GB2258952A - Antennas - Google Patents

Abstract

An antenna suitable for use as a car phone comprises an area of material 2 positioned on the surface 1 of the roof of the vehicle, the feed to the transmitter/receiver 3 being on two positions spaced apart around the periphery of the area of material. As far as the region above the car roof is concerned the antenna behaves like a slot antenna, but the antenna does not radiate or receive beneath the car roof. The antenna is applicable to communications other than for car phones. and the material may be positioned on any shape of metal body. <IMAGE>

Description

Antennas This invention relates to antennas, especially to antennas presenting a low profile.

Slot antennas, often half a wavelength in length and fed on opposite sides of the slot, have been proposed as flush or low profile antennas e.g. on aircraft: in one proposal, a boxed-in slot has been used on an aircraft as a transmitting antenna, the mouth of the slot being covered by insulation.

The invention provides an antenna comprising a metal body, an area of magnetic material positioned on the surface of the body, and a feeder connected to the body at positions spaced apart around the periphery of the area of the magnetic material.

The antenna behaves, as far as the surface bearing the area of magnetic material is concerned, as if the area of magnetic material was replaced by a hole in the metal body, the feed being to positions spaced apart around the periphery of the hole, since the effect of the magnetic material is to increase the impedance between the feed positions in the same way as a bead of magnetic material on a wire increases the impedance of the wire between positions on each side of the beam. This increase in impedance only applies on the surface bearing the magnetic material. The opposed surface is not subject to such an increase in impedance, and so does not radiate and cannot receive.

Thus, the overall effect of the magnetic material is of the boxed-in hole.

The antenna can be used, as can an antenna formed by a boxed-in hole, in a situation where a low profile is called for but does not suffer from the structural disadvantage which such a hole formed in the metal body could cause.

The body may be any shape e.g. a sheet or a rod shape. The magnetic material may be any suitable type e.g.

it may have a permeability of above one thousand, and may be ferrite or other magnetic material.

An antenna for a car phone constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic view of the antenna fitted to a car roof; Figure 2 is an enlarged version of the antenna showing the direction of the radiation pattern; Figure 3 is an end view of the antenna; Figure 4 is a schematic view of an alternative form of antenna; Figure 5 is a schematic view of another form of antenna; and Figure 6 is an end view, partly schematic, of the antenna shown in Figure 5.

Referring to Figure 1, the roof 1 of a car has an antenna 2 mounted on it which is connected to a transmitter and receiver 3 forming a mobile telephone.

The antenna consists of a slab 2 of ferrite (shown in more detail in Figures 2 and 3), and the feeder 4 from the transmitter/receiver is connected to the metal surface of the car roof at positions 5a, 5b on opposite sides of the centre of the slot.

The effect of the slab of ferrite on the upper surface of the car roof is similar to that if a slot shaped like the slab had been cut from the surface of the roof. As is known, such a slot produces a polarised signal, polarised vertically for such a horizontally extending slot. The slab acts in the same way as a ferrite bead on a piece of wire, and increases the impedance for r.f. signals passing between points such as feed points because the magnetic field associated with the r.f. signals is influenced by the (high permeability) ferrite and in turn influences the r.f.

signal. This produces the greatly increased impedance between the feed points when the slab is in position.

The slab of ferrite produces no effect on the lower surface of the car roof and the impedance between the feed points 5a, 5b is determined only by the material of the car roof. Thus, in the transmitting mode, the signal only radiates out of the car, not into the car.

Although the slot could be a half wavelength in length, the impedance at feed points 5a, 5b would be relatively large (in the region of 500 ohms to 600 ohms).

It is preferred that the slot be approximately one wavelength long, which would bring the impedance at the feed points down to something in the region of 20 ohms. For a full wavelength slot, the length of the slot could be from about 145mm to 165mm, preferably approximately 160mm, corresponding to proposed car phone operation from 865 to 965 MHz. The ratio of the length of the slot to its width could be from 50:1 to 20:1, at least. A suitable thidkness could be from 0.5mm to 5mm, depending upon the type of ferrite used. The connection between the antenna and the transmitter/receiver could be via capacitative coupling through the windscreen.

The antenna could in principle be transferred from car to car by affixing it by suitable non-setting adhesive, or by magnetic means. (Ferrite can be made slightly flexible).

If it was desired that the antenna should be a permanent fixture, it could be affixed before the car body is painted.

For larger vehicles, e.g. caravans, a slot of from 210 to 120, depending upon channels required could be provided for reception of television.

Naturally, the invention is particularly suited to use with other vehicles such as aircraft, fighting vehicles, or indeed metal skinned containers of other sorts e.g.

missiles, underwater devices.

Materials other than ferrite may be used such as mumetal at lower frequencies. In any case, a permeability of above 500 is desirable. Of course, shapes of slab other than rectangular may be used. It will be seen from Figure 2 that a rectangular slab has a radiation pattern of vertical polarisation with nulls at the ends of the slab. The nulls in the radiation pattern can be avoided by the slab shown in Figure 4, which has two sections 2a, 2b at right angles, but which is fed at points on either side of the slab in the centre, the total length of the slab being in the region of one whole wavelength.

Alternatively, the slab could be in the shape of a loop (e.g. diamond, circular) with feed at two points 900 apart, allowing the choice of vertical, horizontal and left or right hand circular polarisations (the Applicant's patent application No. 2221096).

The concept of the invention can be applied to any metal body e.g. a rod 6 (Figures 5 and 6) in which two strips of ferrite 7, 8 extend along the length of the rod, each being fed at points on the rod on opposite sides of the centre of each strip from a common transmitter/receiver 3.

The strips may be half or full wavelength long. This would then simulate a dipole (a vertical arrangement of dipole because the illustrated strip is horizontal), and could be used as the drive or driven elements in a Yagi-Uda and similar type aerial arrays.

Claims (12)

1. An antenna comprising a metal body, an area of magnetic material positioned on the surface of the body, and a feeder connected to the body at positions spaced apart around the periphery of the area of the magnetic material.

2. An antenna as claimed in claim 1, in which the magnetic material has a permeability of at least five hundred.

3. An antenna as claimed in claim 1 or claim 2, in which the metal body is a sheet.

4. An antenna as claimed in claim 3, in which the area has constant width and is fed on opposite sides of the area.

5. An antenna as claimed in claim 4, in which the area extends in two straight portions at right angles to each other.

6. An antenna as claimed in claim 4, in which the area extends in one straight portion.

7. An antenna as claimed in claim 5 or claim 6, in which the total length of the area is from 140 to 200mm.

8. An antenna as claimed in any one of claims 3 to 7, in which the sheet is the roof of a vehicle.

9. An antenna as claimed in claim 8, in which the antenna is for radio communication.

10. An antenna as claimed in claim 1 or claim 2, in which the metal body is a rod, and the area is a strip extending along the length of the rod.

11. An antenna as claimed in claim 10, including a second strip of magnetic material extending along the length of the rod and positioned on the surface of the rod on the surface opposed to that on which the first mentioned area is positioned, the feeder being connected to positions spaced on opposed sides of each strip.

12. An antenna substantially as hereinbefore described with reference to the accompanying drawings.