How is this accomplished?
One way to do this is to create a magnetic field and send energy through a tag via inductive coupling. I’m not an engineer, but here’s how I understand it:
Passive low-frequency (LF) and high-frequency (HF) tags use this method. The reader antenna has a coil, and the tag antenna also has a coil. Sending energy through the coiled wire in the reader antenna enables the two coils to form an electromagnetic field. Changes in voltage in the reader antenna coil induce voltage in the coil of the tag antenna, thereby powering up the tag.
Passive ultrahigh-frequency (UHF) tags do not have a coil. They operate in the far field, meaning the tag is usually interrogated from more than one wavelength away. The reader antenna sends out electromagnetic waves, which then reach the UHF tag’s antenna, causing an electrical current to flow through that antenna. This charge can be boosted using a charge pump on the chip, and it is then used to power the tag chip. This method of receiving energy from the UHF reader is sometimes called energy harvesting.
If there are any RF or electrical engineers who can explain these methods of powering up the tags more precisely, please feel free to post your comments below.
—Mark Roberti, Founder and Editor, RFID Journal