GB2275133A - Dipole antenna with a plurality of sub-reflectors - Google Patents

Abstract

An antenna has a dipole box (6) in which there is a dipole (13), a sub-reflector (14) and an additional front reflector (19), mounted forwardly of the sub-reflector (14). The additional front reflector (19) is preferably approximately one quarter wavelength from the dipole (13) and there is preferably a separation of approximately one tenth wavelength between the additional front reflector (19) and the sub-reflector (14). There may be a series of additional front reflectors of different sizes. <IMAGE>

Description

"A Dipole Antenna1 The invention relates to an antenna of the type comprising a support frame which supports a main reflector and an energised component or dipole, and more particularly to such an antenna for operation in the microwave range of lmm to Im.

Such antennae are well known. In general, the main reflector may be of any suitable shape and is most commonly parabolic. The main reflector need not be solid, but can be of mesh construction, in which case the loss is minimised by relating the mesh for use at a particular wavelength to well known ratios of wire thickness and gap size between wire centres. Such a mesh construction has the advantage of reduced wind resistance and reduced weight and cost of roof top support fixtures.

The efficiency of the dipole depends on a match of several variables in its construction such as the linear dimensions of the dipole and balun, and the dimensions and material of the dielectric which separates the core of the coaxial cable from the balun pieces. In general, construction of the antenna depends on the required characteristics of the dipole which besides high gain may include a bandwidth requirement, a cross polarisation ratio requirement, a back-to-front ratio requirement and a beam width requirement.

A well known approach to increasing the gain of an antenna is to use a sub-reflector which is relatively small in comparison with the main reflector and is placed on the opposite side (forwardly) of the dipole or active element.

Such a sub-reflector may be inside a dipole box, but may alternatively be attached outside the box, for example, as described in United States Patent Specification No.

5,191,350 (Conifer Corporation). In this specification, the antenna has a parabolic main reflector and a dipole and there is an apex-shaped sub-reflector mounted forwardly of the dipole outside of the dipole box.

British Patent Specification No. GB 2 211 358 (Kabelmetal) describes an energised dipole which has metal pieces attached to the outer conductor of the coaxial cable.

For competitive high-volume production of dipole antennae, the manufacturer must attempt to improve the operating characteristics and in particular the gain as much as possible, while working within strict production cost requirements. The overall size of the main reflector is generally limited by cost, weight, and wind resistance considerations and accordingly any improvements in gain are achieved by tweaking the various constructional aspects and improving accuracy etc. in the hope of obtaining a slight improvement in gain, possibly of the order of a fraction of a dB. For example, in US Patent Specification 5,191,350 the main reflector is of die cast magnesium material, an expensive attempt to achieve improvements in weight and accuracy.

In summary therefore, the invention is directed towards providing for a significant improvement in gain of a dipole antenna for operation in the microwave wavelength range with only a minimal increase in production costs.

According to the invention, there is provided a dipole antenna comprising a frame member supporting a main reflector, a dipole mounted forwardly of the main reflector, and a sub-reflector mounted forwardly of the dipole, the configuration and mounting of the main reflector and of the dipole defining the radiation wavelength for operation of the antenna, wherein the antenna further comprises at least one additional front reflector mounted forwardly of the dipole, being within a distance of one wavelength from the dipole, and being substantially parallel in the plane of polarisation to a tangent of the main reflector.

It will thus be appreciated that in an extremely simple manner the invention provides for a significant improvement in gain with only a minimal additional cost in production. Because the invention is quite simple it may initially appear to be obvious on the basis that, for example, it is a combination of a Yagi type antenna with a parabolic configuration. However, in the case of an indirect Yagi antenna, directors must be placed between the parabolic reflector and the active element with a back reflector beyond the active element. With a direct Yagi, a small back reflector should be placed between the parabolic reflector and the active element. This latter arrangement would be clearly detrimental to the efficiency of the main parabolic reflector.Therefore, the configuration of a parabolic reflector with a subreflector and at least one additional front reflector does not work as a Yagi antenna. This can be easily demonstrated by the fact that a second and subsequent additional front reflector in an antenna of the invention has no effect on gain, thus demonstrating that the additional front reflectors are not behaving as inductive directors.

In one embodiment, an additional front reflector is in the range of 30% to 40% of a wavelength from the dipole.

Preferably, the separation between a first additional front reflector and the sub-reflector is approximately one tenth of the wavelength.

In another embodiment, an additional front reflector intersects the axis of the dipole and main reflector.

In a further embodiment, an additional front reflector is substantially normal to the axis of the main reflector and the dipole.

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which : Fig. 1 is a perspective view from above of a dipole antenna of the invention; Fig. 2 is a diagrammatic side view of the antenna; Figs. 3(a) and 3(b) are front views showing construction of a dipole box, and Fig. 3(c) is a perspective view showing an additional front reflector; Fig. 4 is a diagrammatic cross-sectional view showing the dipole box in more detail; Fig. 5 is a diagrammatic cross-sectional view showing part of an alternative construction of antenna of the invention; and Fig. 6 is a graph showing the improvement in gain achieved by the antenna of the invention.

