EP1315232A1

EP1315232A1 - Window antenna for the reception of AM and VHF transmissions using the rear window defroster of cars as a radiating structure

Info

Publication number: EP1315232A1
Application number: EP02024378A
Authority: EP
Inventors: Gentili Guido Biffi
Original assignee: Ask Industries SpA; Zendar SpA
Current assignee: Ask Industries SpA
Priority date: 2001-11-26
Filing date: 2002-11-04
Publication date: 2003-05-28
Also published as: ITRE20010111A1

Abstract

An antenna integrated with the conductor grid constituting the rear window defroster of a vehicle, allows the reception of metric band (VHF) emissions using the principle of the space, polarization or beam diversity severally or jointly and, at the same time, the reception of the AM signals with the utmost efficiency.

Description

The present invention refers to a window antenna for the reception of medium wave (AM) and metric band (VHF) transmissions using the rear window defroster of cars as a radiating structure.
Space diversity antennas are already known and used for the reception of modulated signals, preferably in the metric band.
The use of the spatial polarization and beam diversity allows a substantial quality improvement of the FM and
TV reception, particularly on mobile means where non-stationary "multipath" phenomena can cause "fading" effects, even impulsive ones, which strongly reduce the signal/interference ratio.
Car manufacturers presently integrate both the medium wave (AM) reception antenna and the metric wave (VHF) one in the car structure, particularly in windows. This solution, which is not always efficient from the radiative point of view, shows clear aesthetic and aerodynamic advantages. Window antennas are usually made on the rear window, as the windscreen is too close to the engine being, as everyone knows, origin of serious electromagnetic interference. As the most part of the rear window is taken up by defroster grid, according to the engineering and designing point of view, the problem of making the two systems compatible between them arises.
Two solutions are virtually possible:
a first solution that contemplates keeping the antenna separated from the defroster: this is possible when the window is shaped and sized in such a way that a part of it can be kept for the antennas without compromising the rear visibility in case of misting;
the second solution contemplates the integration of the antenna system with the defroster adopting suitable tricks not to limit the original functionalities of each system.
Both the solutions show pros and cons; therefore the optimum solution should be identified depending on the case, according to systems theories involving the vehicle structure, the shape and the place of the windows in addition to the performances of the receivers used.
Window antennas can be manufactured with various technologies but, presently, the one which is mostly used for its low manufacturing cost is based on the lay down of conductive "bus bars" on the glass surface by silk-screen process which is the same technology used to carry out the defroster grid.
As modern FM and TV receivers for cars generally integrate "diversity" functionality, the antenna system for the metric band reception must be structured in such a way to make the most of such feature.
In case of antenna integrated in the defroster, the diversity can be obtained by splitting the grid into two sections which are physically separated between them, as described in US patent no. 4,914,446 with phase centres and FM plugins placed at distances that are suitable for the configuration of the usual solution in space diversity, or providing sections that are more or less physically separated among them or even one section only with a plurality of peripheral plugins. Last solution substantially configures polarisation and/or beam diversity. Diversity antenna systems of this kind are described in patents DE 3220279 and EP 0197650 that do not clearly explain the actual effectiveness of the solution proposed.
The problem related to the radio-frequency insulation of the heating circuit is generally solved putting low pass filter elements between the feed points on the "bus bars" and the wires coming from the battery or from the car body (ground). In order to make the insulation effective even at AM frequencies without highly increasing the dimensions and the cost of the filter, patent UK 1,520,030,1978 discloses the use of a double wire choke wound up on the same core which was previously known to the technicians skilled in the art as "common mode choke". As the direct current flows in the two windings in the opposite direction, the relevant magnetization fluxes cancel thus avoiding even the smallest core saturation. This device is also used in patent EP 0197650 and in other ones deriving from it. Anyway many problems related to the use of the whole defroster as AM antenna due to the losses and parasitic reactance of coils are to be solved. The windings dimension is then influenced by the high inductance values required by the AM frequencies (>1mH) and by the high currents flowing in them that require conductors with a sizeable section. Moreover, the filter elements are not highly efficient with respect to the interferences produced by the ignition system and led by the direct current (DC) feeding wires.
Subject of the present invention is to carry out an antenna integrated with the conductor grid forming the rear window defroster of a car and allowing the reception of metric band (VHF) emissions using severally or jointly the space, polarisation and beam diversity principle and, at the same time, the reception of AM signals with the highest efficiency, in order to avoid the above mentioned problems.
