DRAFT Recommended Practice for Measurements and ...

1/29/98 78 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
to eliminate this error) at each fixed point of interest within the spatial volume being
surveyed. The minimum and maximum values should be recorded and an average value
computed. This can be done manually in near-field situations, e.g., microwave oven
surveys, without introducing error, if the surveyor’s hand is in an area of minimal field
strength with respect to the field at the sensor (probe tip). In situations where uniform
illumination exists over the entire probe body (tip and handle), larger errors are usually
introduced by the presence of the operator’s hand, while performing this rotation, than are
introduced by probe ellipticity. In this case, if precise data are required, the probe should
be supported by a dielectric support and error limits associated with the probe ellipticity
should be assigned to the measurement, rather than attempting to manually rotate the
probe.
5.3.6 Interaction of RF Hazard Probes with Nearby Passive Scattering Objects
(Reradiators) and Active Radiators.
When measurements are made with a hazard probe placed close to conducting or highdielectric-constant
objects, (scatterers or ‘’passive reradiators’’) large errors may result.
Two situations are addressed in this section. One situation occurs when a hazard probe
with an ‘’electrically large’’ antenna (larger than about 0.25 wavelength) is placed close to
field-perturbing objects such as a person’s body, or large conducting objects, e.g., a
metal pole or metal shed. A second error-producing situation occurs when
measurements are made with the probe antennas less than a few probe ‘’antennalengths’’
or ‘’probe lengths’’ from an active RF radiator such as the monopole antenna of
a mobile radio transmitter or a leaking microwave oven. (The term probe/antenna-length
is discussed in 5.3.6.4.)
Inaccurate performance of a hazard probe located near either passive reradiators or
active radiators is due to several factors including the following:
(1) Reflections from a reradiating object that produces standing-waves (or
interference patterns in the EM-fields) that extend a distance of several
wavelengths from the scatterer. When a probe is not tightly coupled (via the
reactive near-fields of the reradiator), the techniques of 5.3.2 can be used to
minimize measurement errors. These techniques remove the effects of
standing-waves through the use of spatial averaging. When the probe is tightly
coupled to the reradiator, the data in 5.3.6.1 can be used;
(2) A perturbing object (whether it is an active radiator or a passive reradiator) ‘’loads’’
or distorts the measurement characteristics of the probe’s antenna/detector
combination [B106]. This occurs when the antenna is large compared with the
wavelength of the RF energy being measured (5.3.6.1 addresses this situation);
(3) An electrically large probe in the reactive near-field of an active radiator alters the
fields being radiated by the source and spatially averages the nonuniform near
fields being measured. This averaging occurs over the effective aperture area of
the probe antenna, i.e., dipole length or loop diameter.
5.3.6.1 The Effects on Measurement Accuracy of the Separation Distance
Between Survey Probes and Nearby Passive Radiators. An analysis can be
performed to determine the degree of probe interaction (or coupling) with nearby objects,
such as passive reradiators or scatterers including exposed personnel. When a probe is
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.