WO1998008268A1

WO1998008268A1 - Vehicle on-screen antenna

Info

Publication number: WO1998008268A1
Application number: PCT/GB1997/002203
Authority: WO
Inventors: Roy Barling
Original assignee: Antiference Limited
Priority date: 1996-08-21
Filing date: 1997-08-15
Publication date: 1998-02-26
Also published as: GB2316538A; GB9617485D0

Abstract

An antenna system comprises a first antenna for reception of L.M.S. band signals and a second antenna for V.H.F. signals arranged on the last screen of a vehicle. The second antenna is arranged adjacent the heater element of the window, so that signals received by the heater element cause an influence in the second antenna for improved V.H.F. reception. The combination of the second antenna and the heater element thus increases the vertical aperture of the antenna element without requiring any direct connection between the heater element and the radio receiver. By omitting the galvanic connection, noise is not tranferred from the power feed of the heater element to the radio receiver. An additional advantage is that fewer components are needed to isolate the heater element from the D.C. feed or earth at any radio frequencies.

Description

Vehicle On- Screen Antenna

The present invention relates to a vehicle antenna, and in particular to an antenna formed as a conductor on a vehicle window.

Vehicle antennas have been known for many years. The simplest, and most common design, is the telescopic rod antenna used on cars and other vehicles . The rod antenna is essentially a monopoie mounted on part of a metal body of the vehicle which, since the metal body acts as a ground plane, can act as one-half of a dipole. Although the rod antenna gives good reception when mounted on a vehicle, there are a number of disadvantages. The rod is unsightly, causes wind resistance, is prone to breakage and is normally mounted by cutting a hole in the metal bodywork of the vehicle leading to a likelihood of corrosion forming around the mounting hole and consequently poor grounding .

To overcome these disadvantages on-screen antennas have been proposed. An on-screen antenna typically comprises a conductor, or set of conductors embedded in, or mounted on the glass of a front or rear screen of a vehicle. In the case of an automobile, attempts have been made to mount the on-screen antenna on the front screen, as this reduces the length of feeder required to the radio receiver which is usually mounted in the front of the vehicle. However, the engine management systems and displays of an automobile create electrical interference which on a front-engined vehicle is readily picked up by an antenna mounted at the front. An on-screen antenna mounted on a front screen is also a visual intrusion which can distract the vehicle driver. For these reasons it is preferred to mount on-screen antennas on the rear screen or backlite.

It is known to use the electrical window heater element of a rear screen as an antenna. Such heater elements generally comprise a pair of bus-bars which extend up the edges of the rear screen and are interconnected by horizontal heater conductors embedded in, or deposited on the glass. In addition to a power supply connection and a connection to ground, a feed is connected to one of the bus-bars to provide an output to a radio receiver for F signals received by the heater element. It has been found that adequate signal response can be achieved for A.M. and F M reception, m such systems However, there are problems in using the heater element in this way Firstly, the heater element must be isolated from the D.C. power feed and earth at the radio frequencies to be received. This requires isolating circuits, particularly for A.M. frequencies, which use costly components such as large inductors, typically m the form of bifilar chokes. A second problem is that, since the element is galvanically connected to a power feed, any mtereference in the D C feed w ll be passed to the radio This can be attenuated to a certain extent, but again requires further components.

A second approach to providing an antenna on a rear screen is to provide a separate conductor pattern for reception of long, medium and short wave band (L.M.S.) signals. Such a separate pattern, which is neither connected to a D.C. supply nor to ground, does not suffer from the problems noted above. However, the size restnctions on the space available on the rear screen limits the size of the separate, conductor pattern, and consequently the signal strength received is also limited. A separate conductor pattern has also been used for both L.M.S. and V.H.F reception. However, the reception of VHF signals with a vertical component of such an antenna s poor due to the lack of vertical space available for the antenna pattern above the heater element on the rear screen and consequent lack of conductor extending in the direction of the vertically polarised component signal.