According to the present invention, this purpose was reached by:
1) splitting the defroster into three sections being connected in parallel for the direct current feeding;
2) providing an RF insulation to the three sections mentioned above from the direct current feeding circuit depending on the car battery, in order to use them as antennas;
3) configuring two of the three above mentioned sections in such a way that the signals received in VHF band from each of them are, as far as possible, not correlated to the interference and "fading" phenomena that produce abrupt variations and distortions of the received signal;
4) maximising the RF insulation between the two above mentioned sections by positioning the diversity plugins in a suitable way and simultaneously keeping the phase centres at the utmost possible distance according to the structural and aesthetical constraints of the car;
5) configuring the remaining section in such a way to efficiently serve as an antenna only for the AM reception.
These and further features of the present invention will be understood by a technician skilled in the art from the following description and the enclosed drawings, which are given by way of non-limitative example wherein:
Figure 1 is a schematic view of an antenna system according to the present invention wherein the defroster is horizontally split into three separate sections which are fed in parallel by the battery of the vehicle through RF chokes; the upper and lower sections are used for the diversity reception on metric band, while the central section is used for the AM reception;
Figure 2 is a schematic view of an antenna system according to the present invention wherein the central section, which is used for the AM reception, is constituted by one horizontal trace; for the direct current this trace is fed by two series choke serving as RF insulating elements; in order to reduce the sizes of the ferrite core, these chokes can be coupled among them;
Figure 3 is a schematic view of an antenna system according to the present invention as shown in figure 1, wherein the central section is not used as heating element but is only used as an antenna for the AM reception; therefore the distance between the horizontal conductors of the grid in the central part of the rear window is adjusted in such a way to allow an effective and smooth defrosting action;
Figure 4 is a schematic view of an antenna system according to the present invention as shown in figure 1, wherein the central section is omitted; therefore, the two remaining sections serve both as antennas for the diversity VHF reception and as antenna for the AM reception;
Figure 5 shows a particular embodiment of the RF chokes of the antenna system shown in figure 4 ensuring a good overall RF insulation and a good decoupling between the AM and VHF plugins.
The invention refers to a window antenna system for the reception of medium wave (AM) and metric band (VHF) transmissions using the rear window silk-screened defroster of a vehicle as radiating structure, which is usually constituted by two wide vertical bus bars connected between them by a grid of narrower horizontal bars wherein the heating direct current flows.
In the basic configuration proposed by the present invention, the defroster is split into three separate sections that are fed in parallel by the vehicle battery, wherein the continuous direct current required for the heating by Joule effect flows. The radio frequency (RF) insulation of the above-mentioned sections from the direct current feeding circuit, which allows using them as antennas, is obtained by chokes connected in series to the feeding circuit. The couples of chokes that are placed on the same part of each section are double-wire wound on one ferrite core and they are crossed by direct currents flowing in the opposite direction, with the result that the magnetization fluxes cancel, thus avoiding the core saturation.
The proposed configuration is suitable for the diversity reception in the VHF range as two of the three sections are configured in such a way that they actually constitute two antennas with separate phase centres. The VHF reception plugins are chosen in such a way to determine the utmost circuitry and electromagnetic decoupling between the two antennas.
In a particular advantageous embodiment the third section, which is only used for the AM reception, is placed at the centre of the rear window and it separates, from the physical and electromagnetic point of view, the two antennas intended for the VHF diversity reception.
With reference to the enclosed drawings, the antenna system according to the invention is based on the state of the art technology related to the rear windows for vehicles, wherein the defrosting function is obtained by a silk-screened conductor grid placed on the glass surface or on an internal layer of the various layers forming the rear window. The heating grid is usually constituted by a plurality of horizontal thin parallel traces ending on two larger vertical traces serving as bus bars. The technicians skilled in the art know that the above described conductor grid, whose original function was the thermal defrosting of the rear window, can be advantageously used as reception antenna for the "broadcast" emissions, even on different bands, if it is properly insulated by the continuous feeding circuit. It is also known that the diversity reception can be obtained using more antennas that are physically separated (space diversity) among them or one antenna with many entrances or separate plugins (polarization and/or beam diversity).
