109 Maximum Exposure Levels to Radiofrequency Fields Radiation Protection Standard – 3 kHz to 300 GHzRadiationProtectionSeriesNo. 3Far-field measurementsIn the far-field the RF power flux density (S), the electric field strength (E), andmagnetic field strength (H), are interrelated by the following expressions:S = E × HE = √(Z × S) = √(377 S), i.e. E2= 377 SH = √(S/Z) = √(S /377), i.e. H2= S /377E = Z × HwhereE = electric field strength, in volts per metreH = magnetic field strength, in amperes per metreS = electromagnetic power flux density, in watts per square metreZ = characteristic impedance of free space, in ohms ≈ 377 Ω.In the far-field of an RF source, relevant E, H and S limits will not be exceeded forfrequencies above 10 MHz if any one of the RF power flux density (S), the electricfield strength (E), or the magnetic field strength (H) can be shown to be less thanthe relevant limits specified in Tables 6, 7 and 8 in Section 2 of the Standard. Atfrequencies below 10 MHz in the far-field, measurements or evaluations of theE field are sufficient to determine compliance with E and H reference levels.Near-field measurementsFor a RF source operating at a frequency with a wavelength in air of λ m, thedistance from the RF source to the reactive field boundary is λ/2π . In the reactivenear-field, the field impedance, Z, will not necessarily be equal to 377 ohms.Therefore both electric and magnetic field strengths should be measured unlessthe impedance of the field is known.However, in the radiating near-field it can be shown that the wave impedance iswithin 10% of the free space impedance at distances greater than about 0.5 λ fromthe antenna so that E, H or S may be measured to determine compliance with thereference levels. However, this approach should be cautiously adopted whenmaking measurements near the reactive field boundary.Many instruments which purport to measure RF power flux density actuallymeasure the square of the electric or magnetic field strengths, but have a metercalibrated to indicate equivalent plane wave power flux density. The quantitysampled shall be deemed to be less than the reference level if such an instrumentregisters a value less than the equivalent level of RF power flux density for a planewave. The expressions given in this Annex may be used to determine theequivalent level. There are instruments currently available that are able tomeasure H fields of frequencies of up to 300 MHz.