3.4 Connecting the probe
Narda NBM-520 25
Recommendation: Try to avoid heating caused by direct sunlight during
measurements with thermocouple probes. Consider an
adequate settling time for stabilization of the probe in case
of temperature changes. A settling time of about 15 minutes
will ensure stabilized conditions. Extremely high steps of the
environmental temperature changes may require longer
settling times.
Strong low frequency fields
The result display when measuring high frequency
electromagnetic fields can be falsified by low frequency
fields. Wideband probes will detect signals even if the
frequency is well outside the specified measurement range
(out-of-band attenuation is 20 dB/decade). A probe
specified to measure from 100 kHz to 3 GHz would
therefore attenuate signals down to 100 Hz by at least
60 dB (= field strength / 1000). However, very high field
strengths of several thousand V/m can occur in the vicinity
of high tension lines. An RF wideband probe would
therefore register several V/m.
Recommendation: Thoroughly inspect every measurement location before any
measurement and make a note of any possible sources of
interference, such as high tension lines in the vicinity. Keep
a critical eye on any possible increase in the minimum
display value (noise floor) which may indicate interfering
factors. Increase the distance from the source of low
frequency interference or use a probe that has a higher
frequency cutoff point at the lower end of its range.
Further useful information can be found under FAQ at
www.narda-sts.com
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