I am a student at the University of Aveiro, in Portugal, and am currently working on a project involving a long-range RFID interrogator (Alien Technology's ALR-8800 reader) and passive tags. After conducting several tests, the workgroup concluded that the distance at which we can read a tag depends on whether the target carrying that tag makes abrupt movements, thus agitating it—for example, if, at a distance, near the threshold of tag detection, we were to shake the tag, the reader would be able to detect it. Why is that? What is the influence of tag movement on the reading process? Is there a scientific explanation?
Since I am not an RF engineer, I reached out to Alien Technology's Victor Vega, who used to teach Alien's RFID Academy course, back when I took it a few years ago. During the course, we mapped the read field. Victor's response, which I've edited, appears below:
"What Tiago is describing is what we used to see, back in 2005, at about 3 meters [10 feet]. Today, it happens much, much further out, because the silicon efficiency and reader sensitivity have improved tremendously. At the outer edge of the read field, the reads are not reliable, and they are heavily influenced by ambient environment (e.g., concrete floor, metal framing studs, overhead metal ducts or roofing, metal vapor deposition coatings on windows, and people moving about). Tiago is just catching enough energy with the movement in a certain spot to activate the tag. It has nothing to do with shaking the tag—he's just catching a ray of energy in an otherwise spotty zone.
"It's probably easiest to show this with some diagrams, like those we used to construct at RFID Academy (view the diagrams). We abandoned these exercises long ago, as UHF [ultrahigh-frequency] performance matured to the point where we no longer had to worry about reliable reads within a few meters of range, but if you look at these old diagrams, you will see we fixed an antenna (circular or linear) 40 inches above the floor. The tag's elevation is plotted at 20 inches, 40 inches and 60 inches, and the reads are plotted on a 30-foot by 40-foot grid.
"The color was an indicator of read intensity, with dark red indicating the greatest number of reads, followed by dark blue, green and light blue, on a light-red background. Notice that the read rates are very much a factor of tag elevation (with respect to antenna elevation), the orientation of the tag or antenna, and the type of antenna.
"To help visualize the effects of moving the tag, look at slide 3. At the 60-inch elevation, at 28 feet from the antenna, there is no read (you're in the light-red area). Then drop the tag to 20-inch elevation, and you are in the green (or light-blue) section, and are reading the tag. This is essentially what Tiago is seeing as he 'agitates' the tag."
—Mark Roberti, Editor, RFID Journal
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