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horsevad
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# Posted: 25 Nov 2016 11:32
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Basic Electromagnetic Field Theory - FAQ

Electromagnetic fields travels with the speed of light:
All electromagnetic radiation travels with the speed of light.

Light is, in fact, electromagnetic radiation

As the frequency of the electromagnetic radiation is the number of complete waves passing a certain point in one second the frequency and the wavelenght of the electromagnetic radiation is closely related:

= c/f
is the wavelength measured in meters
c is the speed of light (299792458 m/s)
f is the frequency measured in Hz



The electromagnetic field and its components:

The electromagnetic field is formed by the combination of its two components, the electric field and the magnetic field.

The electrical field component and the magnetic field component are both oriented perpendicular to each other - and combined they are perpendicular to the direction of propagation for the electromagnetic field.


Field intensity and power flux density:
The strenght of the electromagnetic field is measured as power flux density which is a measure of the energy deposited on a given area by the electromagnetic field.

The strenght of the field components is measured as field intensity. This can be measured both for the electrical field component and the magnetic field component.

All of these measurements are related by a simple formula:

Pd = E²/Z0 = Z0*H²
Pd: Power Flux Density, measured in W/m2
E: Field Intensity for the electric field, measured in V/m.
H: Magnetic field, measured in A/m.
Z0: Impedance of free air = 120 = 377

(It should be noted that this formula only is relevant for far-field conditions)


Near-field and Far-field:

In the region close to the transmitting antenna the coupling between the two field components are more incoherent than in the regions far from the antenna.

The two regions are known as near-field and far-field.

The reactive part of the near-field is calculated by another simple formula:

R < 0.62 * √(d³/l)

R is the distance to the antenna measured in meters
D is the greatest linear diameter of the antenna
l is the wavelenght


In the reactive part of the near-field conditions are rather strange. The two field components can exist rather independently, and absorbtion of RF energy (by a intentional or unintentional receiver) imparts a direct load on the transmitter, whereas a receiver in far-field has no effect on the load of the transmitter.

The radiative part of the near-field is calculated by this formula:

0.62 * √(d³/l) < R < 2D/l

R is the distance to the antenna measured in meters
D is the greatest linear diameter of the antenna
l is the wavelenght


In the radiative part of near-field the normally existing direct coupling between the electric field component and the magnetic field component is slowly established.

In most cases relating to shielding projects the radiative part of near-field can be omitted from the calculations.

To be certain that one are operating in far-field conditions (with the resulting direct coupling between electic field component and magnetic field component) all of the three conditions below has to be statisfied:

R > 2D²/l
R >> D
R >> l

R is the distance to the antenna measured in meters
D is the greatest linear diameter of the antenna
l is the wavelenght



Transmitted and received energy:

In the far-field region the relationship between transmitted and received energy can be calculated with great detail:

Pd = Pt / (4 * * d2)

Pd is the Power Flux Density, measured in W/m2
Pt is the Transmitter Output Power, measured in W
d is the Distance, measured in meters



An online calculation for all of the above calculations can be found at:
http://rf-emf.org/calculations.html



//Kim Horsevad

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