The relative placement and why it matters
Before understanding the role of positioning in an RFID system, it helps to know exactly how the system works.
The antenna emits passive energy in a particular direction. When a passive tag (e.g. HeadlightSticker™) receives this energy, it stores this power to power up its chip. Once enough power is stored, the sticker transmits its signal forward.
This whole process takes a fraction of a second and repeats continually as long as the antenna can power the sticker.
This process is directional. This means that the tag and reader must face each other. The more they face each other, the better the read.
Face to face
Think of it like shining a flashlight at a mirror: your antenna is the flashlight, and the sticker is the mirror. You need the mirror to point in such a way that the light is reflected back at the reader.
It may help to think of it as RF energy “bouncing” off the sticker. (although strictly speaking this is not exactly how it works.) If the energy bounced off the sticker, where would it go? Will most of it bounce back towards the reader, or will it simply bounce up into the sky?
The most intuitive way to remember this is to use the Rule of Thumb.
Windshield vs Headlight: Pick one
Because the antenna and sticker want to be parallel, it is generally recommended that you pick one standard and stick with it. If you try to get both form factors with the same antenna, you will reduce performance and consistency for the entire system.
A low mounted reader will have trouble with most windshield tags. A high mounted reader will have trouble with many headlight tags.
Instead of implementing a hybrid system with both headlight and windshield tags, you should pick a single standard. If a hybrid system is necessary, we recommend discussing your options with our design team.
HeadlightStickers™ work well with a low mounted reader, mostly level and parallel with the approaching headlights. Windshield RFID requires high mounted readers (often directly over the driver’s lane for best results.) Again, as with all RFID, the goal is to have a face-to-face conversation between the tag and antenna.
Overhead mounting for windshield tags has been the go-to solution for most vehicular RFID implementations. Unfortunately, there are several drawbacks with high mount points:
- Readers mounted directly overhead reduce maximum vertical clearance.
- High mount points require a sturdy base, which requires heavy duty poles and pouring concrete.
- Aesthetically speaking, high mounted readers call attention to themselves.
Furthermore, there are two major flaws with windshield mounted RFID:
- Metallized windshields interfere with RF signals and are increasingly common.
- Windshields significantly vary in height and angle. An angle that works for a standard sedan may fail on a higher SUV with a more vertical facing windshield.
Because of the variance in windshield angles, face to face reading is far easier to achieve with a headlight mounted RFID tag than on a windshield.
Simply put, Headlight shape is far more consistent than windshield shape, and headlight RFID completely avoids the conflict with metallized windshields.
With that said, headlights are still three dimensional.
When installing a HeadlightSticker™ one must account for the location of the reader.
So before HeadlightSticker™ placement, ask yourself these questions:
- Where is the antenna?
- During the vehicle’s approach, where will the antenna and sticker best interface?
The major requirements of sticker placement are that they be placed horizontally, and that they be 2″ or further from the car’s metal body. As long as those conditions are satisfied, your top consideration should be ensuring that the sticker is in a place where it will interface with the reader at your ideal range.