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36 Common Diagnostic Techniques www.ets-lindgren.com
To review the physics of the situation: In a far-field that is more than about
one wavelength from the source, the ratio of the E-field and H-field components
to the propagating wave resolve themselves to the free space impedance of
377 ohms. In the far-field the E-field and H-field vectors will always have a ratio
of 377 ohms, but in the near-field that ratio radically changes. The ratio of E-field
to H-field, or field impedance, is determined in the near-field by the source
impedance.
As you probe close to the equipment you can switch between an E-field probe
and an H-field probe. By noting the rate of change of the field strength versus
distance from the source and the relative amplitude measured by the probes, the
relative field impedance may be determined.
Low-impedance sources or current-generated fields initially will have
predominately magnetic fields. The magnetic component of the field will
predominate in the near-field but will display a rapid fall-off as you move away
from the unit. This change may be observed through an H-field probe.
Low-impedance sources also will give a higher reading in the near-field on an
H-field probe than on an E-field probe. Alternately, high impedance sources will
display a rapid fall-off when observed through an E-field probe.
There are two ways to determine the nature and source impedance:
• Map the rate of fall-off of the E-field and H-field. One of these vectors
will fall off more rapidly that the other.
• Measure both vectors at the same point and by their ratio determine
the field impedance.
The equation E/H=Z is calculated and compared to the free space impedance of
377 ohms. Values higher than 377 ohms will indicate a predominance of the
electric field. Lower values will indicate that the magnetic field component is
predomination. From this you can plan your approach to the problem by tailoring
it to a differential model situation or a common mode situation. Field theory leads
us to expect a 1/R fall-off for a plane wave, where
R is the distance from the
source. In the near-field, the non-propagating, reactive field will drop off at
multiple powers of the inverse of the distance
1/RN . Typically, the reactive field
will fall off at something approaching 1/R3 . Therefore, we would predict these
measurements relative to measurements at distance equal to one.