Table of Contents
Advertisement
field is electric, or E-field. A conductor with a substantial current causing an
H field still carries some small voltage that creates an E field, albeit small in
magnitude. Similarly, an antenna or an unterminated 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 Validity of the Near-Field Measurements
The far-field measurements performed at the final compliance test do not
help locate the source of a problem if one exists. 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 a correlation between near- 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 may 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.
‰ A near-field probe has finite dimensions. Slight repositioning of the
probe vs. the device under test (DUT) may put the near-field antenna in
a place where the geometry of the DUT alters field strength readings,
thus rendering repeatability of the measurements next to impossible.
27
Advertisement
Table of Contents
