Re: NEC Evaluations

Dear Richard:

It was almost 50 years ago when the models were rather new.....

More background: the terrain was hilly - far from smooth earth - and
path profiles were a critical part of the information along with the
inherent uncertainties of using "analog" maps and along with the assumption
about almost-straight line propagation. (an aside: we found examples of
unpredictable propagation along string-like valleys that were aligned with
transmitters, but the protected site was in a bowl-like valley.) (I saw one
family in a valley using a rhombic antenna to receive TV signals. Their son
had been in the Signal Corps.)

We were using state-of-the-art Empire measuring systems (run off of a
portable gasoline generator) that were calibrated with an impulse generator
at each measurement. We selected paths that were similar to the expected
paths of interfering transmitters. In other words, the paths were
more-or-less normal to ridge lines not along string-like valleys.

One more qualification: one path was found to have knife-edge
diffraction discovered by the caution of taking measurements spaced a few
meters apart at each data point. It was absolutely classic, but that data
was not used because the protected site did not have such sharp ridges at
its periphery.

With those qualifications, my best recollection is that measurements and
predicted measurements were within something like 3 or 4 dB. I doubt that
repeating those measurements with a GPS receiver, digital topographical map,
averaging near straight-line paths, and using a computer to do the
arithmetic would be any better.

Another note: Because of the expected sensitivity to interference at
the site, I would drive over a few hills, erect a dipole in trees, and work
my father on HF from the back seat of my car. No cell phones in those days.
.... long distance was a big deal too

Let us know how your studies are going. Warm regards, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home: JCM@xxxxxxxxxxxxx
"Richard Clark" <kb7qhc@xxxxxxxxxxx> wrote in message
news:kug0l4hjpl0o2jqiep15gusgjfdph8jogm@xxxxxxxxxx
On Mon, 22 Dec 2008 20:15:09 -0500, "J. Mc Laughlin"
<jcm@xxxxxxxxxxxxx> wrote:

Almost fifty years ago, I led a team who measured field strengths in the
100
to 250 MHz range (FM and TV broadcast transmitters) to verify (qualify)
the
propagation model.

Hi Mac, and season's greetings,

Can you relate the specifics of the measurement? At a minimum, what
you would deem to be your best accuracy compared to an absolute
standard, or to a relative standard (instrumentation, not
computational).

73's
Richard Clark, KB7QHC


.

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