Antenna factor explained

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 = (E)/(V)

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

AF _{dB/m} = E _{dBV/m} - V _{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]

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=

\sqrt{ (\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:AF = (\sqrt{ 377 P _{D})/() } = \frac =\frac

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

\mu _{0} is the magnetic constant\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.

See also

• Antenna effective length

References

• Interpreting Antenna Performance Parameters for EMC Applications
• Interpreting Antenna Performance Parameters for EMC Applications An excellent page with summaries of all the relevant equations
• Antenna Theory
• Antenna Factor Calculator

Notes and References

1. Book: Electronic Warfare and Radar Systems - Engineering Handbook. 2013. US Naval Air Warfare Center Weapons Division. 192. 4th.