Vehicle exterior mirror with antenna

This invention relates to an exterior rear view mirror for a motor vehicle comprising a case containing a mirror glass or other reflective member and an antenna for transmitting and/or receiving radio frequency electromagnetic radiation comprising an electrically conductive layer on a surface of the case.

An exterior mirror of this type is disclosed in GB-A-1590824. The case comprises a body moulded from plastics material having a bright copper layer deposited on substantially the whole of its outer surface for receiving a chromium-plated outer layer. The bright copper layer also serves as an antenna.

According to the invention, in a rear-view mirror assembly of the type described above, the antenna comprises an electrically conductive layer on a surface of a rigid member forming part of the mirror assembly.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 is a perspective view of a vehicle exterior mirror in accordance with the invention;</li><li>Figure 2 is a perspective view of the case of the mirror shown in Figure 1 ;</li><li>Figure 3 is a perspective view of the mirror case shown in Figure 2, from a different angle;</li><li>Figure 4 is a view into the mirror case of Figures 2 and 3 through the opening in which the mirror glass would be mounted;</li><li>Figure 5 is a cross-sectional view taken on the line 5 - 5 in Figure 4;</li><li>Figure 6 is a cross-sectional view taken on the line 6 - 6 in Figure 4; and</li><li>Figure 7 is a diagram illustration dimensions of a theoretical conical antenna.</li></ul>

Figure 1 shows a vehicle rear-view mirror comprising a reflective member 10 fitted in a case 11 which is mounted on a bracket 12 which is secured to one of the front doors 14 of a motor car 16. As can be seen from Figure 2, the case 11 has a clip 18 and guides 20 and 22 by means of which it is secured to an internal frame member (not shown) of the mirror assembly. In accordance with the invention, a first metallic foil element 30 is secured by adhesive to the inside surface of the outboard end of the case 11. As can best be seen from Figure 4, the element 30 is formed in two symmetrical halves 30a and 30b interconnected by a link part 32 which is connected by a coaxial cable 34 to a mobile telephone (not shown). The two symmetrical halves 30a and 30b approximate to a conical antenna. Figure 7 shows a cone of half cone angle θ and length l. Theoretically l should be equal to wavelength λ of the radio signals (i.e. the speed of light divided by the centre frequency). In practice, acceptable results are achieved if l is three tenths of the theoretical value.

The theoretical formula for the feed-point impedance (Zk) of a conical antenna is:<maths id="math0001" num=""><math display="block"><mrow><mtext>Zk = (Zo/π) Ln cotg (θ/2)</mtext></mrow></math><img file="EP0989028A2_D0001.tif" /></maths> where Zo is the free space impedance (377 ohms in air) and "Ln cotg" means "logarithmic cotangent". 30° is a realistic practical value for θ.

The relationship between the angle θ and the impedance of the antenna is linear. Good reception can be obtained if the actual feed-point impedance is between half and twice its optimum value. Provided this condition is met, it can be shown from Figure 8-15 on page 355 of John D Kraus, "Antennas", published by McGraw Hill, ISBN 0-07-0354-22-7, that, because the curve is practically linear in the area used, a practical optimised value Zo for the actual feed-point impedance is:<maths id="math0002" num=""><math display="block"><mrow><mtext>Zo = 1500 (l/λ) - 113·3</mtext></mrow></math><img file="EP0989028A2_D0002.tif" /></maths> It follows that the length l of the foil elements 30a and 30b and the half-cone angle θ is:<maths id="math0003" num=""><math display="block"><mrow><mtext>(Zo/π) Ln Cotg θ/2 = 1 500 (l/λ) - 113·3</mtext></mrow></math><img file="EP0989028A2_D0003.tif" /></maths> In practice, the length l of foil element may be about three tenths of the wavelength λ.

Two other foil elements 36 and 38 which are a mirror image of one another are secured by adhesive to the central and inboard parts of the interior surface of the case 11. These foil elements 36 and 38 are connected by a cable 40 to a radio broadcast receiver, for example an FM radio receiver (not shown).

In addition to the above two antennae, a third antenna 42 is connected by a cable 44 to a transponder (not shown) for an automatic road toll charging system; a fourth antenna 46 is connected by a cable 48 to a controller for the central door locking system for the car 16. Another antenna 50, positioned in the centre of the mirror case 11 is connected by a cable 52 to a digital radio receiver (not shown) while a further antenna 54 is connected by a cable 56 to a GPS receiver (also not shown).

The antennae 42, 46, 50 and 54 are rigid antennae mounted within the case 11. Although the various cables 34, 40, 44, 48, 52 and 56 are shown as parallel to one another, in practice they are gathered together so as to extend through the interior of the bracket 12 into the interior of the car 16, where they are connected to their respective transmitters and/or receivers.

Antennae comprising metal inserts moulded into the case 11 may replace the foil antennae 30, 36 and 38. Another alternative is for the case or cover member to be formed as co-moulding of two different plastics material, only one of which will accept surface metallisation. A layer of metal is then deposited on this part to serve as the antennae.

If the case has a separate decorative exterior cover member of the type described in European Patent Application No. 98302674.1, the electrically conductive layer may be formed either on the inner surface of such cover member or on that part of the outer surface of the mirror case which is enclosed by the cover member.

Alternatively or additionally one or more antennae may take the form of a conductive layer on part of the mounting for the mirror glass or other reflective member.

The mirror assembly may in addition incorporate other antennae for receiving GPS signals, remote operation of the car door locks and transponders for automatic road toll accounting systems. Some of these additional antennae may be formed as self-supporting rigid members.