We have appreciated that the prior art is deficient as noted above In particular we have appreciated that an on-screen antenna used on a rear windscreen should have as large a vertical dimension as possible, but should be isolated from a D.C supply and ground. Accordingly, there is provided an antenna arrangement for a vehicle on-screen antenna comprising an electrical heater element for heating the vehicle screen, and an antenna conductor for reception of electromagnetic signals arranged so proximate the heater element, that electromagnetic signals received by the heater element produce an influence on the antenna conductor to provide improved reception in comparison to the antenna conductor alone.

The invention thus provides a system in which the heater element forms part of the antenna, but there is no galvanic connection from the heater element to the feeder to a radio receiver. Since the antenna conductor and the heater element together form a capacitor of small capacitance (and consequently high reactance) a high pass filter is essentially formed. Consequently, AM noise conducted from the D.C. power feed to the heater element which can be passed to the radio receiver is reduced, since noise is broadband and the influence of the heater element on the antenna conductor is only at the frequency range to be received. Noise of other frequencies which could cause disturbance within the radio receiver or other electronics is thus reduced.

By proximate we mean that the antenna conductor is not galvanically connected to the heater element, but is sufficiently close to be influenced by electromagnetic fields produced by currents created in the heater element by received electromagnetic signals.

In a preferred embodiment, the antenna conductor and heater element are used for reception of V.H.F. signals and there is an additional antenna element for reception of L.M.S. signals.

The heater element needs to be isolated from the D.C. feed and ground at the frequencies to be received by it. By using the heater element for VHF only there is the advantage that large isolating chokes are not required as would be the case if the element were used for the reception of L.M.S. frequencies. The heater element simply requires smaller reactances or resonant circuits to _Drovide isolation at V.H.F. frequencies, in comparison to the size of reactances required to isolate at L.M.S. frequencies. Furthermore, noise from any source and particularly the DC pcwer feed is greater at lower, L.M.S. frequencies. Noise is also predominantly A.M. in nature and so is more problematic at L.M.S. (A M.) frequencies than at V.H.F. (F.M.) frequencies. By providing a separate antenna for L.M.S. reception, the heater element can be grounded at L.M.S. frequencies and so pass less L.M.S. noise to the V.H.F. antenna. It is important to reduce L.M.S. noise in the V.H.F. antenna as L.M.S. noise present in the V.H.F. reception circuitry is liable to be passed to the L.M.S. reception circuitry. It is for this reason that it is desired to cancel L.M.S. noise wherever t occurs.

The high pass filter formed by the capacitative relationship between the V.H.F. antenna conductor and the heater element allows V.H.F. frequencies to influence the antenna conductor, but presents a reactive loss to L.M.S. frequencies. In this way L.M.S. noise on the V.H.F. antenna conductor is reduced.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying figures, m which:

Fig. 1 shows a circuit diagram of a known on-glass antenna system using a heater element as an antenna,

Fig. 2 shows a circuit diagram of a second antenna arrangement in which the heater element is used in addition to an auxiliary antenna, and

Fig. 3 shows a circuit diagram of an antenna system embodying the invention.

The known system shown in Figure 1 comprises heater conductors 10 connected at both ends to respective bus bars 12 to form a heater element 14. The heater element is embedded in or mounted on the glass rear screen of a vehicle and is used primarily to demist cr to defrost the screen. In th s system, however, the heater element serves the additional function of an antenna for a radio receiver in both L.M.S. and V.H.F. frequency ranges. Such a system is known and described in US-A-3 , 484 , 584 (Shaw).

To provide electrical power for heating, one bus bar 12 is connected to a battery 16 by a D.C. feed 18 which includes a switch 20. Similarly, a ground lead connects the other bus bar 12 to a ground point 24. To isolate the heater element at radio frequencies, isolating circuitry, here shown as inductors 26, is included m the D.C. feed 18 and in the ground lead 22. As the heater element is used for both L.M.S. and V.H.F. reception, the circuitry 26 must include large inductors, such as bifilar chokes, to provide isolation at L.M.S. frequencies.

The radio receiver circuit 28 is connected to the heater element 14 by a feeder 30, which feeds radio signals received by the heater element to the radio circuit. Thus, the heater element serves as both a heater and an antenna.

The arrangement described above has a number of disadvantages. The heater element 14 is connected to earth and to a D.C. supply which requires additional circuitry to isolate the screen at L.M.S. and V.H.F. frequencies. This circuitry can be expensive and bulky particularly for L.M.S. frequencies. Furthermore, the D.C. power feed is likely to carry noise which will be transferred directly to the radio by the feeder 30.

