a lens. Preferred dielectric materials are low loss media such as polyethylene. The antenna is cheap and easy to manufacture and has efficiency at least as good as quality precision metal antennae.

Description

ANTENNA SYSTEM

The present invention relates to an antenna system, and more particularly to such a system for use in transmitting and receiving beams of electromagnetic wave of frequencies up to and including the sub-millimetre region of the spectrum. For convenience, such waves are referred to generically herein as waves of the kind defined.

The invention is concerned particularly (but not exclusively) with an antenna system for so called millimetric microwaves typically having a frequency above 10 GHz, but there is also the possibility of the invention being adopted for lower frequencies (radio waves) or higher frequencies (eg. up to 300 GHz (such-millimetre waves)).

Directional millimetric microwave beams are used in many applications, eg. communications by millimetric microwave links, and RADAR. It is general practice for these beams to be transmitted and received by an antenna system which comprises a horn positioned at the focus of a parabolic reflector dish. During transmission, the waves emanating from the horn are reflected by the dish as a collimated directional beam. Similarly an incoming directional beam is reflected by the dish for collection by the horn.

The conventionally used horns and parabolic dishes do, however, have a number of disadvantages. In particular the horns are generally made of metal and it is comparatively difficult and expensive to construct horns to the required shapes and tolerances. Furthermore, the horn will generally be supported (at the focus of the dish) by a strut or the like and this may obstruct the beam. Finally, the parabolic dish and horn may not be very durable having regard to the weather to which they may be exposed, so they may require an extra housing or "radome".

EP-A-0 123 870 discloses a Doppler radar apparatus for use in a velocity measurement system for farm tractors. The apparatus comprises a conical horn formed of aluminium (because aluminium may be readily cast into complex shapes) and a glass filled polyester lens provided over the 'mouth' of the horn so as to assist in focussing of the beam. However metal horns (and indeed any other metal components - such as reflector dishes) need scrupulously clean and uncorroded surfaces to be at all effective at millimetre wavelengths. The metal surfaces are very difficult to preserve and the provision of plastics coatings is not a solution since large areas of plastic coating on metal are notorious for peeling.

It is therefore an object of the present invention to obviate or mitigate the abovementioned disadvantages.

According to the present invention there is provided an antenna system comprising a feeder capable of transmitting or receiving waves of the kind defined, and a lens arrangement through which beams are transmitted from or received by the feeder, said feeder and lens arrangement, being of a dielectric material and being such that an incoming directional beam is focussed, by its passage through the feeder and lens arrangement, at or near a predetermined point within the feeder and conversely the passage of an outgoing beam emanating from the locality of the said point, and then transmitted through the feeder and lens arrangement, produces a collimated beam.

Preferably the dielectric materials, from which the feeder and lens arrangement are produced, are low loss media, examples of which are plastics such as polyethylene, polyester and cross- linked polystyrene (eg. Rexolite - Registered Trade Mark). The use of plastics materials avoids the corrosion problems mentioned above. The dielectric material should be as pure as possible and free of pigments etc.

The dielectric feeder and/or the lens arrangement may however comprise a particle filled polymeric material. The filler may for example be glass (preferably hollow) particles. The particle filled polymeric material may be a syntactic foam. The use of filled polymeric materials (particularly glass filled materials) provides feeders and/or lens(es) of higher strength at elevated temperatures (than in the case of polyethylene feeders and lens arrangements) such as may be required for various uses envisaged for the antenna system.

The feeder/lens arrangements of the invention (particularly those of plastics material) are far more rugged and free from microphony as compared with metal structures. Thermal expansion and vibration problems are eliminated.

The dielectric feeder will for preference, be a tapering horn, particularly a pyramidal or conical horn. It is not however essential that the horn tapers to a distinct apex : it is sufficient that the horn tapers to an 'area' with a maximum cross-sectional dimension which is less than a wavelength. The horn may be of solid or hollow cross-section, e.g. rectangular, square, circular or elliptical. The use of a horn of solid cross-section is particularly advantageous in that the solid horns in addition to acting as a guiding and focussing medium, provides in combination with the lens a self-aligning system. These systems give much greater freedom from cross polarization aberations which plague metal systems.

The horns may be moulded or cast easily and cheaply from low loss plastic dielectric materials, (eg. by injection moulding) thereby obviating the difficulty and expense involved in the production of metal horns.

