Power density
Human exposure to radio frequency energy
Power density (S)
Power per unit area normal to the direction of propagation, usually expressed in units of
watts per square metre (W/m2) or, for convenience, units such as milliwatts per square
centimetre (mW/cm2). For plane waves, power density, electric field strength (E) and
magnetic field strength (H) are related by the impedance of free space, 377 ohms. In
particular,
S = E2 = 377 x H2
_____
377
where E and H are expressed in units of V/m and A/m, respectively, and S in units of
W/m 2. Although many survey instruments indicate power density units, the actual
quantities measured are E or E2 or H or H2.
Power density (S)
Power per unit area normal to the direction of propagation, usually expressed in units of
watts per square metre (W/m2) or, for convenience, units such as milliwatts per square
centimetre (mW/cm2). For plane waves, power density, electric field strength (E) and
magnetic field strength (H) are related by the impedance of free space, 377 ohms. In
particular,
S = E2 = 377 x H2
_____
377
where E and H are expressed in units of V/m and A/m, respectively, and S in units of
W/m 2. Although many survey instruments indicate power density units, the actual
quantities measured are E or E2 or H or H2.
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