DRAFT Recommended Practice for Measurements and ...

1/29/98 61 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
advantages of this approach are convenience, reliability, and simplicity. A potential
source of error when using the transfer standard to calibrate another probe is the
possible difference in the receiving patterns of the two probes. Also, in the near field of a
radiator, the size of the probe's sensor is important. Ideally, the standard and unknown
probes should be nominally identical and the calibration should be conducted in a field
relatively free of spatial variations due to multipath interactions between the probe, the
radiator, the anechoic chamber and other field generating components, and near-field
gradients. In TEM cells or parallel plate transmission systems, capacitive coupling
between the probe and the center plate and walls of the cell can create calibration errors.
The transfer standard probe should be stable, rugged, and not easily burned out; it
should have a large dynamic range, cover a broad frequency range, and possess an
isotropic response. Organizations that cannot justify the construction and maintenance
of a calibration facility could have a transfer standard (probe) calibrated by a reputable
laboratory. This secondary reference standard can then be used to calibrate a fieldgenerating
system at the user's facility. The system, in turn, can be used to calibrate
other probes. The transfer standard should be recalibrated at intervals appropriate to the
particular standard, based on experience with the stability of the calibration factor with
time. See Appendix A for additional calibration techniques.
4.5.6 Evaluation of Field Survey Instruments.
In order to determine electric and magnetic fields accurately, the characteristics of the
survey instruments used should be defined in some detail. A series of tests, when
properly performed, define worst-case uncertainties that can occur when that instrument
is used to make field strength measurements. The tests also indicate what operational
procedures can be used to minimize measurement errors. Details of tests for
microwave instruments (for frequencies greater than 0.9 GHz) can be found in [B57] and
details of electric and magnetic field probe performance evaluations at 27 MHz can be
found in [B98]. The minimum test parameters that should be observed are:
(1) Absolute Calibration -- Should be performed at field strengths that produce
indications that equal or exceed the instrument's mid-scale readout display.
(2) Instrument Linearity -- Measurements should be made at 25, 50, 75, and 100
percent of full scale, on each range of the instrument's readout device.
(3) Amplitude-modulation Response -- This test should be tailored to the desired
instrument usage conditions. For example, if the modulation of concern is from
unfiltered power supplies, such as in industrial microwave heaters, 60 or 120 Hz
modulation with a 50% duty factor would be appropriate. If the instrument is to be
used around radar equipment, duty factors as low as 0.001 should be used during
the evaluation process.
(4) Frequency Response -- The response of the instrument over the frequency band of
interest should be determined. The response should be relatively flat over the
design frequency range, ([R]1 to 3 dB).
(5) Out-of-Band Response -- The sensitivity of the instrument, probe, cable and
readout should be evaluated for exposure to fields at frequencies far from the
specified usable range of the instrument. This is important when instruments are
used in mixed-frequency fields.
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.