If desired, the horn may have a metallic coating (eg. silver) to enhance its waveguiding properties.

In use, the horn will be associated with a source of waves of the kind defined, preferably a source of microwave radiation, eg. a Gunn diode, Impatt diode or a field effect transistor. Likewise, the horn will be coupled to the receiver device, which may be one of the above as a self-oscillating mixer, amplifier, or an additional mixer/detector diode. Any of the above may be coupled to the horn via conventional metal or dielectric waveguide, microstrip, or other transmission line. In one embodiment of the invention, the horn is moulded in plastic material with an integral waveguide in which the source and/or detector of the radiation may be located.

However, in a more advantageous development (for the case where the dielectric feeder tapers towards and apex) the waveguide is located on the dielectric feeder by a locating element in which the apex of the feeder is received.

For preference the waveguide is of rectangular section and there should be a smooth transition in going from the waveguide to the locating element.

The locating element need extend only a minor distance along the face(s) of the feeder from the apex thereof.

The locating element may take a number of forms. For example it may be a tapering 'sleeve' with an open mouth through which the apex of the feeder is received (the apex of the sleeve being fast with the waveguide). Alternatively, the locating element may be of more 'solid' construction formed with a tapering recess. In a further alternative the locating element may be of a plastics material shrunk onto the waveguide.

The use of the locating element is advantageous in that it readily allows the tapering dielectric feeder to be used in conjunction with a rectangular section metallic waveguide element. Without the provision of such a locating element, the bonding of a rectangular section metallic waveguide directly to the dielectric feeder device may provide problems in producing the antenna system.

The function of the lens/feeder arrangement is to focus an incoming beam at or near a particular point in the dielectric feeder. Similarly, the radiations diverging from the locality of that point will be transformed by the lens arrangement into an outgoing collimated beam. The lens arrangement functions in a manner entirely analogous to an optical lens, and the well understood principles of optics may be applied to the design of the dielectric lenses so that they produce focussed or collimated beams as desired above. It will be appreciated that the focussing of an incoming beam is effectively the opposite to the production of a collimated beam.

In one embodiment of the invention, a single lens may be used to focus/collimate the beam of radiation. However, this lens should not be of too short a focal length, since the curvature associated with the lens may give rise to losses in the radiation beam by reflection from the surface of the lens. The use of a single lens will thus, generally, require a comparatively long horn in which the beam may be focussed.

It is more preferred to use two or more lenses (particularly plano-convex lenses) for focussing the beam. Such a combination allows the beam to be focussed over a much shorter distance, thereby allowing the use of short horns. The flare angle of a tapering horn will not be critical (but will affect the length of the horn) although will typically be in the range 40°-50°.

In a particularly advantageous development of the invention, the horn may be integral with a focussing lens. Such a horn/lens arrangement may be produced, for example by moulding a cone of plastic material (eg. polyethylene) with a convex base, and then forming the body of the cone into a generally pyramidal shape (to define the horn) with its quadrilateral base having a gradual transition towards the convex base which forms the lens. These integral feeder/lens constructions have been proven experimentally as a high efficiency antenna. The structures have no frequency dependent disadvantages.

Furthermore, in any of the above constructions a layer of foam dielectric material may be provided between the 'mouth' of the feeder and the lens. Such a foam layer would act partly as a focussing medium and partly for structural purposes.

The invention will be further described by way of example only with reference to the accompanying drawings in which:

Fig. 1 is a cross-sectional view of one embodiment of antenna in accordance with the invention;

Fig. 2 illustrates a combined horn and lens arrangement; and

Fig. 3 is a detail of the apex of a dielectric feeder and associated waveguide.

Referring to Fig. 1 the illustrated antenna comprises a pyramidal horn mounted in a generally cylindrical housing 2. The horn 1 is moulded from a low loss dielectric material (eg. polyethylene, a glass filled plastic, a syntactic foam or Rexolite) and may, if desired, have a metallic coating on its triangular faces.

A waveguide 3 is mounted at the apex of the pyramidal horn and has a flange 3a affixed to a bracket 2a on an end cap of the housing. The waveguide 3 and is associated with a source (not shown) of radiation.

