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:
![{\displaystyle AF={\frac {E}{V}}}](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
If all quantities are expressed logarithmically in decibels instead of SI units, the above equation becomes
![{\displaystyle AF_{\mathrm {dB/m} }=E_{\mathrm {\mathrm {dBV/m} } }-V_{\mathrm {dBV} }}](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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][clarification needed]
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 PD Ae = Pr = V2/R and E2=
PD ~ 377PD (E and V noted here are the RMS values averaged over time), the antenna factor is developed as:
![{\displaystyle AF={\frac {\sqrt {377P_{D}}}{\sqrt {50P_{D}A_{e}}}}={\frac {2.75}{\sqrt {A_{e}}}}={\frac {9.73}{\lambda {\sqrt {G}}}}}](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Where
- Ae = (λ2G)/4π : the antenna effective aperture
- PD is the power density in watts per unit area
- Pr 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 (units: meter) is used to measure the ratio between voltage and electric field.
See also
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
- Antenna Factor Calculator