Method and apparatus for eliminating resonance in a vehicle antenna system

An automotive antenna system for receiving FM broadcast band radio signals includes a conformal antenna and a glazing panel having an electrically conductive coating. A conductive trace integrated with a front windshield panel, for example, forms an active slot antenna which is connected by a coaxial cable to the automobile's FM radio receiver. A transparent conductive layer is applied over a substantial portion of the rear window opening which is typically dimensioned such that it would otherwise be resonant within the FM broadcast band and parasitically coupled to the active antenna, which would cause the active antenna to have an undesirable directional pattern. The transparent layer has a sufficiently high conductivity and is applied over a sufficiently large portion of the resonant window opening to substantially reduce or eliminate resonances within the FM broadcast band.

FIELD OF THE INVENTION 
This invention relates to conformal vehicle antenna systems and more 
particularly to methods and apparatus for eliminating deleterious side 
effects resulting from the resonance of vehicle window openings at 
frequencies near those used by motor vehicle radio systems. 
BACKGROUND OF THE INVENTION 
Conformal automobile FM radio receivers typically are formed in the front 
or rear window opening of the vehicle. These antennas are designed to 
provide good reception of signals within the FM broadcast band, 88 to 108 
Mhz (3.5 to 2.8 meters wavelength). 
Unfortunately, the front and rear window openings which are typically 
defined by the conductive vehicle body have resonances within the FM 
broadcast band, and these resonant openings create undesirable distortions 
from the omnidirectional antenna pattern desired. 
It is a principal object of the present invention to reduce the adverse 
effects which would otherwise result from these undesired electromagnetic 
resonances. 
SUMMARY OF THE INVENTION 
In a principal aspect, the present invention takes the form of methods and 
apparatus for substantially reducing or eliminating undesirable effects on 
a conformal antenna of a motor vehicle radio system which are caused by 
the electromagnetic resonance of one or more vehicle window openings 
within the frequency band used by the radio system. In accordance with the 
invention, the glazing panel, typically a pane of glass, plastic, or a 
laminate of such materials, mounted within a potentially offending window 
opening is coated or otherwise provided with a transparent, electrically 
conductive layer which extends over a sufficient portion of the panel 
surface to effectively alter the opening size and reduce or eliminate the 
undesired resonance. 
The principles of the invention may be applied to particular advantage when 
for a conformal window antenna in either a vehicle windshield or backlite 
(i.e., rear window) with the conductive coating disclosed here being 
applied to the other of these glazing panels, to reduce or eliminate the 
directional pattern distortions that would otherwise result from such 
other window opening. Vehicle windowshield and backlite dimensions are 
such that they typically are resonant within the FM broadcast band and, 
while being spaced from each other, are parasitically coupled to an active 
antenna in the other. 
In accordance with one preferred embodiment, a motor vehicle antenna system 
has a magnetic current receiving, conformal, substantially rectangular, 
truncated slot antenna. The slot antenna has a horizontal length L equal 
to one-half a wavelength in the FM broadcast frequency band. The vertical 
dimension of the slot antenna, referred to here as its width, is 
substantially less than dimension L, preferably being only 3% to 20% of 
dimension L, more preferably 10% to 15% of dimension L. The slot antenna 
preferably is defined in part by the sheet metal or other conductive 
periphery of a window opening in the vehicle body. Specifically, the slot 
antenna is defined in part by a substantially horizontal portion of a 
conductive periphery of the window opening, typically being formed by the 
edge of a sheet metal body panel and/or metal frame components for the 
window opening. The slot antenna is also formed in part by a conductive 
trace formed integrally with a glazing panel mounted in the window 
opening. The conductive trace, which may be formed on or imbedded in the 
glazing panel, extends laterally from a first side point to an opposite 
side point substantially parallel the aforesaid horizontal portion of the 
conductive periphery of the window opening. In addition, suitable means 
are provided for effectively connecting each of the two side points of the 
conductive trace to the conductive periphery of the window opening. An 
electrical lead is provided for carrying signals from the slot antenna to 
a radio receiver. A first conductor of the electrical lead is connected to 
the conductive periphery of the window opening, and a second conductor of 
the electrical lead is coupled to the slot antenna either directly (i.e., 
by an electrical junction) or by a feed network disposed in the slot area. 
The conformal antenna may advantageously be a conformal windshield antenna 
and, hence, be positioned near the radio receiver, with the rear window 
panel containing a conductive coating to eliminate the resonance defined 
by the rear window opening within the FM band, thus providing a more 
omnidirectional reception pattern. 