Two plano-convex lenses 4 and 5 are provided as shown between the 'mouth' of the horn 1 and an open end 6 of the housing 2. Each lens 4 and 5 is of a low loss dielectric material (eg. polyethylene or Rexolite). The path of a millimetric beam being focussed or collimated by the lenses 4 and 5 is illustrated by the dashed lines 7.

The antenna can, if desired, be covered in a foam shroud for shock absorption.

The illustrated antenna is cheap and easy to manufacture. Efficiency measurements have shown that it is at least as good as quality precision metal antennae. By way of example, an antenna of the type illustrated in Fig. 1 made of polyethylene and in which the horn has a flare angle of 40°-50° and a "mouth" of 10 cm diameter showed attenuation of less than 30%. The antenna does not have the disadvantages of obstructing its own beam by the horn, feeder or support bracket rods nor does it suffer problems of corrosion.

In an alternative construction to that shown in Fig. 1, the pyramidal horn could be replaced by a conical horn optionally with a metallic coating.

In a modification of Fig. 1 it is possible to replace the horn 2 and separate lens 4 by one piece moulding 10 (see Fig. 2) which combines both functions. This moulding incorporates a pyramidal horn section 11 and lens 12 connected to the horn by a transition region 13. At the apex of horn 11 is an integrally moulded spigot-like extension 14 of the horn 11 with an aperture 15 in which a source/detector of radiation may be positioned. The 'apex' of the horn will not in this case be a distinct point but will be a small area (integral with spigot 14) with a maximum cross-sectional dimension of less than a wavelength. Alternatively, this "bulb" could couple at its apex to a waveguide/transmission line, e.g. by means of a locating element as described below with reference to Fig. 3. Furthermore, the tapering horn section 11 need not be pyramidal but could, for example, be of circular, rectangular, or elliptical cross-section, as could the spigot 14.

Fig. 3 shows the use of a locating element 20 for positioning a waveguide 21 on the apex of a conical dielectric feeder horn 22. The metal waveguide 21 is associated with the frustoconical locating element 20 in which the tip of the horn 22 is located. There is a smooth transition between the locating element 20 and the waveguide 21. Once again the horn need not converge to a distinct point but could converge to an 'area' with a cross-sectional dimension less than a wavelength.

Claims

1. An antenna system comprising a feeder capable of transmitting or receiving waves of the kind defined, and a lens arrangement through which beams are transmitted from or received by the feeder, said feeder and lens arrangement being of a dielectric material and being such that an incoming directional beam is focussed, by its passage through the feeder and lens arrangement, at or near a predetermined point within the feeder and conversely the passage of an outgoing beam emanating from the locality of the said point, and then transmitted through the feeder and lens arrangement, produces a collimated beam.

2. An antenna system as claimed in claim 1 wherein the dielectric materials are low loss media.

3. An antenna system as claimed in claims 1 or 2 wherein the dielectric material of the feeder and/or that of the horn are in a plastics material.

4. An antenna system as claimed in claim 3 wherein the plastics material is polyethylene, polyester or cross-linked polystyrene.

5. An antenna system as claimed in claim 2 or 4 wherein the plastics material is a glass filled plastics material or a syntactic foam.

6. An antenna system as claimed in any one of claims 1 to 5 wherein said lens arrangement comprises at least one lens which is separate from said feeder.

7. An antenna system as claimed in claim 6 comprising two lenses each separate from the feeder.

8. An antenna system as claimed in any one of claims 1 to 5 having a lens arrangement moulded integrally with the feeder device.

9. An antenna system as claimed in any one of claims 1 to 8 wherein the dielectric feeder is a tapering horn.

10. An antenna system as claimed in claim 9 wherein the horn is pyramidal or conical.

11. An antenna system as claimed in claim 9 or 10 wherein the horn is of solid sections.

12. An antenna system as claimed in any one of claims 9 to 11 wherein the horn has a flare angle of 40°-50°.

13. An antenna system as claimed in any one of claims 9 to 12 wherein the horn tapers to an area with a maximum cross- sectional dimension less than one wavelength.

14. An antenna system as claimed in any of claims 9 to 13 having a waveguide mounted on the horn by a locating element which locates over the apex of the horn.

15. An antenna system as claimed in claim 8 provided with an integrally moulded spigot adapted to receive a source or detector of radiation.

16. An antenna system as claimed in any one of claims 1 to 15 wherein a layer of foam material is provided between the feeder and the lens arrangement.