Antenna factor
In electromagnetics, the antenna factor is defined as the ratio of the electric field strength to the voltage V (units: V or μV) induced across the terminals of an antenna. 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]
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Antennas 

Common types 
Safety and regulation

Radiation sources / regions 
For an electric field antenna, the field strength is in units of V/m or μV/m and the resulting antenna factor AF is in units of 1/m:
If all quantities are expressed logarithmically in decibels instead of SI units, the above equation becomes
For a magnetic field antenna, the field strength is in units of A/m and the resulting antenna factor is in units of A/(Vm). 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} = V^{2}/R and E^{2}= P_{D} ~ 377P_{D} (E and V noted here are the RMS values averaged over time), the antenna factor is developed as:
Where
 A_{e} = (λ^{2}G)/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
 is the magnetic constant
 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 is used to measure the ratio between E and V.
Notes
 Electronic Warfare and Radar Systems  Engineering Handbook (4th ed.). US Naval Air Warfare Center Weapons Division. 2013. p. 192.
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