Global Positioning System Reference
In-Depth Information
6.1.4
Far-Field and Near-Field Radio Systems
Very close to the antennae there is a complex set of electric and magnetic fields,
the field strengths of which decay very rapidly (within a very few wavelengths) to
the point where they can be ignored. This region is known as the “near field” (in
contrast to the “far field” of normal radio propagation). The near field has been
recently exploited by RFID technologies and for appliance to appliance near-field
communications (NFC). Both are very useful proximity positioning techniques. In
RFID systems, the RFID tag is used within the near field that is used first to carry
energy to power the device, and second, to carry messages (especially identity
messages) by being modulated with information. The best way to think of the near
field is not in radio propagation terms but as either a magnetic transformer or an
electrostatic capacitor both with large air gaps.
Figure 6.6
Near-field communications by magnetic induction.
If the magnetic approach is adopted, the tag transmitter uses loop (or coil)
antennae (as the primary windings of the “transformer”) with secondary coils in
the passive tags. Figure 6.6 shows how a magnetically coupled RFID tag system
(at the bottom of the diagram) works on the same principle as an air gap
transformer shown at the top. Both have closed conducting loops that carry
alternating electric currents. The magnetic fields induced by the power transmitter
are shown as dashed lines and these carry the energy through the air. A classic
transformer is made from two adjacent coils but in the case of the RFID tag the
