# Why do lower frequency RFID readers have a smaller range for reading tags?

Low frequency RFID readers only go a few inches. UHF can read much farther (on the scale of meters). As far as I know, lower frequency waves of the same energy actually radiate better/farther.

Is there a reason that the distance scales up with frequency? Is it an arbitrary choice that lower frequency readers only work with low power?

• Antenna length. Aug 3, 2016 at 7:09
• Please explain a bit more. I don't see how smaller antennas means more power or a longer read range. Aug 3, 2016 at 7:53
• – abc
Mar 24, 2017 at 17:35

A UHF emission is likely to be received as a true radiated electromagnetic wave. Close up to the transmitting antenna it's called "near field" and the individual E and H fields aren't really combined/lined-up until beyond about one wavelength. A wavelength of 300 MHz is 1 metre so if you are metres away you will be receiving a true EM wave.

True EM waves reduce in received power as distance squares. As individual fields, E and H reduce linearly with distance - remember this....

For a low(er) frequency RFID reader such as one operating at 14 MHz, one wavelength is 21 metres hence you are never going to be receiving a fully-fledged EM wave. Instead you will receive just the magnetic field of the transmission (because it uses a coil and there is very little E-field content transmitted).

Beyond a distance that is approximately the diameter of the coil$^1$, the magnetic field reduces with distance cubed.

Hence scenario 1 (UHF) the magnetic field reduces linearly with distance and in scenario 2 (14 MHz e.g.) the magnetic field starts to reduce as distance cubed.

$^1$ - here's the background for the magnetic field reducing with distance cubed: - If z gets much bigger than R, the "R" within the parenthesis in the denominator can be ignored leaving just $z^3$

• I thought the "low frequency" RFID was the 125kHz range, although that seems to be on the way out? Aug 3, 2016 at 10:24
• I've just chosen an RFID frequency (14 MHz) that is substantially lower than UHF. Clearly for 125 kHz the same answer applies and even more so because of the wavelength of 125 kHz being 2.4 km. I've changed "low" to "low(er)" in my answer. Aug 3, 2016 at 10:30