New computation method of electric field strength in close vicinity of half-wavelength dipole antennas

A simplified equation for calculating electric field strength, based on the formula of free space propagation loss, 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, IEEE Std. 1309, and IEC 61000-4-3 Amendment 1. 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 toward to the far field. The accuracy attained by the conventional equation to calculate the field strength 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 expressed by Padé approximation of the half-wavelength dipole antenna. Numerical results yielded by the conventional and proposed equations were compared with the experimental data. Substantial improvement in accuracy was achieved with the proposed equation.