This invention relates to antenna or aerial systems for motor vehicles. BACKGROUND ART

It is known to use a motor vehicle heated rear window (defroster) unit as a radio frequency transmitting or receiving aerial while permitting the window unit to be used simultaneously (if so desired) for heating (demisting) purposes. Thus, U.K. Patent 1520030 describes a system using a bifilar coil which separates RF signals from the usual dc heater circuit for a heated rear window. U.K.Patent 1600987 describes a system using matching and isolating circuitry to enable a heated rear window to be used as a transmitting aerial while dc power is supplied to the window for heating purposes. U.K.Patent Application 2152760A describes a system using tuning and isolating circuitry to enable a heated rear window which is essentially a periodic and non-resonant at VHF frequencies to be used as an efficient VHF receiving aerial while dc power is supplied to the window for heating purposes, the window being brought to series or parallel resonance near the centre of the VHF band by the tuning and isolating circuitry. It is desirable to be able to use a window heater structure which is convenient to manufacture and which gives

good heating characteristics with the above mentioned prior patent specifications 1520030, 1600987 and 2152760A, the described window heater structure corresponds to that conventionally used solely for heating purposes and comprises multiple horizontal wires which connect with bus-bars at opposite upright side edges of the windows, such busbars providing a means of linking the wires to each other and to leads of the heater circuit. This structure can give most heating in the region where this is required (i.e. the horizontal central band of the window) with a relatively small spread of parallel wires - because the wires run between the busbars along such region. Such a small spread of wires is advantageous from the point of view of aerial characteristics because relatively short bus-bars can be used whereby adverse parasitic capacitive and inductive effects can be minimised. In this respect it is to be understood that to avoid intrusion into the field of view it is normally desired to locate the bus-bars as close as possible to the edges of the window and this inevitably gives rise to parasitic capacitance to the body of the vehicle. Parasitic series inductance arises as a function of the length of the bus-bars.

Where it is desired to receive or transmit vertically polarised VHF signals a window heater structure having vertical wires (i.e wires running in the direction from the bottom edge to the top edge of the window rather than from

one side edge to the other side edge) could be more appropriate. However, with this arrangement in order to achieve adequate heating along the central horizontal band of the window, the parallel wires would have to spread along all or a major part of the window and insofar as this would require the use of relatively long bus-bars it is to be expected that serious problems due to parasitic capacitance and inductance would arise.

U.K.Patent 1439689 describes the use as an aerial of a heated front window having parallel vertical wires with long top and bottom bus-bars. However the main aerial components are not the vertical wires but constitute horizontal parts of the bottom bus-bar, which parts are spaced well away from the edge of the window and are extended to define substantial structures in themselves. DISCLOSURE OF THE INVENTION

The present invention is based on the realisation that it is in fact feasible to use vertical wire of a heated window as an aerial. According to the invention therefore there is provided a motor vehicle aerial system having a motor vehicle window with a heater structure comprising a series of heater wires running alongside each other between opposite bus-bars, a linking and isolating circuit arranged for linking said heater structure to a dc power supply and to radio apparatus of said motor vehicle so that said heater structure can act

as an aerial for said radio apparatus whilst it is powered by said dc supply to heat said window, characterised in that said heater wires extend in the vertical direction and said linking circuit is arranged to effect efficient transfer of vertically polarised VHF signals between said radio apparatus and said wires.

With this arrangement, effective transmission and/or reception of vertically polarised VHF signals can be achieved by use of the vertically extending hsater wires as used herein the term vertical is used to mean wires which run in the direction from the bottom edge to the top edge of the window as distinguished from wires which run from one to the other of the window side edges. Thus it will- be understood that in practice the window may be sloping and accordingly the wires may be at an angle, possibly a large angle to the true vertical.

The wires may be straight or substantially straight over their entire lengths between bus-bars. Preferably however, the wires are straight or substantially straight in their central regions and at at least one and (preferably both) the end regions of at least the outermost wires are inwardly inclined, i.e. curved or bent towards the vertical central plane extending transversly through the window, at at least one side of said plane (preferably both) so that the separation of the ends of the wires and the length of the associated bus-bar at the or each said end is less than the

maximum horizontal separation of the central regions of the wires. In a particularly preferred embodiment the inward inclination of the end regions of the wire is such that at at least one end (preferably both) the separation of the ends of the wires and the length of the associated bus-bar is no greater than one half (and preferably less than one half) of the said maximum horizontal separation.

Preferably the bus-bars terminate at or close to the connections wiih the outermost heater wires. Although the term bus-bar is used, it is possible to use structures which are not straight strips e.g. curved strips or other shapes.

