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 {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} }

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 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= ${\sqrt {\frac {\mu _{0}}{\varepsilon _{0}}}}$ P_D ~ 377P_D (E and V noted here are the RMS values averaged over time), the antenna factor is developed as:

AF={\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
$\varepsilon _{0}$ 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.

Notes

Electronic Warfare and Radar Systems - Engineering Handbook (4th ed.). US Naval Air Warfare Center Weapons Division. 2013. p. 192.

gollark: I'm not sure of the exact details, but using lots of tools and fewer people.

gollark: Aliens? Magic cranes?

gollark: So how *did* they build them if not huge amounts of slave labour?

gollark: I'm not sure how else they would have been built, with the technology of the time.

gollark: Well, yes, lots of slaves, sure.

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

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[1] Electronic Warfare and Radar Systems - Engineering Handbook (4th ed.). US Naval Air Warfare Center Weapons Division. 2013. p. 192.