The present invention is now described with reference to figure 1 showing a particularly advantageous embodiment wherein the defroster grid that is supposed to have a rectangular shape, by way of non-limitative description, is horizontally split into three different antenna sections ( SECT 1,2,3). Each of the above-mentioned sections is further split into two subsections that are fed through RF block inductance couples. The above-mentioned purposes are obtained by the present invention using two of the above-mentioned sections and, exactly, the upper one (1) and lower one (3) as antenna for the diversity reception in VHF band and the third central section (2) only for the AM reception. The three above-mentioned sections can have different shapes, sizes and positions; numbers, shapes layouts of the subsections composing each of said antenna sections can also vary with respect to the ones mentioned in figure 1 without leaving the protection field of the present invention. The direct current feeding for the heating of the three above-mentioned sections is obtained connecting the ends of the bus bars of each subsection to the conductors of the relevant feeding circuit through block inductance couples for the RF. The feeding polarities are advantageously chosen, according to the invention, in such a way that the direct current crossing the corresponding couples of chokes flows in them in the opposite direction. The inductance values that the above-mentioned chokes take in order to carry out an efficient RF insulation action of the grid sections to be used as antennas can change in size fromµH to mH depending on the frequency band to be covered.
In order to reduce the number of turns and consequently the dimensions, particularly for the highest inductance values, the correspondent couples of chokes are double wire wound on a ferrite core with high magnetic permittivity. This trick that is widely used in the field of common mode chokes, allows avoiding the saturation of the core provided that the chokes are crossed by equal and opposite currents. This condition turns into a constraint on the value of the resistance among the direct current feed points of the two subsections composing each section that should be the same. Therefore, a certain unbalancing level between the currents can be tolerated provided that the resulting differential current is lower than the one that would determine the core saturation.
A further advantage deriving from the use of a multisection grid derives from the fact that, for the same power required for the defrosting action, the total current distributes among the various sections placed in parallel with respect to the direct current, therefore the current crossing the chokes of each single section is reduced of a factor which is inversely proportional to the corresponding continuous resistance.
By way of example, it should be supposed that the AM section is constituted only by two traces and that the same number of traces equally constitutes the VHF sections.
Marking with N the total number of traces and with I the total current required for the defrosting action, a current corresponding to I/N would flow in every trace. The direct current crossing each of the AM subsections will correspond to I/N, while the one crossing each of the VHF subsections will take the I/N x (N-2)/2 value. Considering that for the sizing of the wound chokes, reference is essentially made to the parameter defined "area product", from the charts provided by the core manufacturers, it can be noticed that for the AM chokes the proposed solution leads to a size reduction factor of 1.6 x N and for the VHF chokes a size reduction factor of about 1.6 x (1/N x (N-2)/2) is obtained. The above-mentioned factors are rough and are to be meant as reduction indexes of the whole choke volume, with respect to the reference solution that is constituted by a single grid crossed by the same total current. Size reductions of this entity can be practically obtained provided that the currents flowing in the two wire windings are actually equal and opposite, as, only in this case, core sizes do not highly contribute to determine the total volume of the core. With the use of suitable cores, the AM core volume reduction, let's suppose, for a 20 traces grid, can be comprised between 30 and 40 times. Though, the sectioning and the subsectioning of the grid produce further relevant advantages described here below:
1) the possibility of using, for each antenna section, the more suitable core in terms of magnetic permeability (AL), area product (AP) and losses for magnetic hysteresis (depending on the frequency);
2) the possibility of using, for each antenna section, cores with proper shapes and sizes for the chosen type of assembly and the optimum "winding packing factor" (KW) in order to minimize the parasitic reactances connected to the winding;
3) the possibility of connecting the correspondent two wire choke terminals directly to the "bus bars" of the relevant subsections, without introducing additional lengths of wires taking part to the radiative phenomena;
4) the possibility of using chokes with sizes being compatible with the ones of the modern devices for the surface mounting, thus creating all the RF feeding and filtering network on a low cost and small-sized printed circuit.
