The Validity Of The Near-Field Measurements - 3M ScanEM-C User Manual

creates an E-field, although small in magnitude. Similarly, an antenna or an
un-terminated wire still carries some current via parasitic capacitance to
the ground.
Electric and magnetic fields attenuate as distance from the source increases.
Each field begins to produce its complementary field. As a result of this,
the wave characteristics change with the distance from the source. At some
distance from the source, at what is known as the transition region, the
fields reach constant ratio that thereafter stays the same. The field before the
transition region is called the near-field, and the field beyond it is called the
far-field.

The Validity of the Near-Field Measurements

The far-field measurements performed at the final compliance test do
not help in locating the source of a problem, should one exist. Near-field
measurements are the only way to pinpoint sources of offensive radiation.
But how good and reliable are these tests and how does one interpret their
results? Since near-field readings are greatly dependent on the geometry of
the source and its properties, any attempt to provide correlation between
near-field and far-field measurements will not deliver usable results.
The only correlation is that, in general, the stronger the field near the
source, the stronger it will register in the far-field. In addition to the basic
correlation problem, the following issues are inherent with the precision and
repeatability of near-field measurements:
‰ Small movements of the near-field probe produce a much greater relative
change of distance from the source than the same movement in the far-
field. Readings of the field strength will be affected accordingly.
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