antenna factor

{{Antennas|characteristics}}

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:

: $AF\; =\; \backslash frac\{E\}\{V\}$

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

: $$

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.{{cite book|title=Electronic Warfare and Radar Systems - Engineering Handbook|date=2013|publisher=US Naval Air Warfare Center Weapons Division|page=192|edition=4th}}{{clarify|date=May 2020}}

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= $\backslash sqrt\{\backslash frac\{\backslash mu\_0\}\{\backslash varepsilon\_0\}\}$P_D ~ 377P_D (E and V noted here are the RMS values averaged over time), the antenna factor is developed as:

: $$

AF =

\frac{\sqrt{377 P_D}}{\sqrt{50 P_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
• $\backslash mu\_0$ is the magnetic constant
• $\backslash varepsilon\_0$ 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.