1-8 General Information
1.7 Anisotropy
1) The IEEE 1309 [3] defined the anisotropy (A) as the maximum deviation
from the geometric mean of the maximum response and minimum response
when the probe is rotated around the ortho-axis (e.g. "virtual handle") as
shown in the example in figure below.
⋅
⋅=
minmax
max
10
log20
SS
S
A
dB equation (1)
where S is the measured amplitude in field strength units.
2) The IEC 61786 [2] "Measurement of low-frequency magnetic and electric
fields with regards to exposure of human beings - special requirements for
instruments and guidance for measurements" does not define the anisotropy
and suggests, for three-axis probes, the calibration of each axis when each
element is aligned with the incident field.
The calibration should also be checked for a specific orientation where
approximately there is the same indication for each one of the three axis
(XYZ measurement).
Following this suggestion some laboratories find the minimum and the
maximum values of the X, Y, Z and XYZ measure and calculate the
anisotropy using equation (1).
3) We calculated the anisotropy with equation (1) but with 3D mesh
measurements to cover 4π steradians,that is to say in a much more severe
condition than the orthotropic one.
Fig. 1-2 3D mesh measurements of magnetic probe
Each x marker in the fig.1 indicates the coordinates surface of the spherical
coordinates (r,
θ
,
ϕ
).
The anisotropy is evaluated with 30 degrees steps for
θ
and
ϕ
, and r
shows the calibration factor at each position.
The typical value of anisotropy is 1,4% (0.12 dB) for magnetic probe and
6,5% (0,54 dB) for electric probe.
The anisotropy calculated in this way is worse respect to other cases above
described and it is more representative of the reality.
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