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:

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

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.^{[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:

Where

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.