I'd like to be able to interrogate tags measuring 2 millimeters by 2 millimeters, from a distance of between 30 centimeters and 50 centimeters.
To answer your question, I reached out to Victor Vega, the director of RFID NFC solutions at NXP Semiconductors, a leading manufacturer of the chips used in proximity cards. Before I get to his answer, let me explain that proximity cards are designed to have a short read range—about 10 centimeters (3.9 inches)—because they are often used in financial transactions. Most proximity cards employ the ISO 14443 air-interface protocol standard. Vicinity cards have a read range of 50 centimeters (19.7 inches).
Victor responds: "My assumption is that the questioner is referring to high-frequency (HF) tags with 2-millimeter coils. Increasing the read range to 50 centimeters is a very tall order. The rule of thumb, as I recall, is that an HF tag's read range may be expected to be approximately twice the diameter of the tag coil, and up to about two times the diameter of the reader coil. We have some tag ICs that are more sensitive than others, but still, expecting nearly 20 inches of range out of such a small tag is, in my opinion, not feasible. The reader is governed by regulatory restrictions, so you are limited in how much energy the reader can emit. My suspicion is that the best chance for maximizing range would likely be to use something like a ferrite core-based reader antenna to concentrate/focus/extend energy as much as possible, but my gut says the range would still fall far short of 50 centimeters."
Here's a typical field pattern for a ferrite core-based antenna. The field is concentrated axially, which is good for smaller tags.
—Mark Roberti, Founder and Editor, RFID Journal
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