electromagnetics In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

, the antenna factor (units: m⁻¹, reciprocal meter) is defined as the

ratio In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...

of the electric field ''E'' (units: V/m or μV/m) to the

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

''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 unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...

s, 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 Vacuum permittivity, commonly denoted (pronounced "epsilon nought" or "epsilon zero"), is the value of the absolute dielectric permittivity of classical vacuum. It may also be referred to as the permittivity of free space, the electric const ...

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

{{reflist

References

Interpreting Antenna Performance Parameters for EMC Applications

An excellent page with summaries of all the relevant equations
Antenna Theory
Antennas (radio)

See also

Notes

References