Antenna factor

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In telecommunications, the antenna factor AF (unit: 1/m) is defined as the ratio of the incident electromagnetic field strength E (units: V/m or µV/m) to the voltage V (units: V or µV) on the line connection of an antenna:

$AF = \frac EV$

If all quantities are expressed logarithmically in decibels instead of SI units, the above equation becomes

$AF_{\mathrm{dBm}^{-1}} = E_\mathrm{\mathrm{dBV/m}} - V_{\mathrm{dBV}} = E_\mathrm{\mathrm{dB}\mu\mathrm{V/m}} - V_{\mathrm{dB}\mu\mathrm{V}}$

In a 50 Ω system, the antenna factor is related to the antenna gain G and the wavelength λ via:

$AF = \frac{9.73}{\lambda \sqrt{G} }$

The variable G means antenna gain (dimensionless) and it can be written in terms of effective aperture A to obtain the final expression considering the impedance of the free space (376.6Ω) and the load as 50Ω. The antenna effective aperture can be written as A = (λ²G)/4π.