These and other features and advantages of the present invention will be 
better understood by considering the following detailed description of 
certain preferred embodiments which are presented with the attached 
drawing.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS 
The front and rear window openings of a typical automobile are commonly 
resonant at frequencies within the FM broadcast band. As described in more 
detail in commonly assigned and concurrently filed patent application 
entitled "Motor Vehicle Antenna Systems" (attorney reference 94-263), a 
vehicle window, typically the front or rear window, may be used to 
advantage as part of a motor vehicle antenna system as shown generally in 
FIG. 1. The conformal antenna may advantageously take the form of a 
horizontal slot antenna formed by a metallized trace 12 applied along a 
path in the windshield 14 which extends approximately one-half wavelength 
in the FM broadcast band, parallel to an upper perimeter of the windshield 
opening 16. Together with ground connections or equivalent vertical 
extensions at each end of the trace, these elements cooperate to define a 
slot antenna. The center conductor of a coaxial cable 18 is electrically 
connected to the trace 12 at junction 20, and the outside sheath conductor 
is connected to the conductive perimeter surface of the automobile body at 
22. The other end of coaxial cable 18 connects to the antenna connector of 
a conventional FM receiver mounted behind the vehicle dashboard as 
indicated at 23. 
Ideally, an automobile antenna should be essentially omnidirectional. 
Although some distortions from this desired omnidirectional property are 
unavoidable, it has been found that the rear and front window openings are 
often resonant within the FM broadcast band and, when parasitically 
coupled to the receiving antenna, have a substantial deleterious effect on 
the directional pattern of reception. The resulting "blind spots" and the 
like can cause a received station to fade in and out as the orientation of 
the car changes with respect to the incoming signal direction. 
In the preferred embodiments of the invention, this effect is substantially 
reduced or eliminated by coating the window pane mounted within an 
offending window opening with an electrically conductive layer which 
extends over a substantial portion of the pane surface. Preferably, the 
coating is applied to the entire surface everywhere except at its edges. 
As shown in FIG. 1, the rear window panel 30 is coated over substantially 
its entire area with a conductive coating 32, typically before the window 
pane is installed in the vehicle. 
The coating may be any suitable metallized film, of which various 
commercially available and known types will be apparent to those skilled 
in the art in view of this disclosure. It should, of course, be 
sufficiently conductive to alter the electromagnetic characteristics of 
the window opening to substantially reduce the extent to which it would 
parasitically alter the effective directional pattern of the antenna. 
Preferably, it is substantially transparent to visible light. Preferred 
conductive films are 500 to 10,000 angstroms thick and are formed of 
silver, zinc oxide, fluorine-doped zinc oxide, cobalt oxide, iron oxide, 
indium-tin-oxide, chrome oxide, fluorine-doped tin oxide and/or like 
materials, yielding less than 10 ohms per square resistance, preferably 
1-2 ohms per square. Both float deposition processes and finished product 
deposition processes are suitable, such as sputter coating, chemical vapor 
deposition, pyrolytic processes and like techniques. Alternative suitable 
materials and deposition techniques will be apparent, whether presently 
commercially available or developed hereafter, in view of the disclosure 
provided here. In accordance with one alternative embodiment, the 
conductive film comprises an electrically conductive film adapted for 
electrical resistance heating of a glazing panel, together with an 
electrically isolated additional film cooperating therewith to provide the 
desired performance characteristics. 
The coating 32 should have sufficient electrical conductivity to 
effectively alter of the effective electromagnetic dimensions of the 
window opening and reduce or even eliminate resonance at frequencies 
within the FM broadcast band. Electrically resistive films of low 
conductivity of the type typically used to provide electrothermal heating 
to the windshield to remove fog, frost and ice typically have suitable 
conductivity to effectively alter the resonant properties of the 
windshield opening and to satisfactorily meet the objective contemplated 
by the invention. 
It is to be understood that the foregoing description of the preferred 
embodiment is merely illustrative of one application of the principles of 
the present invention. The invention may also be used to advantage to 
improve the omnidirectional reception pattern of other conformal antennas 
by coating either or both the front or rear window panels. In some 
vehicles, the side window openings may also exhibit unwanted resonances 
within the FM broadcast band which can similarly be substantially reduced 
or eliminated by coating all or a substantial part of the side panel 
surface(s) with a conductive material. Numerous modifications to the 
methods and structures described may be made without departing from the 
true spirit and scope of the invention.