DRAFT Recommended Practice for Measurements and ...

1/29/98 83 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
measurement problem. This problem involves the worst-case uncertainties that occur
when measurements are made at a minimum distance from an active radiator or passive
reradiator. Table 5.3 provides information on this subject, in terms of ranges for which
the probe sizes and frequencies are valid for the particular analysis technique. Also, the
minimum separation distance is expressed with respect to wavelength and in terms of
the probe-antenna length (the maximum dimension of a single antenna or the array of
orthogonal antennas of the hazard probe).
From the data in Table 5.3, estimates can be made regarding the errors that will result if
the minimum probe to source or scattering object separation distances are not
exceeded. The largest error is due to the gradient of the radial field over the probe
antenna aligned with it. This error is an extreme worst case, and many situations will not
produce this large of an error, even at smaller separation distances. In general, for a
typical hazard probe with an array of 5 or 10 cm dipoles or loops, a separation of 3 to 5
probe lengths will ensure a maximum error of 3 dB at frequencies lower than 500 MHz,
while the maximum error at higher frequencies may be lower due to the fact that the
gradients in the radial components are less steep at distances greater than 0.15
wavelengths. Therefore, for most situations, when appropriate near-field survey
instruments are used, a minimum separation distance of 20 cm is reasonable.
Table 5.3 Worst-Case Errors Associated with the Minimum Probe to RF Source
Separation Distance
Separation Distance
Analysis Method Antenna Length Wavelength Frequency Range
MHz
TEM Cell 1