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NFER® or "near field electromagnetic ranging" technology involves an often overlooked aspect of radio frequency ("RF") behavior whose existence generally has been regarded as a nuisance by RF engineers. The vast majority of conventional radios operate in the "far field." NFER® technology exploits peculiarities of near field behavior. The behavior exploited by NFER® technology manifests itself within about a half wavelength of an antenna. Hence, for NFER®technology, "near field" means within about a half wavelength.
The physics behind NFER® technology is easy to grasp. Radio signals are electromagnetic waves. A radio wave is a combination of an electric wave and a magnetic wave. When these waves leave the antenna, they are offset by ninety degrees. By the time the electronic and magnetic waves have traveled about a half wavelength from the transmitting antenna, the phase offset has decreased to almost zero. NFER® technology exploits this peculiar, yet predictable, phenomenon.  The figure (above) provides a graphic explanation of how the phase of the electric and magnetic waves behave in the near field around a small electric antenna. The green blue curve illustrates the change in phase of the magnetic wave as that wave travels away from the transmitting antenna. Similarly, the red curve illustrates the change in phase of the electric wave as the electric wave travels away from the transmitting antenna. The blue curve is what enables NFER® technology. The blue curve is the difference in the phase between the magnetic wave and the electric wave (the green curve minus the red curve).
Example:
The following is a simple illustration of how a NFER® system is able to measure distance: Let's begin with a receiver that is capable of detecting both electric and magnetic waves. Next add a transmitter that is an unknown distance away from the receiver. The transmitter periodically emits a signal at 1000 kHz. The receiver measures the difference in the phases of the electric and magnetic waves to be 30 degrees. Using the red curve in the figure, it is possible to extrapolate that the distance between the transmitter and receiver is 0.185 wavelengths. An RF wavelength at 1000 KHz is 983.8 feet. Multiplying 0.185 times 983.8 reveals that the distance between the transmitter and receiver is 182 feet.
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