ABSTRACT
Electromagnetic (EM) energy at both low and high frequencies can be transmitted into biological materials through the use of antennas or applicators. Antennas launch the EM energy into the medium. They serve to couple the generating source of EM energy into the medium, which surrounds it. The spatial distribution of EM energy from an antenna is directional and varies with distance from the antenna. At distances sufficiently far from an antenna, so that local field distribution changes predictably and varies mostly with distance, the region is called a far field or radiation zone. In the near field or near zone close to the antenna, the EM energy distribution varies as a function of both angle and distance. Moreover, the behavior of electromagnetic fields (EMFs) and their coupling and interaction with biological systems are very different, depending on whether they are in the near or far zone. In fact, these differences constitute the major variances between radiofrequency (RF) and low frequency energy deposition into biological systems. As shown in subsequent sections, the induction of electric and magnetic fields, deposition of EM power, absorption of EM energy, and their penetration into tissue, all are functions of the source and its frequency or wavelength. In general, when considering the interaction of EMFs with biological systems, it is necessary to account for the frequency or wavelength and its relationship to the physical dimensions of the body.