Figure 2 is a schematic view of a further embodiment of the defroster-antenna system wherein the AM antenna is carried out using only one conductor of the grid that is continuously fed through two simple inductances placed at the end of this conductor. In this case, the core sizing of each coil is carried out in such a way to avoid its saturation due to the direct current effect flowing in the winding. Even in this case, for the maximization of the "common mode choke" properties, the two chokes are two wire wound on the same core and the correspondent terminals are connected to the opposite ends of the unique printed conductor serving as AM antenna. But, in such a way, lengths of additional cables external to said printed conductor and influencing the antenna properties, particularly its input impedance, are necessarily introduced. A size reduction of the chokes can be, to some extent, obtained increasing the continuous resistance of the unique conductor serving as AM antenna, that is to say correspondently reducing its width. This does not have consequences on the RF behaviour of said printed conductor but it causes a direct current power reduction dissipated in it by the Joule effect. The consequence is a possible unevenness in the temperature distribution on the rear window that can be however easily compensated acting on the width or on the distance of the adjacent conductors belonging to the subsections assigned to the VHF reception.
An even more advantageous solution than the previous one for production costs consists in the complete elimination of the RF block inductances only in the AM section, as schematically shown in figure 3. In this case, the problem of unevenness in temperature in the centre of the rear window is made more serious by the fact that the central conductor does not take part in the heating. This drawback can be therefore solved modulating the width and the distance of the adjacent conductors of the VHF sections as proposed in the previously case, or putting the AM section in the upper or lower part of the window.
Figure 4 schematically shows another particularly advantageous solution from the point of view of the complexity of wires outside the rear window. In this configuration, the rear window is split only in two sections that simultaneously serve as diversity AM and VHF antennas. The above-mentioned sections are fed by direct current through the "common mode chokes" with the same criteria used in the first configuration (Figure 1), that is to say in such a way that the magnetization flux in the core is cancelled.
A potential drawback of this solution is due to the difficulty in producing chokes having a good simultaneous RF insulating efficiency both in the AM and in the VHF bands. A possible solution to this problem consists in building the "common mode choke" as shown in figure 5.
Two series "common mode chokes" are practically used; the first one is optimised for the block of the frequencies in the VHF range and the second one for the block of the frequencies in the AM range. In this case, the AM plugin is advantageously carried out by connection to the conductors placed inside the two chokes by previously inserting a high capacity capacitor being able to behave as a good RF short-circuit in all the frequency range.
In all the above described solutions, the position of the FM1 and FM2 plugins on the correspondent sections of the grid, together with the length and the layout of the printed traces used for the connection to the circuits that are external to the glass, must be determined by way of experiment and/or using suitable electromagnetic simulation codes and objective criteria measuring the decorrelation level among the signals received by each antenna section.
In the optimisation process considerations on the shape and size of the metal body of the vehicle cannot be set aside as the body of the vehicle affects the radiation diagrams and the input impedances of the above-mentioned sections. Once the positions of all the plugins, including the AM one, are decided, it is necessary to insert proper passive and/or active matching networks between them and the inputs of the relevant receivers. These networks are not discussed here, as their circuitry configuration and their functioning are part of the technical/scientific knowledge of the technicians skilled in the art. All the external circuitry can be housed in two small metal boxes that are screened and provided with passthroughs for the connection to the DC and RF feeding conductors of the sections and subsections of the grid. These conductors will end on special connection pads that will be grouped in two peripheral areas of the glass; these areas will be properly chosen according to the constraints imposed by the car manufacturer. The drastic size reduction of the "RF choke", that can be carried out through the grid sectioning and subsectioning technique, allows making the most of the advantages of the present printed circuit integration technologies. These technologies allow reducing board sizes and costs and ensure, at the same time, a high reliability and quality of the product.
Even though the present invention has been described and illustrated here with reference to some embodiments that are given only by way of non-limitative example, it is clear that various changes to antenna, sections, position, layouts, circuits, components and other combinations can be made by people skilled in the art according to the above-mentioned description.
It is therefore clear that the present invention is meant to include all the changes and variants falling within the spirit and the protection (purpose) field of the following claims.

