Re: NEC Evaluations

Dear Richard Fry:
Thank you for the 1985 reference, which I had not seen before. Too many
IEEE groups exist!

A closed-loop system much like that shown in Figure 15 was built by me
and a student and used by the mid 70s to subject DUTs to up to at least 200
v/m at frequencies up to about 200 MHz. This was for automated evaluation
of the EMC of relatively small DUTs and was the prototype of a much larger
system implemented by a major manufacturer that allowed the testing of
entire cars. This was well before PCs, but after 488 signal sources and
wattmeters were available. Confidence to about 1 dB was felt because of the
tight correlation with a short voltage probe extending into the TEM cell.
Unfortunately, the small effective volume of the TEM cell precluded
measurements of antennas. The large room at NBS allowed them to measure
antennas and I saw them measuring a large UHF antenna with a near-field
probe in the early 1970s.

Jumping to HF antennas of 0.5 WL size or more: I am convinced that even
with a helicopter being used to measure a pattern, one can have more
confidence in the result of the intelligent use of NEC4 than in any
measurements.

The measurements made in late 50s (to gain confidence with VHF
propagation models) involved cherry-picking the paths to correspond with the
goal of understanding propagation of possible interference into the
radio-astronomy site. They also involved averaging a series of measurements
taken within a few meters of each other. The measurement sites were all
very rural and free of significant reflecting surfaces.

Warm regards, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home: JCM@xxxxxxxxxxxxx
"Richard Fry" <rfry@xxxxxxxxx> wrote in message
news:4b1e63fe-882c-47a3-bd2b-6cdce282c0f8@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Richard Clark wrote:
...what you would deem to be your best accuracy compared
to an absolute standard, or to a relative standard (instrumentation,
not computational).
______________

You weren't asking me, but still you may be interested in the link
below which leads to a good presentation of this by the NIST. A table
on Page 3 there shows a measurement uncertainty at the NIST test
facilities of ±1/4 to ±1 dB, depending on the DUT and the frequency
range.

Field intensity measurements made using uncontrolled path conditions
are more a measure of the propagation environment and the pattern/
location of the receive antenna than they are of the absolute
performance of the transmitting antenna system. Such measurement
errors can be gross, and difficult to quantify.

http://ts.nist.gov/MeasurementServices/Calibrations/upload/im-34-4b.pdf

RF


.

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