Field strength in free space is a term in telecommunications. It is the field strength

Field strength

In physics, the field strength of a field is the magnitude of its vector value.In theoretical physics, field strength is another name for the curvature form...

 caused by a half wave dipole

Dipole

In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

 under ideal conditions. The actual field strength in terrestrial environments is calculated by empirical formulas based on this field strength.

Power density

Let N be the effective power radiated from an isotropic antenna and p be the power density at a distance d from this source


Power density is also defined in terms of electrical field strength;

Let E be the electrical field and R be the impedance of the free space


The following relation is obtained by equating the two,


or by rearranging the terms

Numerical values

Impedance of free space is

Since a half wave dipole is used, its gain over an isotropic antenna ( ) should also be taken into consideration,


In this equation SI units are used. But these units are impractical in actual antenna field computations. So

kW instead of W in power,

km instead of m in distance and

mV/m instead of V/m in field strength should be used.

Effectively this is equivalent to multiplying the expression on the right by . In this case,

See also

• Antennas
• Effective radiated power
  Effective radiated power
  In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...
  
  
• Electric field
  Electric field
  In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
  
  
• Field strength meter
  Field strength meter
  In telecommunications, field strength meter is a measuring device which measures the electric field caused by a transmitter.-The relation between the electric field and the transmitted power:...