Electronic – Why does low frequency RFID have a short read range

rfid

If I want to get the wavelength of a radio wave, I must divide the speed of light by the frequency? So having a 125 kHz RFID means an estimated 2km wavelength?

If its wavelength is 2km long, why does this low frequency RFID have a short read range?

Best Answer

Because RFID doesn't work based on wave propagation. It's thus not actually a radio system (despite working at "RF"=Radio Frequency).

Think of an RFID tag more as the secondary side of an air-core transformer, where information is transmitted by the tag changing the amount of power it draws from the primary side of the transformer, or by charging a energy storage (capacitor), and then exchanging the role of secondary and primary side of the transformer.

Because we're not talking about a wave propagating away from antenna, but about a coil coupling into a magnetic field, the decay in power is even worse than the distance┬▓ for free space loss, and after a couple of cm, practically no effect of the tag on the reader can be made.