If anybody is interested. Here's a E-ZPass taken apart. The parts are horizontally aligned.
If you are looking at the E-ZPass from the front (where the barcode/label is) note that the antenna part is on the right. So when you mount it make sure the right part of the E-ZPass is on the non-coated section of the windshield.
Disclaimer, I'm no expert on EM systems or radios, I just like to read a lot.
According to various documents on the Internet (
http://www.root.org/talks/BH2008_HackingTollSystems.pdf), it seems that the E-ZPass transponder is vertically polarized (perpendicular to the surface of the Earth). So while mounting the E-ZPass sideways (vertically) may work, it is
probably better to mount it horizontally. Or in Tesla's case, upside down with the right over the black non-coated area.
Additionally, the communication protocol for the transponder systems seems to be governed by something called "Title 21". The toll booth antenna sends a carrier signal to your transponder. And if the antenna of your transponder is in a position to receive it, it will modulate (backscatter) the a hardcoded frame back. This can take up to 1s on normal toll booths (not the express ones). While this may sound like its quick, the Tesla is quicker.
With that said, I did all this research to see if I can come up with a more elegant way to mount a transponder than a) on the winshield b) on the bumper c) taking apart a nosecone on the refresh by going through the frunk d) mounting in the rear trunk. (Mounting on the front will allow earlier detection aka less slow down at the booth, thusly less "unable to read" anxiety.)
My current idea is to create some sort of sticker antenna that I can mount on the non-coated part, and paint it black. With the chip/transistors hidden away inside the rearview/autopilot cradle. It'll likely fail, but should be a fun project/learning experience nonetheless.
