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Magnetic field strength (H):
A field vector that is equal to the magnetic flux density divided
by the permeability of the medium. Magnetic field strength is
expressed in units of amperes per meter (A/m).
The meter uses of magnetic field strength for measurements
only in the far-field area of the source.
Power density (S):
Power per unit area normal to the direction of propagation,
usually expressed in units of watts per square meter (W/m
2
) or,
for convenience, units such as milliwatts per square
centimeter (mW/cm
2
).
The characteristic of electromagnetic fields:
Electromagnetic fields propagate as waves and travel at the
speed of light (c). The wavelength is proportional to the
frequency.
λ
(wavelength) =
f (frequency)
If the distance to the field source is less than three
wavelength, then we are usually in the near-field. If the
distance is more than three wave length, the far-field
conditions usually hold.
In the near-field, the ratio of electric field strength (E) and
magnetic field strength (H) is not constant, so we have to
measure each separately.
In the far-field, however, it is enough to just measure one field
quantity, then the other could be computed accordingly.
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