Some of these disadvantages are overcome in the second known system shown in Figure 2 and described in GB-A-2 , 266 , 189.

In Figure 2, like components are given the same numbering as Figure 1. The system comprises heater conductors 10 and bus bars 12 forming a heater element 14 as before One C ΛS bar 12 is connected to ground by a ground lead 22 and the other s connected to a tuner or radio receiver 28 by a feeder 30. However in this system an additional amplifier module 32 is included between the feeder 30 and the element 14 to provide improved ga and signal to noise ratio. The module 32 includes a V.H.F. amplifier 34 connected to one bus bar 12 of the heater element 14 by V.H.F. feeder 44. The D.C. power feed 18 includes isolating circuitry 26 which comprises inductors 25 and capacitors 27.

The significant difference with this system over the system of Figure 1 is the provision of an additional antenna element 40, mounted on or embedded in the screen in a similar manner to the heater element, for reception of L.M.S. signals only. The additional antenna 40 is connected to an L.M.S. amplifier m the module 32 by an L.M.S. feeder 42. The two amplifiers 34, 36 are then connected to a summing point 38 which provides signals to the radio receiver 28 via feeder 30. This system has an advantage over that shown m Figure 1 in that the heater element is used to receive V.H.F. signals and not L.M.S. signals and so is not required to be isolated from the DC supply at L.M.S. frequencies. Consequently, the isolating circuitry 26 need not include large chokes for L.M.S. isolation. The additional antenna 40 for L.M.S. reception is not connected to a D.C. feed or ground and so requires no isolation.

The above systems are deficient, however, as previously noted in that the heater element receives noise from the D.C. feed and requires isolation from both the D.C. feed and ground to operate as an antenna. The heater element usually takes up a large amount of the area of a vehicle rear screen as can be seen m Figure 2. The additional antenna is useful for L.M.S. reception, but its restricted vertical height makes it unsuitable for reception of VHF signals with a vertical component. These disadvantages are overcome m the em-oodiment of the invention shown in Figure 3 Once again like components are given the same numbering The embodiment of the invention shown in Figure 3 has many features in common with that of Figure 2, and reference is made to the description above for an explanation of the common features. The embodiment shown has, additionally, an antenna conductor or element 50 comprising a wire or strip mounted on or embedded in the glass. This wire 50 is not electrically connected to the heater element 14 but is arranged adjacent and parallel to the topmost of the heater conductors 10.

The present embodiment thus comprises an antenna element 50 for reception of V.H.F. signals and an antenna element 40 for reception of L.M.S. signals. Due to the closeness of the VHF antenna 50 to the heater element 14, V.H.F. signals received by the heater element influence the VHF antenna 50. Accordingly, V.H.F. reception is achieved by a combination of the antenna element 50 and the heater element, without any direct electrical connection being made between the heater element and the radio receiver. The embodiment of the invention thus benefits from the vertical height of the heater element to provide good gain by coupling to the second antenna, without suffering from AM noise directly conducted from the D.C. power feed which could be transferred to the radio receiver.

The ■ influence of the heater element on the V.H.F. antenna may be through capacitative coupling, inductive coupling or both. In essence, the heater element acts as a passive element in the system, receiving V.H.F. signals and causing an influence on the V.H.F. antenna. The vertical dimension and thus the quality of reception of a vertical component of VHF signals achieved by the combination is higher than that achievable by the single wire or strip 50 on its own, without the presence of the heater element 14.

The L.M.S. antenna 40 and V.H.F. antenna 50 are connected to an amplifier module 32 by feeder lines 42 and 44 respectively. The module includes separate amplifiers for L.M.S. and V.H.F. to provide gam prior to the feeder 30 to a radio receiver. The module has a separate ground wire 21 and power supply 17 Each antenna will now be discussed turn.

The L.M.S. antenna of the present embodiment comprises a single conductor wire mounted on or in the glass screen Other patterns may be suitable, such as the pattern of Figure 2, for example, and these are within the scope of the invention. In addition, the L.M.S. antenna element does not need to be mounted on the rear screen, it could be mounted elsewhere.

