The near field of dipole and helical antennas

This paper describes some recent E-field measurements performed in the close vicinity of antennas commonly used in conjunction with portable communication equipment. The study was motivated by the criteria of the proposed ANSI Radio Frequency Protection Guides, which are based on the measurement of mean squared electric (|E|²) field strength. In the far field of radiators the criteria are sound because the value of |E|²is directly related to power flow and so to a potential hazard. In the close proximity of sources the safety criteria are no longer valid because the value of |E|²is not directly related to power flow, but, rather, to the level of electric energy stored around the radiator. This reactive energy, because of its high impedance, is not all available for penetration and deposition in human tissue. So, in the near field of radiators a Safety Standard based only on the value of |E|²can be exceedingly restrictive, as shown by the measurements presented in the paper.