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How much energy is extracted by a passive tag from the reader?

How much energy is extracted by a UHF tag from the reader? Will the tag receive and use all of the energy emitted by the reader? On what factors, energy absorbing is dependent?

Dear Shabana:

Tags use very little energy, as little as 500 miliwatts (a milliwatt is equal to one thousandth of a watt). The tag’s antenna can only capture a tiny portion of the energy of the reader. Much of it bounces away from the tag or is absorbed by material in the environment. The tag stores a small amount of energy and then changes the load on the antenna to send back a signal. Some of the energy powers the chip and some is reflected back to the reader.

The size of the antenna is a big factor in determining how much energy the tag can harvest from the reader and use for reflecting back a signal. A bigger antenna captures more energy. This improves the read range, since more energy means the tag can reflect back a stronger signal to the reader.

There are other factors that affect the amount of energy the tag can capture. One is the amount of water and other materials that absorb RF energy in the environment. If there is a lot of water or objects that contain water (such as fruit and meat), there will be less energy available for the tag to use. If there is a lot of metal in the environment, it could reflect energy away from the tag or block the amount of energy reaching the tag.

—Mark Roberti, Founder and Editor, RFID Journal

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