GB2034124A - Improved antenna - Google Patents

Abstract

A Yagi antenna has its director elements 3 embedded in and supported by a foam plastics material 11. The driver element 8 is in the form of a waveguide section excited by a probe 9 and having an aperture outlet 13, there being no reflector. The foam fills the interior of and partially surrounds the waveguide section. The antenna is suitable for the 1,500 MHz band and can be directly connected to an unbalanced feed 7. <IMAGE>

Description

SPECIFICATION Improved antenna This invention relates to antennas, or aerials.

One well-known antenna is the Yagi antenna, which comprises a driver element with a series of director elements to one side of the driver element, and usually has a reflector element to the other side of the driver element.

The basic Yagi antenna has been known for many years and is illustrated in Fig 1 of the drawings. It comprises a driver element 1, which is shown as folded but may alternatively be a simple dipole, a parasitic reflector 2 behind the driver element 1, and a series of parasitic directors 3 all mounted in a line along a boom 4. The more directors there are, the more directional the antenna becomes, (at least until a point is reached where ohmic losses start to outstrip any increase in gain).

The driver element 1 and the reflector 2 may be regarded as creating a forward travelling wave which is trapped by the directors and travels down them at a velocity which is less than that of an electromagnetic wave in free space. This "slow" wave is gradually released into the surrounding space, and it is this property of the slow release of energy over its length which gives the antenna its directional properties. In other "slow" wave structures each director can consist of a pair of crossed elements in a common plane orthogonal to the boom, or of a circular disc in the same plane, this being known as a Cigar antenna.

Another way of visualising the operation of the antenna is to regard each director as an individual radiating element which receives its energy from being coupled to the other elements, including the driver element.

In any event, the antenna has a plurality of interacting elements arranged in a line, and mounted on the boom. However, the existence of the boom 4 affects both the radiating performance of the antenna, and the ease with which power is coupled into the antenna from the, feeder, and if the antenna is to be used over wsde frequency bandwidths it is not possible to compensate for this by standard matching techniques.

Commonly it is desired to connect the antenna to the receiving and/or transmitting equipment with a coaxial cable, referenced 7 on Fig 1. A further problem arises here in a Yagi antenna because the currents in the coaxial line are "unbalanced" while the currents at the input 5 of the driver element 1 are "baíanced".To join the two together, a balanced-to-unbalanced transformer 6, generally known as a "balun," is required. It becomes increasingly difficult to make these for frequencies above a few hundred megahertz, and particularly difficult above 750 or 1 ,000 MHz.

There is, however, an increasing demand for Yagi antennas in the 1,500 MHz band for "line-of-sight" radio communication, primarily for private use.

The invention in its vanous aspects is defined in the appended claims to which reference should now be made.

The invention will now be described in more detail, by way of example, with reference to the remaining figures of the drawings, of which: Fig 2 illustrates a Yagi antenna of the type shown in Fig 1 when potted in a foam plastics material.

Fig 3 shows a preferred driver element in the form of a waveguide aperture, with an array of director elements; Figure 4 shows an embodiment of the invention in where the elements of Fig 3 are embedded in foam plastics material; and Fig 5 is another view of the antenna of Fig 4 showing the mounting arrangement.

In the generally elongate antenna of Fig 2 the driver element 1, reflector 2 and directors 3 of the Yagi antenna of Fig 1 are encapsulated or embedded in a foam plastics material 11 which supports the elements without the need for a boom 4 as shown in Fig 1. The foam plastics material used in this example is a low-density polyurethane foam of closed cell type. The density is low so as to reduce losses in the foam. The foam has relatively little effect on the radiation properties of the antenna which are still principally determined by the Yagi elements; even so the dimensions and spacings of the elements need to be slightly reduced, typically by about 5%, to allow for the wave slowing action of the foam. A closed cell foam is preferred to make a weatherproof package for the elements.

Fig 3 shows the elements of an improved and preferred antenna in which the driver element is in the form of a short length 8 of circular waveguide, one end 10 of which is closed and the other end 12 of which has an outlet feed aperture 13 in the shape of a slot facing the directors 3. There is no reflector element. The waveguide section 8 is excited by a probe 9 which is directly connected to the central conductor of the (unbalanced) coaxial cable 7 without the need for a balun. While a waveguide of circular cross-section is shown, a rectangular or square guide can be used as long as the dimensions are such that the dominant waveguide mode is generated and higher order modes are either sufficiently suppressed or generated in carefully controlled relative phase and amplitude with respect to the dominant mode with a view to improving feed efficiency.

