RadioBanter - View Single Post - Measuring the fieldstrenght nearby is not reliable.

J. Mc Laughlin · January 11th 08, 01:30 AM posted to rec.radio.amateur.antenna

Dear Jim: Please see below....

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J. McLaughlin; Michigan, USA
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On Jan 9, 5:35 pm, "J. Mc Laughlin" wrote:
Dear Jim Lux W6RMK:

I was not able to examine the probes that I saw in use at NBS in what was
probably the mid-70s.

The probes that I did see and use comprised three, orthogonal, very short
doublets with attached means for rectifying. The resulting DC was
conveyed
away through a resistive, plastic transmission-line crafted to be almost
transparent to RF. These probes were used to estimate the size of strong
EM
signals in the vicinity of equipment so as to be able to put better
numbers
on EMC capabilities.

Yes.. The older works (50s and 60s) used carbon loaded string or
thread, but newer stuff uses conductive plastic. If the sheet
resistance of the material is 377 ohms/square then it's sort of like
lossy freespace.

Exactly. With a high input resistance voltmeter at the far end, the net
resistance of the resistive line has little (and predictable) effect. What
I saw was not loaded string - it was a conductive plastic.

Those three axis probes work fine for measuring the magnitude and
direction of the field, but they can't measure the phase, and to do
the nearfield to far field conversion you also need the phase.

Exactly. I did not intend to leave a suggestion that the probes used to
estimate the size of strong EM fields could provide phase information. The
utility of the probes that I saw and used resided in their insensitivity to
the local polarization and their useful sensitivity to the magnitude of E.
The testing of such polarization sensitivity and the calibration of
magnitude of E is straight forward when one has a TEM cell, a power
amplifier, a calibrated high-power attenuator, and a trustworthy power
meter. Still, the results of EMC testing will have uncertainty.

Once upon a time, a major effort was made to estimate the strongest E to be
found in the environment. Testing took place on SW broadcast sites and a
host of other places. I ran a mobile test on a drive from the East Michigan
area to Chicago. My testing found CB signals and VOR signals to be the
strongest. The former because our van was passed by a car transmitting at
27 MHz and the latter because the (about) 200 watt VOR was at ground level,
near the highway, and its antenna was surrounded by a very large
ground-plane. Others found a particularly strong signal from an FM
broadcast station with a mis-designed antenna having a major lobe straight
down! Today, many more RF hazards exist. Thanks for your expansion
of what I wrote. 73, Mac N8TT