# Why Can’t RFID Systems Use 2.4 GHz Inductive Coupling to Communicate?

Published: December 9, 2010

High-frequency (HF) tags and readers use inductive coupling to communicate. Ultrahigh-frequency (UHF) tags can also employ inductive coupling in the near field. Why can’t 2.4 GHz inductive coupling be utilized?

—Name withheld

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That’s an interesting question, and the answer is beyond my knowledge of RF engineering, so I reached out to Neeraj Sood, a researcher at the University of Toronto and a member of McMaster University‘s RFID Applications Lab. Here is an edited version of Neeraj’s response:

“I assume that we are talking about a passive system using inductive coupling, and not an active system. Inductive coupling works in the near field of the reader antenna. The electrical current flowing through the reader antenna produces a magnetic field. When the tag’s antenna is placed in this magnetic field, it produces a current in the tag antenna that allows tag-reader communication to take place.

“The strength of the magnetic field is proportional to 1/r^3, where r is the radial distance from the reader antenna. This means that as we increase the radial distance from the near-field reader antenna, the field’s strength decays as the cube of this distance—a very rapid rate of decay [see the graph below]. Therefore, to use inductive coupling, the tag should be placed less than 0.16 wavelengths away from the reader.

“For passive HF RFID systems, the wavelength is around 22 meters [72 feet], which allows for the tag to be placed a reasonable distance from the reader antenna. However, at 2.4 GHz, the wavelength is quite small—around 12.5 centimeters [4.9 inches]. That means the tag would need to be placed less that 2 centimeters [0.8 inch] from the reader antenna, which would not be useful for most data-collection needs.”

This graph, provided by Neeraj Sood, illustrates how quickly a signal decays as a function of distance, and shows the difference between linear (1/r), quadratic (1/r^2) and cubic (1/r^3) rates of decay. The magnetic field in the near field has a cubic rate of decay with distance.