In the antenna of Fig 3, the elements could be held in place by a metallic or dielectric rod running down the antenna centre line. However, it is preferred to use an arrangement as shown in Fig 4. Fig. 4 shows the elements of Fig 3 embedded in a foam plastics material of the type described with reference to Fig 2. The foam surrounds part of the waveguide section and fills its interior. The elements are suspended in the foam in a two-step operation. First, a small foam substrate is made to hold the elements in their relative positions, the elements meanwhile being held in a jig, and then the subassembly thus formed is potted in a second moulding operation.

While a two-part polyurethane foam has been used, any other plastics foam can be employed provided it has suitable electrical and mechanical characteristics.

Another possibility is to use both the rigid foam of Fig 4 and a central rod to assist in supporting the elements.

Fig 5 shows how the antenna of Fig 4 can be mounted. A short length 14 of tube is fixed to the closed end 10 of the waveguide section 8 as by welding, and a scaffolding clamp 1 5 connects the tube 14 to a convenient upright 16. The waveguide feed end may equally well be sand or even die cast in one piece. The outer and inner conductors of the coaxial feeding cable 7 are connected to the waveguide feed body 8 and the probe 9 respectively by means of a coaxial plug and socket pair 17. Conveniently the socket is fixed to the part of the waveguide feed body at which the probe conductor emerges.

It is seen that the use of a waveguide section as the driver element is particularly suited to an antenna in which the directors are embedded in foam because it provides a very convenient way of mounting the antenna. The waveguide section can easily be firmly clamped to the support, while it provides a good structure for the foam to key onto.

The physical characteristics of the antenna illustrated in Figs 4 and 5, having regard to manufacturing difficulties and strength requirements, make it particularly suitable for use at a minimum frequency of about 900 MHz. It can however, be used at frequencies as high as 5,000 MHz. In theory, though, it could be used at frequencies outside this range if suitable foams and manufacturing techniques can be developed.

The antenna can of course be used for transmission or reception.

The potting techniques is applicable generally to antennas which have a plurality of interactive elements arranged in a line, particularly of the type in which the elements are conventionally mounted on a boom which is itself supported at a single position along its length. It is applicable in antennas in which the individual elements are joined together, as for example in a helical or zigzag antenna in which the turns of the helix or sections of the zig-zag are arranged along a line, or any other periodic or quasi-periodic slow wave structure.

Claims (16)

1. An antenna of the type comprising a driver element and a plurality of director elements arranged in a line, in which the driver element is in the form of a waveguide section.

2. An antenna according to claim 1, wherein the waveguide section has an aperture outlet.

3. An antenna according to claim 1 or 2, wherein the director elements are spaced from one another.

4. An antenna according to claim 1 or 2, wherein the director elements are in a helical or zig-zag arrangement.

5. An antenna according to claim 1 or 2, wherein the antenna is a Yagi antenna.

6. An antenna according to any preceding claim, wherein at least some of the director elements are substantially enveloped in a foam plastic material.

7. An antenna of generally elongate shape comprising a plurality of interactive elements arranged in a line, and in which at least some of the elements are substantially enveloped in and are supported substantially solely by a foam plastics material.

8. An antenna according to claim 6 or 7, wherein the plastics material is a closed cell foam.

9. An antenna according to claim 8, wherein the plastics material is a polyurethane foam.

10. An antenna according to claim 7, 8 or 9, wherein the antenna elements are spaced from each other.

1 An antenna according to claim 7, 8, 9 or 10 wherein the antenna comprises a driver element and a plurality of director elements.

12. An antenna according to claim 11, wherein the driver element of the antenna is in the form of a waveguide section having an aperture outlet.

13. An antenna according to claim 6 or 12, wherein the foam plastics material extends within the waveguide section.

14. An antenna according to claim 6 or 11, wherein the plastics material at least partially encapsulates the driver element of the antenna.

1 5. A Yagi antenna substantially as herein described with reference to Fig 2 of the drawings.

16. A Yagi antenna substantially as herein described with reference to Fig 3 of the drawings.

1 7. A Yagi antenna substantially as herein described with reference to Figs 4 and 5 of the drawings.