Antenna factor

In telecommunications, the antenna factor is defined as the ratio of the incident electromagnetic field strength to the voltage V (units: V or µV) on the line connection of an antenna.

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:

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

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

$AF_{\mathrm {dBm} ^{-1}}=E_{\mathrm {\mathrm {dBV/m} } }-V_{\mathrm {dBV} }=E_{\mathrm {\mathrm {dB} \mu \mathrm {V/m} } }-V_{\mathrm {dB} \mu \mathrm {V} }$

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.

In a 50 Ω system, the antenna factor is related to the antenna gain G and the wavelength λ via:

$AF={\frac {9.73}{\lambda {\sqrt {G}}}}$

The variable G means antenna gain (dimensionless) and it can be written in terms of effective aperture A to obtain the final expression considering the impedance of free space (376.6Ω) and the load as 50Ω. The antenna effective aperture can be written as A = (λ²G)/4π.

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
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