In
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 ...
, 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 t ...
''V'' (units: V or μV) induced across the terminals of an
antenna.
For an electric field antenna, the resulting antenna factor ''AF'' is:
:
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 Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
s, the above equation becomes
:
The voltage measured at the output terminals of an antenna is not the actual field intensity due to actual
antenna gain
In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes h ...
,
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 The impedance of free space, , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, , where is the electric field strength and is the magnetic field ...
.
For a 50 Ω load, knowing that P
D A
e = P
r = V
2/R and E
2=
P
D ~ 377P
D (E and V noted here are the
RMS values averaged over time), the antenna factor is developed as:
:
Where
* A
e = (λ
2G)/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
*
is the
magnetic constant
The vacuum magnetic permeability (variously ''vacuum permeability'', ''permeability of free space'', ''permeability of vacuum''), also known as the magnetic constant, is the magnetic permeability in a classical vacuum. It is a physical constant ...
*
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
Monopole may refer to:
* Magnetic monopole, or Dirac monopole, a hypothetical particle that may be loosely described as a magnet with only one pole
* Monopole (mathematics), a connection over a principal bundle G with a section (the Higgs field) o ...
and
dipoles
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system ...
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
In electromagnetics and antenna theory, the aperture of an antenna is defined as "A surface, near or on an antenna, on which it is convenient to make
assumptions regarding the field values for the purpose of computing fields at external points. Th ...
Notes
{{reflist
References
Interpreting Antenna Performance Parameters for EMC ApplicationsAn excellent page with summaries of all the relevant equations
Antenna Theory
Antennas (radio)