Electric field strength estimation in boundary region between near and far fields

A simple equation for calculating electric field strength has been widely used for electromagnetic applications. This equation has also been applied to the calibration of electric field probes, which has been specified in various standards, such as ISO/TR 10305-1, ISO/TR 10305-2, and IEEE Std. 1309. It is, however, valid only in the far field region. It is well known that the far field distance is given by λ/2π, where λ is the wavelength. This formula, however, gives a distance where the magnitudes of three terms - electrostatic, induction, and radiation fields - are equal. The lowest test frequency specified in ISO 11452-2 is 80 MHz (λ = 3.75 m) and the test distance is 1 m, which slightly exceeds λ/2π. The accuracy attained by the conventional equation deteriorates at this marginal distance. In this report, a new equation for calculating the electric field strength is proposed, embracing the effects of variations in space impedance. Numerical results yielded by the conventional and proposed equations were compared with the experimental data measured in a radio anechoic chamber. Substantial improvement in accuracy was achieved with the proposed equation.