In the embodiment shown, the L M.S antenna element lies approximately midway between the top of the heater element 14 and the top of the screen. This position is chosen because the top of the screen has a metal surround which is grounded (the vehicle bodywork) and similarly the heater element, which is not isolated at L.M.S. frequencies represents a grounded conductor. If the L.M.S. antenna element were placed nearer either the top of the screen or the heater element, the efficiency of the antenna would be reduced. The arrangement thus provides the best reception achievable in the space available. The L.M.S antenna element 40 feeds to an input connection of the L.M.S. amplifier the module 32 The L.M.S. amplifier is of high input impedance and low output impedance which gives an optimum signal voltage recovery from the L.M.S. antenna which is electrically very short. The amplifier module 32 is placed approximately midway vertically between the L M S. antenna element and the top of the heater element 14 to reduce the length of the L.M.S. feeder 42 and consequently minimise losses and noise pick up m the feeder 42. The module 32 may be mounted on the screen glass or hidden behind the C pillar or roof lining of the vehicle for cosmetic reasons and to minimise deleterious effects on the antenna feeders 42 , 44.

The V H.F. antenna element 50 of the present embodiment is shown as a single conductor or wire which may be mounted on, or embedded n the glass screen. The antenna wire 50 runs parallel and close to the top conductor 10 of the heater element 14. The close proximity of the V.H.F. antenna element 50 to the heater element 14 provides good coupling (capacitative and inductive) between the two so that the influence of the V.H.F. signals received by the heater element on the antenna element is large. There is, however, no electrical connection between the V.H.F. antenna and the heater element. The V.H.F. antenna 50 feeds to a V.H.F. amplifier in the module 32, and the common L.M.S. and V.H.F. output is fed by the feeder 30 to a radio receiver. The module 32 can be separately grounded by a lead 21, and a power supply provided by the lead 17, or these can be provided the common R.F. feeder 30. The V.H.F. antenna element 50 is a driven element of a complex antenna arrangement, and is influenced by V.H.F. signals received by the heater element 14, and influenced by the electrical currents generated the vehicle bodywork and apertures by impinging V.H.F. signals.

The heater element is decoupled from the ground connection 22 and power feed 18 at V.H.F. frequencies by a decoupling circuit 26 comprising inductive 25 and capacitative 27 components. This circuit 26 passes the high current needed for heating the screen, but isolates the heater element at V.H.F. frequencies. The values of impedance 25 required will depend upon the degree of isolation required at the R.F. impedance of the heater element. To minimise any L.M.S. noise which could be conducted to, and then radiated from, the heater element, a low reactance capacitor 27 is used to decouple the battery input to ground. The capacitor is of such value as to ground the heater element at L.M.S. frequencies.

The benefits of the whole system are thus provided the following ways. The V.H.F. antenna element is influenced by inductive or capacitative coupling (or by re-radiation) to fields produced by currents created in the heater element by reception of V.H.F. signals. The heater element is grounded at L.M.S. frequencies and so does not Dass anv L.M.S. noise to the V H.F antenna. Furthermore, the antenna conductor and heater element have a small capacitance between them (and thus high reactance) at L.M.S. frequencies, which further reduces any L.M.S. noise which would otherwise be transferred to the V.H.F. antenna. Since noise is generally by nature A.M. rather than F.M., noise is more of a concern in the L.M.S. band of the spectrum which is the range at which A.M. is broadcast. Accordingly, A.M. noise received on the V.H.F. antenna is further filtered by the F.M. receiving circuitry and does not have a large effect on the F.M reception. However, L.M.S. (A.M.) noise passed to the V.H.F. antenna will be passed to the amplifier circuitry and may consequently be passed to the L.M.S. receiving circuitry. It is for this reason that it is desired to reduce L.M.S. noise wherever it occurs, and particularly such noise which could be passed to the V.H.F. antenna.

It could be possible to omit the isolating components 25 if the heater element is self isolating. This can occur at particular wavelengths at which the heater element acts as a half wave dipole. In this situation, there will be a low impedance point at either bus bar and so the heater will appear isolated. However, this situation occurs only at particular frequencies and so the use of reactive components 25 is preferred.

