DRAFT Recommended Practice for Measurements and ...

1/29/98 70 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
5.1.3 Determination of Type of Instrument Required.
Although many instruments designed for potentially hazardous fields are broadband in
nature, there are none that cover the entire frequency range of interest and all
parameters of potential interest. Some general considerations in the selection of an
instrument include the following:
(1) Frequency–Frequencies should be determined in advance so that proper
instruments and measurement methods can be selected. The presence of
several frequencies dictates the use of a broadband device with true RMS
response. (If the operating frequency cannot be readily determined, e.g., from the
equipment label or user information, the use of a spectrum analyzer or frequency
counter should be used to determine the frequency.)
(2) Response Time–It is usually desirable to begin a survey using a hazard
instrument with a response time (integrator time-constant) of one second or less
(the ‘’fast’’ setting on some commercial instruments). This enables a coarse
measurement or the detection of pulse-modulated or intermittent fields, e.g.,
those created by a scanning radar beam. A ‘’peak hold’’ feature on some survey
instruments can provide an accurate indication of moderately fast bursts of RF
energy (duration greater than several milliseconds). Once a high- field-strength
zone is located, a slower time constant (3 s or more) should be used to obtain the
time averaged value of the field strength. If the hazard meter still indicates that an
intermittent field exists, other means of recording and averaging should be used.
Data-logging systems are available specifically for use with RF hazard meters.
(3) Peak Power Limitations–A knowledge of the peak power limitations of the
instrument is necessary to protect probes from damage in some low-duty-factor
pulsed fields, such as those associated with radars.
(4) Polarization–A knowledge of the polarization of the fields enables a surveyor to
use a non-isotropic probe for hazard surveys. In the absence of such knowledge,
an isotropic probe is highly desirable both for ensuring accuracy and ease of
performance of the survey in a reasonable period of time.
(5) Dynamic Range–The maximum anticipated field strengths should be estimated
before measuring emissions from an RF source. A survey instrument capable of
withstanding continuous exposure to field strengths (E 2 or H 2 ) of at least ten
times the predetermined value should be chosen in order to avoid destruction of
the probe sensing elements or the high-resistance leads connected to those
elements. In addition, adequate sensitivity is required to ensure a reasonable
signal-to-noise ratio when the minimum expected field strengths are being
measured.
(6) Near Field Measurement Capabilities–If a leakage situation exists, or if the fields
in close proximity to a source are to be measured, care should be taken to select
a suitable instrument (see 5.3.6).
5.2 Safety Precautions
Personnel should take appropriate safety precautions while conducting surveys, and the
degree of care exercised should increase in proportion to the power levels associated
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.