Claims

A window antenna using as a radiating structure the rear window defroster of cars constituted by two vertical conductive "bus bars" connected between them by a grid of horizontal bars wherein the direct current for the heating flows, characterized by the fact that said defroster is horizontally split into two separate sections that are fed in parallel by the vehicle battery through RF chokes; said sections being used for the diversity metric band (VHF) reception and AM reception.
The window antenna according to claim 1 characterized by the fact that said separate sections are split into subsections that are fed through RF block inductance couples.
The window antenna according to claims 1 or 2, characterized by the fact that said sections can have different shapes, sizes and positions.
The window antenna according to any of the previous claims, characterized by the fact that the subsections composing said sections can change in number, shape and layout.
The window antenna according to any of the previous claims characterized by the fact that the radio frequency insulation (RF) from the direct current (D.C.) feeding circuit that allows using the above-mentioned sections and subsections as antennas, is obtained by chokes connected in series to the feeding circuit; the inductance values to be taken by said chokes according to the above-mentioned RF insulation of the grid sections that can be used as antennas vary in size fromµH to mH, according to the frequency band to be covered.
The window antenna according to any of the previous claims characterized by the fact that the couples of chokes placed on the same part of each section and subsection are two wire wound on one ferrite core and they are crossed by direct currents circulating in the opposite direction to cancel the magnetization fluxes.
The window antenna according to any of the previous claims characterized by the fact that said defroster is horizontally split in three separate sections that are fed in parallel by the car battery through RF chokes; said upper and lower sections are used for the diversity reception on metric band (VHF) and the central section is used for the AM reception.
The window antenna according to any of the previous claims characterized by the fact that two of the three sections constitute antennas for the diversity reception in the VHF range with separate phase centres; the VHF reception plugins being chosen according to the maximum circuitry and electromagnetic decoupling between the two antennas.
The window antenna according to any of the previous claims characterized by the fact that the third section, used for the AM reception, is placed in the centre and it physically and electromagnetically separates the two antennas for the diversity VHF reception.
The glass antenna according to one of the previous claims from 1 to 6 characterized by the fact that the central section for the AM reception is constituted by a unique trace fed by direct current through the two chokes with the function of RF insulating elements.
The glass antenna according to one of the previous claims from 1 to 6 characterized by the fact that the section for the AM reception is moved from the central position to the upper or lower part of the window.
The window antenna according to claim 1 characterized by the fact that the two chokes are coupled between them according to the size reduction of the ferrite core.
The window antenna according to any of the previous claims from 1 to 6, characterized by the fact that the central section is not used as heating element, but it is only used as antenna for the AM reception; in said configuration the distance among the conductors of the grid placed in the central area of the rear window is adjusted according to the defrosting action.
The window antenna according to any of the claims from 1 to 6, characterized by the fact that the central section is omitted and the two remaining sections are used both as diversity VHF reception and as AM reception antennas.
The window antenna according to any of the claims from 1 to 6 and 14, characterized by the fact of comprising two "common mode chokes" in series, the first one is optimised for the frequency block in the AM range wherein the plugin is carried out by connection to the conductors placed inside the two chokes by previously inserting a high capacity capacitor being able to behave as a good RF short-circuit in all the frequency range.
The window antenna according to any of the previous claims, characterized by the fact that the position of the FM1 and FM2 plugins on the correspondent sections of the grid, together with the length and the layout of the printed traces used for the connection to the circuits that are external to the glass, are determined by way of experiment and/or using suitable electromagnetic simulation codes and objective criteria measuring the decorrelation level among the signals received by each antenna section.
The window antenna according to any of the previous claims, characterized by the fact that, according to the shape, the sizes of the metal body of the vehicles and according to the positions of all the plugins, included the AM one, passive and/or active networks for the fitting to the entrances of the relevant receivers are inserted.