Whitepaper
EMC Antenna Parameters and their Relationships
This paper by John D. M. Osburn of EMC Test Systems provides a rigorous mathematical derivation of two fundamental EMC antenna parameters: the Antenna Factor (AF) and the Transmit Antenna Factor (TAF). The AF, expressed as the ratio of incident electric field strength to the voltage at the antenna terminals (AF = E/V_L), is the primary parameter used in radiated emissions testing to convert measured voltages into field strength values. Starting from basic antenna theory — effective aperture, the Friis transmission formula, and Ohm's Law — the paper derives the AF in a 50-ohm system as AF = 9.73 / (λ√G_r), and its dB equivalent: AF = 19.8 - 20·log(λ) - 20·log(G_r). The TAF is derived as the complementary parameter for radiated immunity testing, relating the input voltage of a transmitting antenna to the electric field it generates at a known distance. The paper demonstrates that while AF and TAF share the same units (m⁻¹), they are neither identical nor reciprocal — their relationship depends on antenna gain and the distance at which calibration is performed. A conversion formula between the two is provided, valid under ground-plane or semi-anechoic conditions. The paper's intent is explicitly educational: these parameters are widely used daily in EMC labs but their derivations are rarely understood. By grounding the formulas in first principles, the author provides engineers with the conceptual foundation to understand why the values are what they are and how to apply them correctly across both emissions and immunity test setups.