In electromagnetics, the antenna factor (AF, units: m⁻¹, reciprocal meter) is defined as the ratio of the electric field E (units: V/m or μV/m) to the voltage V (units: V or μV) induced across the terminals of an antenna:

${\displaystyle AF={\frac {E}{V}}}$

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

${\displaystyle AF_{\mathrm {dB/m} }=E_{\mathrm {\mathrm {dBV/m} } }-V_{\mathrm {dBV} }}$

The voltage measured at the output terminals of an antenna is not the actual field intensity due to actual antenna gain, aperture characteristics, and loading effects.^{[1][clarification needed]}

For a magnetic field, with units of A/m, the corresponding antenna factor is in units of A/(V⋅m). For the relationship between the electric and magnetic fields, see the impedance of free space.

For a 50 Ω load, knowing that P_D A_e = P_r = V²/R and E²= ${\displaystyle {\sqrt {\frac {\mu _{0}}{\varepsilon _{0}}}}}$P_D ~ 377P_D (E and V noted here are the RMS values averaged over time), the antenna factor is developed as:

${\displaystyle AF={\frac {\sqrt {377P_{D}}}{\sqrt {50P_{D}A_{e}}}}={\frac {2.75}{\sqrt {A_{e}}}}={\frac {9.73}{\lambda {\sqrt {G}}}}}$

Where

• A_e = (λ²G)/4π : the antenna effective aperture
• P_D is the power density in watts per unit area
• P_r is the power delivered into the load resistance presented by the receiver (normally 50 ohms)
• G: the antenna gain
• ${\displaystyle \mu _{0}}$ is the magnetic constant
• ${\displaystyle \varepsilon _{0}}$ is the electric constant

For antennas which are not defined by a physical area, such as monopoles and dipoles consisting of thin rod conductors, the effective length (units: meter) is used to measure the ratio between voltage and electric field.

• Antenna effective length