To improve reception of a vertical component of polarisation of a V.H.F. signal, additional conductors which are galvanically connected at equipotential points of the heater element may be included which extend up the screen. Since the conductors cross the heater wires 10 at equipotential points, there is no effect on the • heating efficiency of the heater element as the conductors draw no current. The additional conductors have a vertical component which couples to vertical polarisation in the V.H.F. signal. This can assist in producing good omnidirectional sensitivity for horizontally and vertically polarised waves. The vertical lines also facilitate noise reduction. The antenna system described thus provides a geometrically simple arrangement of antenna conductors which is easily applied during manufacturing of the screen with the heater element. Because the antenna elements have no galvanic connection to the vehicle bodywork or electrical supply, noise from these sources is minimised.

Providing separate antennas for L.M.S. and V.H.F. means that there is no need to deplex the signals minimising deplexmg losses. As the heater element does not need to be isolated at L.M.S. frequencies simple reactances may be used to provide isolation rather than bulky, costly chokes. The length and proximity of the

V.H.F. antenna element to the heater element, and the tapping points of the battery and ground leads provided optimum omnidirectionality of the system.

Whilst the invention has been described in relation to an embodiment having a rectangular heater element and wire antennas, it is clear to the man skilled in the art that other configurations of the heater element and V.H.F. antenna are possible which maintain their proximate relationship and are within the scope of the invention. Similarly, the screen has been described as glass, though other suitable materials may be envisaged.

Whilst the additional LMS antenna element is described as mounted on the 'same screen as the heater element, it would be possible to mount the LMS antenna element elsewhere, for example on another glass pane, or on a body panel in the case of a car body of plastics or other non-conducting material. Also, the position of the VHF antenna element could be below the heater element, to one side or even lying within the heater element pattern. These may all be proximate relationships.

Claims

1. An antenna arrangement for a vehicle on-screen antenna comprising:

an electrical heater element for heating the vehicle screen, and

an antenna conductor arranged so close to the heater element that electromagnetic signals received by the heater element influence the antenna conductor to provide improved reception.

2. An antenna arrangement according to Claim 1 , wherein the antenna conductor runs adjacent to a conductor of the heater element .

3. An antenna arrangement according to Claim 1 or 2 , wherein the heater element comprises a set of conductors connected at each end by bus bars, and the antenna conductor extends adjacent and generally parallel to one of the heater conductors.

4. The antenna arrangement according to Claim 3 in which the antenna conductor is adjacent the topmost conductor of the heater element .

5. An antenna arrangement according to any preceding claim, wherein the antenna conductor provides V.H.F. reception only.

6. An antenna arrangement according to claim 5 , further comprising a second antenna conductor for L.M.S. reception.

7. An antenna arrangement according to Claim 6 , in which the second antenna conductor is not galvanically connected to the first mentioned antenna conductor and is not galvanically connected to the heater element.

8 An antenna arrangement according to claim 6 or 7 , m which the second antenna conductor is on the same vehicle screen as the heater element

9. An antenna arrangement according to Claim 8, wherein the second antenna conductor is arranged above the heater element.

10. An antenna arrangement according to either or claims B or 9 , wherein the second antenna conductor is arranged substantially midway between the top of the heater element and the top of the screen.

11. An antenna arrangement according to claim 9 or 10 in which the first mentioned antenna conductor lies between the heater element and the second antenna conductor.

12. An antenna system for a vehicle on-screen antenna comprising:

an electrical heater element for heating the vehicle screen connected to a power supply and ground

an antenna conductor arranged so close to the heater element that electromagnetic signals received by the heater element influence the antenna conductor to provide improved reception and connected to the input of a first amplifier to provide amplified received signals to a radio receiver.

13. An antenna system according to Claim 12, further comprising a second antenna conductor for reception of L.M.S. signals connected to the input of a second amplifier, wherein the first mentioned antenna conductor is used for V.H.F reception and the second for L.M S. reception.

14. An antenna system according to Claim 12 or Claim 13, comprising a module having the first and second amplifiers wherein the first mentioned antenna conductor connects to a first input of the module, the second antenna conductor connects to a second input of the module, and the module has a common output for L.M.S. and V.H.F. signals.