So, as we move away from either a time-varying electric source or a time-varying magnetic source, we start getting electromagnetic waves. How far away? That’s the question. From Figure 3 we see that at distances less than 1/6th the wavelength, we will be in the “near-field” region. The attenuation will then be significant, requiring us to come “closer” to the source of the field if we want to be able to deliver any significant amount of energy wirelessly. However, at distances greater than 1/6th the wavelength, we will be more in the “far-field” region, and we will then be able to send energy over far greater distances using wave effects (distances being relative to the wavelength of course).
Let’s do the “distance math” for MPT. Its selected frequency of 2.45 GHz corresponds to a wavelength of 300/f MHz = 300/2450 = 0.122 meters, or 12.2 cm. For this frequency, “far-field” would refer to distances greater than 1/2π of 12.2 cm, i.e. 19.4 mm. In other words, even at distances barely greater than about 2 cm, we will start to get a beam of propagating electromagnetic energy (microwaves). And in fact, that is exactly what we depend on to heat up