eld as an active device does. Instead a passive device transfers data back to an active device through a process called load modulation. These concepts are described in the following sections. Near Field and Far Field Communication The space around an antenna can be divided into two regions based on the differing nature of the electromagnetic fields generated by the antenna. The boundary between the two regions is known as the radian sphere and has a radius of 2/2, where n is the wavelength of the propagated electromagnetic wave. Radio Communication Basics The primary magnetic field begins at the antenna and oscillations in this field induce an electric field in the surrounding space. This region, inside the radian sphere, is within the influence of the primary magnetic field and is called the near field of the antenna. The electromagnetic field equations in this region reflect energy storage in the magnetic field and are described by near field coupling volume theory. The region outside the radian sphere is called the far field of the antenna and here the fields separate from the antenna and propagate into space as an electromagnetic wave. The electromagnetic field equations here represent energy propagation rather than storage, and propagation is described in terms of the concepts covered in the Section "RF Signal Propagation and Reception, p. 106". For NFC operating at 13.56 MHz, a = 22 metres, SO that the radius of the radian sphere is 2/2n = 3.5 metres. In the near field region, the magnetic field strength is inversely proportional to the cube of the distance between the antennas while the power in the magnetic field, which is used to energise passive NFC, decreases as the inverse sixth power of the separation. This is equivalent to an attenuation of 60 dB for a ten-fold increase in distance. Inductive Coupling Near-field inductive coupling uses an oscillating magnetic field to transfer RF energy between devices. Each device includes a resonant circuit tuned to the RF carrier frequency, and a loosely coupled