User's Manual

Safety
18
E =
P
r
h
=
h
P
t
4pd
2
E = E
x
2
+E
y
2
+E
z
2
2.4 Exposure evaluation
Exposure assessment requires the identification of electric field values in places where
the presence of people may occur.
The field values must refer to the type of stay and the exposed subjects, according to the
provisions of current legislation.
The identification of the electric field values can be carried out in various ways, depen
ding on the need for precision and the fact that one intends to perform a prediction, or a
subsequent verification.
We intend to provide below elements that may be useful in the evaluation and measure-
ments of electromagnetic fields, with particular reference to sound and television broad-
casting systems.
2.4.1 Calculation in the free space
A first approximate evaluation can be made by placing the following hypotheses:
i) source located in far field
ii) absence of obstacles
The first hypothesis (i) must be previously verified by applying the equation:
1.
where “d” represents the distance between source and evaluation point, “D” the physical
dimension of the source and “l” the wavelength of the field.
Under these hypotheses, the electric field at distance “d” from the source is:
2.
where “Pt” is the power transmitted (ERP) in the direction of the evaluation point.
Introducing the polarization of the electric field, impressed by the particular type of
antenna used in transmission, the relation 2 must be applied to the three components Ex,
Ey, Ez of the field; the expression of the total electric field becomes:
3.
d w
D
l