GB2310319A - Antenna - Google Patents

Description

IMPROVEMENTS IN OR RELATING TO ANTENNAS This invention relates to antennas.

More especially, although not exclusively, the invention relates to antennas for use at frequencies above 500 MHz. There is for example a requirement for an antenna suitable for operation at about 950 MHz, which is versatile in that it may be secured to a flat surface incorporated within communication equipment, or alternatively enshrouded for use on land or waterborne craft.

It is therefore an object of the present invention to provide an efficient antenna which will satisfy these criteria.

According to the present invention an antenna comprises an elongate insulative substrate on which is carried in spaced apart relationship two substantially parallel conductive tracks which are mutually co-extensive from one end thereof, one of which tracks being substantially three times the length of the other track.

The said one track may be substantially 3 B/4 in length, where x is a wavelength which corresponds to a frequency which lies within a predetermined frequency band of operation.

The said one end of each track may be adapted to receive signal conductors.

The signal conductors may comprise an inner conductor and an outer conductor of a pair of mutually co-axial conductors, which inner conductor is connected to the longer of the two tracks and which outer conductor is connected to the shorter of the two tracks.

The substrate may comprise a printed circuit (PC) board.

The PC board may be contained within a cylindrical plastics tube, The inner diameter of the plastics tube may correspond substantially to the width of the substrate so that the substrate is a sliding fit therein.

At least one end of the tube may be closed as by means of a plastics closure cap.

The tube may be filled with a plastics filler in which the substrate is potted.

The plastics filler may be a foam plastics material.

One embodiment of the invention will now be described solely by way of example, with reference to the accompanying drawings, in which, FIGURE 1 is a sectional side view of an antenna assembly and wherein, FIGURE 2 is a sectional plan view on a broken line A-A' as shown in Figure 1.

Referring now to the drawings, wherein corresponding parts of the two Figures bear, as appropriate, the same numerical designations, an antenna assembly comprises a cylindrical plastics tube 1, which is closed at one end by means of a plastics closure cap 2. Fitted within the plastics tube 1 is an elongate printed circuit board 3, which carries a pair of printed circuit tracks 4 and 5. As can be seen from Figure 1, the track 4 is three times the length of the track 5. The length of the tracks 4 and 5 are arranged to correspond to 3 A/4 and A/4 respectively, where A is a wavelength which corresponds to a frequency which lies within a band of operation for the antenna. As can be seen from Figure 1, the two tracks 4 and 5 are arranged to be mutually parallel and co-extensive over the length of the shorter track 5. Ends 4a and Sa of the tracks 4 and 5 respectively, are adapted to receive the inner conductor 6 and the screen 7 of a co-axial feeder cable 8.

The antenna shown is eminently suitable for use aboard a boat and for this purpose the printed circuit board 3 may be potted within the tube 1, using a plastics foam potting material.

Optionally, an end of the tube 1 adjacent the feeder cable 8 may also be closed by means of a further closure cap (not shown) which may be similar to the closure cap 2 but which is suitably pierced or moulded to provide access for the co-axial feeder cable 8.

Various modifications may be made to the antenna shown without departing from the scope of the invention and for example, the antenna may be unshrouded and may simply comprise a flat substrate corresponding the substrate 3, carrying tracks which correspond to the tracks 4 and 5, so that the antenna may easily be secured to a flat surface and incorporated in communication equipment for example. In the present example, the PC board may be made of fibre glass or any other suitable substrate material and the tracks 4 and 5 may be made of copper.

It will be however appreciated that the material from which the tracks are fabricated is not important and that any other material or materials having suitable conductive properties may be used.

In operation of an antenna, as shown in Figure 1, the mutually co-extensive region, of length B/4, serves as a transmission line via which signals collected in an upper portion of length B/2 of the track 4, are fed to the feeder cable 8.

Impedance matching of the tracks 4 and 5 to the feeder cable 8 is determined in accordance with the spacing between the tracks.

The spacing is not particularly critical and is not especially frequency dependent.

One important feature of antennas according to this invention, which stems from this lack of criticality, is that the substrate may be trimmed at each end for tuning purposes to suit a predetermined frequency of operation. For this purpose an antenna may be fabricated wherein the track lengthlspacing are suitable for the lowest operation frequency envisaged and simply trimmed thereafter to suit a particular application for use at a higher frequency.

In order to provide an antenna for operation at or about 950 MHz, an antenna having an overall length of about 24 cm would be appropriate and having a spacing between the tracks of about 1.5 cm to afford impedance matching of about 50Q to a 50D co-axial feeder cable. Thus, it will be apparent that a particularly compact and convenient antenna for use at this operating frequency is afforded.

Claims (10)

1. An antenna comprising an elongate insulative substrate on which is carried in spaced apart relationship two substantially parallel conductive tracks which are mutually co-extensive from one end thereof, one of which tracks being substantially three times the length of the other track.

2. An antenna as claimed in Claim 1 wherein, the said one track is substantially 3 B/4 in length, where X is a wavelength which corresponds to a frequency which lies within a predetermined frequency band of operation.

3. An antenna as claimed in Claim 1 or Claim 2 wherein, the said one end of each track is adapted to receive signal conductors.

4. An antenna as claimed in Claim 3 including signal conductors which comprise an inner conductor and an outer conductor of a pair of mutually co-axial conductors, which inner conductor is connected to the longer of the two tracks and which outer conductor is connected to the shorter of the two tracks.

5. An antenna as claimed in any preceding claim wherein the substrate comprises a printed circuit (PC) board.

6. An antenna as claimed in Claim 5 wherein the PC board is contained within a cylindrical plastics tube.

7. An antenna as claimed in Claim 6 wherein the inner diameter of the plastics tube corresponds substantially to the width of the substrate so that the substrate is a sliding fit therein.

8. An antenna as claimed in Claim 6 or Claim 7 wherein at least one end of the tube is closed by means of a plastics closure cap.

9. An antenna as claimed in Claim 8 wherein the tube is filled with a plastics filler in which the substrate is potted.

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