Referring to the drawings, there is shown a dipole antenna of the invention, indicated generally by the reference numeral 1. The antenna 1 comprises a support frame 2 comprising an elongate support arm 3 of square crosssection and a base support 4. The support arm 3 supports a dipole box 6 mounted near the focal point at a distance from the main reflector 5 determined precisely by testing and optimising for each design. In this case, the antenna 1 is designed for radiation having wavelengths between 2.5GHz and 2.686GHz and accordingly the mesh size of the main reflector with wire of 2mm diameter is 12.5mm in the plane of polarisation, the nearest standard mesh size yielding low losses in gain.

As shown most clearly in Figs. 3 and 4, the dipole box 6 comprises a plastics housing 10 which has various integral supports 11. A coaxial cable 7 extends through the support arm 3 into the dipole box 6 and is held in place by integral plastics clamps 12 in the dipole box 6. The two conductors of the coaxial cable 7 connect with dipoles 13. There is a sub-reflector 14 mounted forwardly of the dipole 13 at a distance of 25% of the wavelength of the radiation. In more detail, a copper braid 15 is crimped to baluns 16 which are integral with the dipoles 13. An inner insulator 17 surrounds the core 18 which is connected to the right-hand dipole half 13.

In addition to the above components, there is an additional front reflector 19 mounted at a distance of 10% of the wavelength from the sub-reflector 14 and thus at a distance of approximately 35% of the wavelength from the dipole 13. The additional front reflector 19 is similar to the sub-reflector 14 and is a rectangular strip of brass having similar dimensions to the sub-reflector 14, namely 55mm x 8.75mm x 0.5mm. Both reflectors are held in place by the integral supports 11 in the dipole casing 10.

Referring now to Fig. 6, the improvement in gain which is achieved by the additional front reflector 19 is shown.

It will be appreciated that there is an improvement in gain of approximately 0.9dB throughout the range of frequency from 2.5GHz to 2.7GHz. The horizontal axis 30 represents radiation frequency and the vertical axis 31 the gain in dB. The line 32 represents gain of a conventional dipole antenna without an additional front reflector, whereas the line 33 represents the gain performance of the dipole antenna of the invention.

It will be appreciated that the additional front reflector 19 may be inserted at practically no additional cost and yet there is a major improvement in gain. Heretofore, to increase gain the approach has generally been to increase the size or improve the materials of the known reflectors, namely the main reflector and the sub-reflector. It has never been appreciated that there could be such a considerable improvement in gain by use of an additional front reflector mounted forwardly of the dipole. It has generally been thought that the radiation is focused by the main reflector and any "overshoot" is directed back towards the dipole by a sub-reflector. It was never appreciated that an additional front reflector would provide any benefit. Thus, what the invention has achieved is a considerable improvement in gain at virtually no additional cost or complexity of construction.

It will be appreciated that the invention is not limited to the embodiments described above. For example, the additional front reflector may be in the form of an apexshaped reflector 21 shown in Fig. 5 on a radiation antenna 20. In this embodiment, there is a sub-reflector 14 and the additional front reflector 21 is mounted outside of the dipole box. The central part of the additional front reflector 21 is parallel to the dipole 13 and the subreflector 14. In addition, the central part and the two "wings" are parallel to three different tangents to the main reflector 5.

An important aspect of the additional front reflector is that it is mounted within one wavelength of the dipole.

There may be a series of additional front reflectors which may be of different sizes and in this case the first of the series should be approximately 30% to 40% and preferably about 35% of the wavelength from the dipole.

Each additional front reflector is preferably substantially parallel to a tangent of the main reflector.

This is the case for both the additional front reflectors 15 and 21. It is preferable that the additional front reflectors intersect the axis of the support arm 3 and of the dipole 13. It has been found that where the additional front reflector is of similar construction to the dipole as is the case in the dipole box 6, then the additional front reflector is ideally approximately 5-10% shorter than the dipole.

In this specification, distances are generally referred to as a function of wavelength. This "yardstick" is set by construction of the antenna as described in the introductory part of this specification.

The additional front reflector and the sub-reflector may take the form of components or conductors on a printed circuit board.

The invention is not limited to the embodiments hereinbefore described, but may be varied in construction and detail.

Claims (6)

1. A dipole antenna comprising a frame member supporting a main reflector, a dipole mounted forwardly of the main reflector, and a sub reflector mounted forwardly of the dipole, the configuration and mounting of the main reflector and of the dipole defining the radiation wavelength for operation of the antenna, wherein the antenna further comprises at least one additional front reflector mounted forwardly of the dipole, being within a distance of one wavelength from the dipole, and being substantially parallel in the plane of polarisation to a tangent of the main reflector.

2. A dipole antenna as claimed in claim 1, wherein an additional front reflector is in the range of 30% to 40% of the wavelength from the dipole.

3. A dipole antenna as claimed in claims 1 or 2, wherein the separation between a first additional front reflector and the sub-reflector is approximately one tenth of the wavelength.

4. A dipole antenna as claimed in any preceding claim wherein an additional front reflector intersects the axis of the dipole and the main reflector.

5. A dipole antenna as claimed in claim 4, wherein an additional front reflector is substantially normal to the axis of the main reflector and the dipole.

6. A dipole antenna substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.