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.

While one can make a reasonable estimate of field strength inside of a TEM
cell (a big piece of transmission line) from external measurements, it is
desirable to be able to estimate FS at spots inside of the cell. It is
important to note that even with care, significant uncertainties remain in
the estimates. All measurement comprise a minimum of two numbers: an
estimate of the value and an estimate of the uncertainty of that estimate.
You, and most on the group, know this, but it needs to be repeated.

Safety of life was involved with the testing then done. With the profusion
of transmitters in close proximity of safety equipment today, it is a wonder
that more lives are not lost with inappropriate actuations just within
present cars. A reminder that the need for savvy RF engineers will not
diminish, and sending that work off-shore to a low-bidder is dangerous and
probably criminally negligent. The major US car makers, to pick an
industry, expend a large, expensive effort to see that their cars are safe
using resident engineers.

Warm regards, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:
"Jim Lux" wrote in message
...
:
J. Mc Laughlin wrote:

...
To use this technique at, say, 10 MHz with a yagi would be improbable,
but not impossible.

73, Mac N8TT






The challenge is that one needs a probe that measures 3 axis E and H.
There are some clever designs out there based on a sphere with 6 half
loops sticking out. They've been used to make near field measurements of
broadcast stations, among other things. See, for instance, Gassman and
Furrer, 1993. Silva, et al., published an interesting fiber optic probe
scheme in 97. Driver and Kanda published a optically linked sensor for
making Poynting vector measurements in the near field in 1988 (IEEE Trans
EMC). In the microwave area, the probe is usually an open ended section of
waveguide.

There are some clever techniques (see, for instance, the work of Bolomey)
where you put an array of small (non resonant) dipoles with switches in
the near field, and turn them on and off. You look at the antenna's
feedpoint impedance and from that, you can tell what part of the field is
affected by that dipole.

Once you've got your near field data, you need to post process. A
gentleman, A.C. Newell, literally wrote the book(s) on this technique at
NIST/NBS.