The electric field of a human's (or animal's) body can be used to carry and transport informations. The range of possible applications is quite large and includes a host of automobile applications, such as intelligent squeeze protection, driver / passenger discrimination for in-car electronic devices such as air con, or decoupling of function and haptics. These properties generate new degrees of freedom for automotive interior designers.
1. Definition of Near Field
Electric fields are always generated close to electric charges, which can be distributed as a point charge or in a linear or two-dimensional fashion, for instance at planar electrodes. Applying a direct voltage makes the charges constant over time, resulting in a static field, as illustrated in Fig. 1 for an electrode arrangement in dipole form.
Replacing the direct voltage by an alternating voltage makes the charges vary over time and thus the electric fields. According to the theories of electrodynamics this leads at the same time to magnetic fields and thus to electromagnetic waves, which are radiated by electrodes like an antenna. If the charge varies sinusoidally with frequency f, the electromagnetic wave is characterized by wavelength λ = c/f, where c is the speed of light.
In the case of wave radiation it has to be distinguished the area very close to the antenna and the area farer away. A near field at a distance of d < λ/2π has a field distribution equal to a purely static field. In this case we speak of a quasi-static field. At a frequency of 1 MHz for example, this region will be at a distance of several hundred meters from the antenna.
A fundamental difference between the near field and the far field is that in the near field, because of a phase shift of 90° between the E field and H field, no energy is transported, in contrast to the far field. This makes it in principle possible to transmit information without power in the near field. Another difference from the far field is that the intensity of the near field decreases at l/d, meaning that it can be used to advantage only within a very limited space.