DRAFT Recommended Practice for Measurements and ...

1/29/98 77 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
frequencies. Therefore, fiber optically coupled instruments are preferable at frequencies
below 300 MHz.
RFI also becomes a significant problem at lower frequencies, since it is more difficult to
shield readout electronics and cables at frequencies below about 500 MHz. An
operational test of the system can be made by totally shielding the sensor tip of an
electric-field probe with metal foil. This procedure allows the determination of the
existence of RFI or capacitive coupling between the cable and readout and nearby
radiating objects during the measurement. Similarly, reorienting the probe leads in the
field, with the probe sensing element fixed, should not alter the reading of the meter
appreciably (±6 dB) if RFI is not problematic. It is not uncommon to find large errors in
readings from older designs of isotropic instruments when they are used to measure
electric and magnetic fields below about 1000 MHz, especially when the readout device
and probe cable are exposed to a field of the same magnitude as that which illuminates
the probe [B16, B109]. This often is due to faulty RFI gaskets in the readout device or
poor system RFI design. Fiber-optic cables, high-resistance cables, or double-shielded
coaxial and signal cables can be used by designers of hazard survey instruments to
connect the probe to the readout device, in order to minimize the effects of RFI. Also, a
self-contained (active antenna) probe with integrated antenna, sensor, and readout in the
same housing can minimize the problem of RFI otherwise induced in cables or readout
[B19].
Instrument linearity as a function of the peak-to-average power ratio of amplitudemodulated
fields should be considered when performing measurements in an unknown
situation. Pulse duty factor or amplitude-modulation waveform can be quantified through
the use of a conventional antenna with a coaxial output, e.g., log periodic, dipole, and a
coaxial attenuator and diode detector. The detector output can be fed via coaxial cable
into an oscilloscope of appropriate bandwidth. The duty factor of pulsed sources and the
waveform of the amplitude modulation can then be observed, and the ratio of the peak to
average power or energy density can be ascertained. It is not necessary to maintain
square-law operation of the detector for the evaluation of 100% rectangular amplitudemodulated
waveforms. For other pulse modulated waveforms, one can only be assured
that both the peak and average field strengths are correct if the detector is operated in the
square law region [B118]. Also, with square law detection, the ratio of the peak to
average equivalent power density, (E 2 or H 2 ) can be quantified.
Measuring the effects of a slowly time-varying field generated by a rotating radar antenna,
or a microwave oven with a mode stirrer, can be accommodated by use of the
appropriate time constant of the readout instrument. Some instruments provide ‘’fast’’
and ‘’slow’’ time constants. Any observable periodic variation of the field strength, as
indicated by the instrument in the fast mode of operation, should be considered as lowfrequency
field modulation and the slow time constant should be used. Care should be
taken to avoid mistaking slow variations in reflected signals (due to objects moving in the
field) for a true time-varying source modulation. This implies that any movement of the
operator, probe, or surrounding objects, should be eliminated during the measurement
procedure where time-varying fields are involved.
An ellipticity (non-isotropic reception condition) in the receiving pattern of multi-axis or
isotropic probes is always present to a certain degree, (typically 2% to 20%). Only if the
ellipticity of an instrument is accounted for, can highly accurate data be taken. In fields
where components do not exist along the direction of the probe handle, a rotation of the
probe about the handle axis should be performed during a survey (if the surveyor desires
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.