Engineering:Antenna factor

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:

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

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

$A{F}_{\mathrm{d}\mathrm{B}/\mathrm{m}}=E_{\mathrm{d}\mathrm{B}\mathrm{V}/\mathrm{m}}-V_{\mathrm{d}\mathrm{B}\mathrm{V}}$

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

$AF=\frac{\sqrt{377P_D}}{\sqrt{50P_D{A}_e}}=\frac{2.75}{\sqrt{A_e}}=\frac{9.73}{\lambda \sqrt{G}}$

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
• ${\mu }_0$ is the magnetic constant
• ${\epsilon }_0$ 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