In accordance with common practice a folded configuration is used whereby one bus-bar is continuous and the other is divided into two lengths so that in use the heater current flows from one divided length through some of the wires to the continuous bus-bar and then back to the other divided length via the other wires. With this arrangement preferably the configuration is symmetrical about a vertical plane between the divided lengths.

To give efficient transfer of VHF signals, the heater structure is preferably matched to the radio apparatus (i.e. to the transmitter/receiver and/or the characteristic impedance of the feeder cable which connects the heater structure to the transmitter/receiver) by means of matching and tuning elements of said linking and isolating circuit.

Such elements are preferably such as to bring the heater structure including the wires to series or parallel resonance at or near the centre of the VHF bands. Where wires with inclined end regions are used as mentioned above, by selection of a wide range and smooth gradation of the length of the wires a relatively slow variation of aerial impedance with signal frequency can be achieved thereby simplifying the design of even relatively broadband matching circuits. By suitable choice of dimensions for the heater wire pattern the matching circuitry can be made extremely simple or even eliminated.

Most preferably the heater wires are of a conventional nature with regard to thickness and spacing so that manufacture can be effected in a convenient manner using normal production techniques. With regard to the bus-bars, these are preferably located close to the window edges to minimise intrusion into the field of view. Where an inclined arrangement is used as mentioned above, the possible consequent shortening of the bus-bars permits location close to the vehicle body at the edges of the window without undue parasitic capacitance and series inductance.

The linking and isolating circuit of the invention may incorporate any suitable isolating arrangement. Where the aerial system is to be used for AM reception additionally to VHF reception/transmission, a bifilar coil arrangement may be used, as described in the above mentioned prior patent

specification 1520030, 1600987 and 2152760A.

It is visualised that the present invention will find particular application in the context of a heated rear window of a motor car however other windows and other kinds of vehicles may also be appropriate. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further by way of example only and with reference to the accompanying drawings in which:- Figs. 1 to 3 are diagrammatic views of different heating structure patterns in accordance with the present invention; Fig. 4 is a circuit diagram of a complete aerial system according to the invention; and Figs.5 and 6 are circuit diagrams of alternative forms of a part of the system of Figure 4. BEST MODE OF CARRYING OUT THE INVENTION

With reference to Fig. 1, this shows a heater structure 1 of a motor car heated rear window 2. The structure comprises multiple thin heater wires 3 embedded in or bonded to the window glass. The wires 3 run between top and bottom bus-bars 4, 5 and all are substantially straight, parallel to each other and vertical, although, as indicated, in fact the wires are slightly inclined towards the central vertical plane which extends transversely through the window, such inclination increasing for successive wires from such plane

outwardly towards the outermost wire on both sides. This progressive increase in inclination generally follows the window profile. The outermost wires 3 are close to the upright side edges of -the window 6,7 and the bus-bars 4,5 are 5 very close to the top and bottom edges 8, 9.

The bus-bars 4,5 are in the form of generally straight strips. The top strip 4 is continuous whereas the bottom strip 5 is divided in the middle (at the vertical centre plane) into two parts 5a, 5b. All wires 3 are electrically

10 connected at their top ends to the top bus-bar 4. The wires 3 are separated into two equal sets, each being connected at the bottom end to a respective one of the bottom bus-bar parts 5a or 5b, thereby giving a 'folded' configuration. Power supply leads 10 are connected to the heater structure 1

15 at terminals 11 at the spaced inner ends of the twobottom bus-bars parts 5a,5b.

The arrangement of Fig. 2 differs from that of Fig.l in that the wires 13 of the heater structure 12 have generally straight, parallel vertical central regions 14 and are

20 inclined inwardly (towards the central vertical plane) at their top and bottom end regions 15, 16 and the top and bottom bus-bars 17,18a, 18b are shorter than (by about half) the maximum horizontal spacing .of the two outermost wires 13. The arrangement of Fig. 3 differs from that of Fig. 2 in

25 that the wires 19 of the heater structure 20 have end regions 21,22 with inclinations which are greater, and the

bus-bars 23,24a,24b are much shorter. These bus-bars 23,24 may be slightly curved rather than straight strip shaped to accommodate the close connections of the wire 19.

