Can you please explain why this is so, in terms of induced volts, reader transmission power, backscatter energy, and so forth? Could it be read via UHF from a distance of a few meters?
I am not an electrical engineer, so my answer might not be as technical as you would like, but I will do my best. The read range of passive RFID transponders that use backscatter to communicate (as opposed to inductive coupling) is determined by the amount of power the chip in the transponder can receive, store and reflect back to the reader antenna. The amount of energy is influenced by a number of factors.
One is the power output of the reader antenna. If the reader antenna can blast out a lot of RF energy, the tag can capture more energy than if the antenna emits only a small amount of energy. In the United States, the Federal Communication Commission limits the power output of devices operating in the 2.4 GHz band (see FCC 2.4 GHz BAND RULES [POINT-TO-MULTIPOINT] Maximum = +36dBm [4watts]). Other countries have similar rules. So the amount of energy from the reader is limited by regulation.
Another key influencer is the size and shape of the tag antenna. A larger antenna can capture more energy. But the mu-chip’s antenna is etched into the chip itself, so it is extremely small. That means most of the energy from the reader passes it by. It can only capture the miniscule amount of energy that hits its tiny antenna. In the case of the mu-chip, this is the single biggest reason it has a short read range.
Another influencer is how efficiently the chip briefly stores the energy from the antenna and uses it to send back a signal to the tag. If two passive UHF tags have the same antenna design but different chips, the one with the more efficient chip will have a slightly longer read range. In the case of the mu-chip, this would only have a tiny influence on reader range.
The frequency used by the tag and reader does influence read range. Generally, a device emanating radio waves at 2.45 GHz would have a longer read range than an identical device operating at 915 MHz (as passive UHF tags do). But given the limited energy that can be harvested by the mu-chip’s tiny antenna, its read range is extremely short. You might think of it this way. You have a six-inch speaker, and I have a two-inch speaker. You would thus expect your speaker to be heard from further away. But if your speaker is hooked up to a 3V lithium cell battery and mine is hooked up to a car battery, mine would likely be heard from a greater distance.
As for a mu-chip being read by a UHF reader from a few meters, this is not possible. Tag antennas are tuned to receive signals at a particular frequency (2.45 GHz in the case of the mu-chip). The antenna would receive almost no energy from a reader operating at 915 MHz because it is not tuned to that frequency (just as you would not receive a specific AM station using an FM radio).
So to summarize, the mu-chip can only capture and store a tiny amount of energy due to the extremely small size of its antenna. This is what restricts its read range to a few millimeters. It cannot reflect back a strong enough signal using backscatter for the reader antenna to receive from more than a few millimeters.
—Mark Roberti, Founder and Editor, RFID Journal
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