Hi Carl and others,
Before I quote or accept data, I like to know where ir comes from and
HOW it was derived. The fact it was in print means nothing without
that info.
> This system produced a
> FYI, here is a quote from a technical paper delivered by Clarence
> Beverage (nephew of THE Beverage) at the 49th NAB Broadcast
>SNIP
> 85 kilometers. The measured RMS efficiency was 287 mV/m
> [normalized] to 1 kW at 1 kilometer, which is the same measured
> value as would be expected for the tower above 120 buried radials."
Here's how the engineer arrives at an "RMS Field". I think everyone
will find this interesting. If you want to read it first hand, get
out your FCC Rulebook and read part 73 carefully. It's in print.
1.) They install a temporary tower (it does not have to be a big
efficient tower with a big ground), go out on radials and measure
the FS intensity along each radial. (Radial refers to a radial
direction, not a radial wire)
2.) A plot is made of FS along each radial path, and a
curve is plotted on graph paper using those readings.
3.) The average SLOPE of that curve is matched to the slope of a curve
in a graph supplied by the FCC for the frequency range in question,
and the curves are "matched" to estimate soil conductivity.
4). The estimated soil conductivity, as determined by the radial
measurements, graphs, and comparisons above is then used to determine
estimated minimum FS is at one km over LOSSLESS soil and a perfect
path.
So the number you hear bantered about is the value obtained for
lossless soil, and the result of all these steps and estimated
values. It also assumes everything, including soil and
propagation, is uniform or averages out.
Of course the real world FS is ALWAYS much less than the theoretical
maximum over perfect soil at one km, so the mV/m/km value you read
about is never a "real" measured value.
It's very important to understand this point. If you measure the data
and plot it on a graph, and estimate the soil as 2 mS/m (as they did
in this case), you get to "fudge" or normalize the readings up to a
much higher "Inverse FS" than if the soil estimates to be something
better, say 15 mS/m.
This does not imply any "falsification" on the part of the engineer,
but rather is a normal artifact of multipath, varying soil, wires,
pipelines, hills, and so on along the path.......and how that
particular person averages out the readings when he draws his curves
or plugs in his numbers. People just do things differently when data
isn't exact, and they have to toss some data out.
You see Carl and others, the 195 mV/m per mile (or 286.4 mV/m per km)
per kilowatt bantered about is the estimated minimum FS that
would be measured at that distance over perfect lossless soil and a
lossless path, NOT what is really measured in the real world.
With all the various steps and built in errors, all you get from
this exercise is a estimate of the worse case minimum (poorest) FS
you should have. IMO, meeting a minimum estimate that is very
forgiving hardly proves anything, except you "just got by".
The only real way to prove an efficiency change is to change only one
thing, and measure the FS change.
73, Tom W8JI
w8ji.tom@MCIONE.com
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