vaguelyaware.com

When I first saw the news of the new epass (a small sticker to be affixed to your windshield) i thought it would be great for my motorcycle since it would be more trouble than its worth to try and steal, waterproof, compact, and have little chance of it getting bounced or blown off. The marketing material said it used the windshield as an antenna which i thought was total BS since I knew it was just a passive version of the old active rfid sunpass tags (and since when is glass used as an antenna? maybe you could say waveguide for fiber optic).

later marketing material corrected the omission that the tags do NOT work on motorcycles that they must be affixed to glass. well, there went that idea, so i figured i would find out how the tags work and find a work-around.

well after much searching i found the patent application detailing WHY the tag only works on glass. its all about impedance matching, that probably means more to an electrical engineer than to most of the people reading this, but then again its not a hard concept to grasp.

Here is the excerpt from the patent that explains the whole setup:

The sticker transponder comprises a flexible circuit substrate having an antenna formed thereon and a transponder circuit disposed on the substrate and coupled to the antenna. An adhesive layer is coupled to a first surface of the flexible circuit substrate. An indicia layer is coupled to a second surface of the flexible circuit opposite from the first surface. The indicia layer comprises a space permitting indicia to be printed thereon. The antenna has a characteristic impedance defined in part by a dielectric constant of the flexible circuit substrate in combination with a dielectric constant of the glass surface. As a result, a proper impedance match between the antenna and the transponder circuit is achieved only when the sticker is affixed to the glass surface.

kudos to the engineers for a very cool way to secure their device. i was quite impressed even if it did foil my motorcycle plan.

for a workaround you would need to substitute the windshield with either another piece of windshield that was portable, or another material with the same dielectric constant (some web sources:3.7-10 others: 7.6-8) Don’t have an old text-book on hand to look it up.

Off to find a piece of neoprene or rubber to try this out. We’ll see if it works…

Link to Patent