Antenna factor

In electromagnetics, the antenna factor (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.

For an electric field antenna, the resulting antenna factor ''AF'' is:

: $AF = \frac EV$

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

: $AF_ = E_\mathrm - V_$

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.

For a magnetic field antenna, the field 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 = V²/R and E²= $\sqrt$P_D ~ 377P_D (E and V noted here are the RMS values averaged over time), the antenna factor is developed as:

: $AF = \frac = \frac = \frac$

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

Notes

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References

Interpreting Antenna Performance Parameters for EMC Applications


An excellent page with summaries of all the relevant equations

Antenna Theory

Antennas (radio)