The arrangement of Figs. 1 to 3 are all symmetrical about the vertical central plane (i.e. both the vertical wires and the bus-bars are symmetrical) and it will be noted ^~~ that the bus-bars are as short as possible (i.e. they terminate level with the connections to the outermost wires) and they are very close to the top and bottom window edges. Fig. 4 shows the circuitry 25 used to connect a window heater structure of the kind shown in any one of Figs. 1 to 3

(although the configuration of Fig.2 is illustrated) to radio apparatus having a VHF receiver and/or transmitter and if desired also an AM receiver. The power supply leads from the heater terminals 11 are connected to inputs 26 of the circuitry which is within a housing which is located as close as possible to the terminals 11. The circuitry 25 has an earth connection 27 which is connected to the vehicle earth, a power supply connection 28 which is connected to the vehicle dc power supply, and VHF and AM aerial outputs 29,30 which are connected to an aerial feeder cables of radio apparatus via an inductance/capacitance arrangement 31 which recombines the VHF/am signals. The circuitry 25 comprises two VHF chokes 33,34 (which may be coupled as windings of a bifilar coil) in series with

coupled windings 35,36 of a ferrite pot cored bifilar coil interposed between the inputs 26 and the outputs 27,28 whereby dc power can flow freely to the heater structure 12 (when switched on) to heat the window 2. The VHF chokes 33,34 and associated capacitors 37,38 act to isolate VHF signals at the heater structure 12 from the dc power supply. The bifilar coil windings 35,36 and associated capacitor 39 act to isolate AM signals at the heater structure 12 irom the dc power supply. The isolated AM signals are fed to the AM output 30 via an optional AM preamplification stage 40 of the circuitry 25.

The capacitor 38 of the VHF isolating part of the circuitry 25 is connected to the VHF output 29 via a matching and tuning circuit 41. In this respect it is to be understood that the window is essentially aperiodic and non-resonant at VHF frequencies and matching and tuning is necessary to give efficient operation of a ^■ VHF transmitter and/or receiver connected to the output.

The matching and tuning circuit is preferably such as to bring the heater structure 12 and particularly the vertical wires 13 to parallel or series resonance at or near the centre of the VHF operating band of frequencies.

Figs. 5 and 6 show respectively parallel and series tuned arrangements. With both of the circuits the two terminals 11 of the heater are coupled together via the capacitor 37 which has

negligible impedance at the frequencies at which the aerial is to be used. In the parallel-tuned matching circuit a reactive tuning element 42 (inductance or capacitance) connected in parallel between one heating structure terminal 11 (or both) and the body of the vehicle is employed to bring the combination to resonance at the centre of the operating band of frequencies. In the series-tuned matching circuit a reactive tuning element (the capacitance 38 alone or in series with an inductance 43) is connected in series with one (or both) of the heating structure terminals 11 so that the input impedance of the combination is resonant at the centre frequency of the operating band of frequencies. The impedance of the resonant arrangement achieved in either the parallel-tuned or series-tuned circuit is then transformed to the desired terminal value by means of a tuned (or untuned) transformer 44.

The actual choice of circuit will depend on the detailed impedance characteristic of the heating structure and the impedance to which it is to be matched and the frequency of operation. The matching circuit may also be required to provide other functions such as a direct current path to the source of a field-effect amplifier transistor (e.g. in circuits 40,41).

For fuller details of suitable circuits reference is made to U.K. Patent Application 2152760A which describes VHF matching and tuning circuits and to U.K. Patent 1600987 which

describes matching circuits for matching a heater structure to a VHF transmitter. The bifilar coil presents a high impedance to AM signals and, for further details reference is made particularly to U.K. Patent 1520030 but also to Patent 1600987 and Application 2152760A.

With the embodiment described above efficient reception and/or transmission of vertically polarised VHF signals can be achieved.

The invention is not restricted to the details of the above embodiment. Thus for example, the arrangement need not be symmetrical about the division in the bottom bus-bar but instead may have a degree of asymmetry. If desired, a continuous, rather than divided bottom bus-bar may be used in which case connections are taken from the top and bottom bus-bars respectively with, an appropriate compensatory filter arrangement as required. It is also possible to use a continuous bottom bus-bar and a divided top bus-bar with the circuitry close to the top bus-bar. Instead of the bifilar coil other isolating arrangements involving mutually coupled coils or the like may be used as described in the prior patent specification referred to.

Capacitor 37 gives common mode operation. Where common mode operation is not desired, capacitor 37 would be omitted. Thus, capacitor 37 would be omitted where diversity reception or transmission is required as described in U.K.Patent Application No. 2173644A. That is, capacitor 37 would be

omitted where the heater wires are to be used as a radio aerial in two different configurations permitting diversity reception or transmission. As described in the above mentioned application, in this context, provision is made for switching between the two configurations so that fading in one configuration can be compensated by better receiving or transmitting characteristics in the other. The two configurations may be separated by a centre-tapped transformer which gives balanced and unbalanced aerial characteristics. A switching circuit may be provided to switch repeatedly between the two configurations. Alternately, switching may be effected under the control of a circuit which monitors signal strengths.