How is Windshield Glass Used As Antenna For Transponder?

[thender]

I'm an automotive technician working at a dealership and I am working on a car with intermittent issues with wireless communication with the tire pressure sensors which are mounted inside each wheel.

I went searching for the location of the particular antenna that serves the tire pressure monitoring system. There are a number of antennas on the vehicle for different things, GPS/satellite navigation, radio, tire sensors, and more. When I did find the tire system antenna I found it was located basically smack dab top center of the windshield.

And,

Right next to a Florida Sunpass transponder. A sticker with some kind of RF antenna/transponder device built into it, for passing through toll roads.

Now I would like very much to think my technical problem has nothing to do with that transponder. But I don't know.

The manufacturer's technical assistance personnel state the specific problem I have is caused by some source of EMI on the vehicle, commonly aftermarket accessories, and EMI only.

Further it is clear that some types of windshields or glass tints affect RF/EMI and wireless functionality:

http://www.eham.net/ehamforum/smf/index.php?topic=71368.0

This leaves me pretty confused.

Does a windshield enhance the operation of an antenna?

The other issues that I have had have been intermittent problems with the vehicle's only unshielded network (single wire). Low speed communication, no real protection from EMI.

https://www.sunpass.com/specialWindshields

That site even lists some vehicles with special windshield considerations for the transponder.

Here the claim is made that the Sunpass has to be attached to the windshield:

https://www.sunpass.com/sunPassInstallation

Installing SunPass Mini Sticker Transponder
The SunPass Mini Sticker Transponder cannot be hand-held or moved from car to car. It must be permanently mounted to the inside of the windshield or it won’t work, because the windshield acts as an antenna. Once removed, the Mini will no longer function and cannot be reattached to the windshield. indicates a fileClick here to see Mini Transponder Installation Instructions.

Can someone explain the electrical technical theory about how the windshield has any relationship to radio waves?

This car has run me ragged with a multitude of electrical problems of an illusory, transient, and intermittent nature.

If the windshield had no bearing on antenna operation, why did the vehicle engineers place the tire monitoring system antenna there?!

Thanks for any help understanding this technical issue.

 

[thender]

Here is an interesting forum reply apparently from a General Motors RF Engineer.

http://www.terrainforum.net/index.php?topic=2704.0

gmiemigrad
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Re: what's the black box on the inside of the windshield?
« Reply #16 on: April 21, 2011, 01:06:47 PM »

Quote from: psywzrd on April 21, 2011, 11:05:59 AM
Antenna for the tire monitors? Please explain why the tire monitors need an antenna.

The TPMS (tire pressure monitor system) uses RF (radio frequency) to communicate from the tire sensors to the BCM (Body Control Module) and typically (not always) use the remote keyless entry receiver to do that.

Steve Ochs - Senior Manufacturing Engineer
General Motors - Parma Plant
The statements and opinions expressed here are
NOT an official statement of the General Motors Company

That's exactly it. I believe the antenna is hard wired straight into the BCM. I am not really sure how it works, I think it is four wires, power, ground, and two signal wires for the antenna.

I have intermittent fault records for the remote start, keyless entry, keyless start, and tire pressure monitoring system.

This sticker transponder thing is going to make me pull my hair out. I plan to call and ask if they've had issues with it. It may be just by the nature of its placement that it can cause a problem.

Or it may have NOTHING at all to do with my problems. My problems are vehicle-wide and can't be isolated to specific systems, modules, sensors, or wiring. It can't even be determined if there is a real fault for sure, because some intermittent RF noise/issues is normal.

Bald I tell you.