1. Field of the Invention
The invention pertains to a coated substrate, preferably transparent, with a communication window allowing information to be transmitted therethrough.
2. Description of the Related Art
Patent DE 195 03 892 C1 discloses measures for reducing the shielding of coated panes in regard to microwave rays transmitting information. Such panes with electrically conducting and optically transparent coatings find their application as thermal insulation panes reflecting infrared rays (IR) and/or as electrical heating panes both for the glazing of buildings and for the glazing of vehicles.
In vehicles, they form together with a metal bodywork a Faraday cage, which protects the interior space of the vehicle against electromagnetic fields. In construction aloft also, premises can be protected electrically through the use of panes with an electrically conducting coating and a corresponding electrically conducting structure of the other walls. With shieldings of this nature, sensitive apparatus such as control computers can be protected, in the region of construction, against the disturbing influences due to radiophonic emitters or to radar installations of large power.
Moreover, neither does the shielding allow through electromagnetic radiation in the microwave region, which is used as carrier wave for information. If an emitter and/or a receiver is situated alongside the antenna in a protected (vehicle) space, transmission problems occur. For example, systems for determining the position of the vehicles, for remote control, for identification, for fixing tolls, or the like are disturbed.
It is known for systems of layers to be structured a posteriori, mechanically or thermally, by removing lines of the layer deposited initially in a continuous manner. In particular, extraordinarily narrow slots can be produced in the layer with laser rays. In the aforesaid state of the art, there is devised as remedy in the electrically conducting layer at least one slot forming a radiating slot, with a length tuned to the wavelength of the microwave radiation and a very small free surface area, through which the energy radiating in the microwave range picked up by the conducting layer can again be decoupled. A so called communication window is thus formed.
If the working frequency for the transmission of information equals for example 5.8 GHz, as is envisaged for the automatic fixing of tolls on motorways (“remote payment”) (DSRC standard with a mean frequency of 5.8 GHz with circular polarization) and if the slots are provided mainly for the transmission of the microwaves of this frequency, they will be appropriately designed for the resonant length of λ/2, taking account of the dielectric constants of the glass.
Should the information be transmitted with circularly polarized microwaves (that is to say the instantaneous plane of oscillation of the waves rotates about its axis of propagation, so that the waves oscillate inside a circular envelope curve), appropriate provision is made for gaps in the form of cruciform slots in the layer. The length of the two slots is again tuned in an appropriate manner to the wavelength of the microwaves used and corresponds to the value μ/2 of the microwaves used, taking account in a corresponding manner of the dielectric constants of the glass.
Comparative measurements of the damping of a microwave radiation of frequency 5.8 GHz indicate, in this state of the art, that with a laminated pane exhibiting radiating slots in the coating, appreciably weaker damping of transmission, for high frequency radiation, is obtained than with a fully coated laminated pane, and that it is possible to approach the damping of an uncoated laminated pane.
Document DE 198 17 712 C1 describes a similar substrate with a communication window in a coating, which is produced by the creation of a structure in the form of fine lines or fine patterns in a limited part of the surface of the coating and which is visually very discreet.
The formation of resonating structures in the conducting coating poses problems however. Results of trials have shown that the high frequency currents necessary for compensation, in the overall system formed by the dielectric substrate and the conducting coating, were not able to flow on account of the high surface resistance or of loss of the usual conducting coatings.
Document WO 00/72635 A1 describes a transparent substrate with a coating that reflects IR and a communication window made by the surface removal or the omission of the coating. In contradistinction to communication windows made solely in the form of fine lines, discussed in the introduction, this variant forms a disturbance of the coating which is markedly perceptible to the eye through a difference in color at the boundary of the coating.
This disturbance poses a problem in particular, when the coating is used at the same time for the electrical heating of the substrate. For this purpose, a voltage is applied to the coating with the aid of at least one pair of electrodes (in the form of bands), the currents having to be introduced and distributed as uniformly as possible in the surface of the layer. For vehicle panes, which are appreciably wider than high, most of the time the band shaped electrodes are situated along the longer sides of the pane, so that the heating current can flow over the shortest path over the height of the pane. At the same time, the communication windows are situated on the upper rim of the pane and here extend over a width of several centimeters.
Manifestly, each communication window affecting the homogeneity of the coating forms a disturbance of the current flow. Local temperature spikes (“hot spots”) appear which may lead to damage to the substrate (thermal stresses) and to the coating itself. This is not only the case when the coating is of large extent, but also when the communication window is formed by a larger or smaller number of discontinuous individual slots. The latter also form in the zone of the surface considered an appreciable increase in the resistance of the layer and also in addition cause the appearance of the hot spots mentioned above.
The document cited last proposes, as a measure aimed at reducing the disturbing effect of the large communication window, that provision be made on the edge of the latter for an electrically conducting band, which exhibits a much smaller ohmic resistance per unit area than the heating layer. This ought to cause the currents to bypass the cut-out. Preferably, a communication window is entirely flanked by such a band. The band may be fabricated by printing and baking a conducting silk screen printing paste, containing silver. It may, however, also be applied by depositing an electrically conducting lacquer or by laying a metallic band. In all cases, an electrically conducting functional bond is naturally necessary between the band and the coating.
The band may be masked from view by the superposition of an electrically nonconducting, opaque masking band, made for example of black enamel. Such masking bands are as a general rule composed of a nonconducting substance, black in color, which may be baked (silk screen printing paste). An infrared radiation is not reflected, but absorbed, by this substance.