Dear Group: What a delight it is to see a computer doing the calculations
for VHF propagation.
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. Of course, I used a slide rule and log tables to perform
the calculations and manually extracted path profiles from topo. maps. The
goal was to place confidence in the model for estimating expected
interference levels at a radio-astronomy site located in a valley. The
result from extensive filed measurements and data reduction was that we
could be confident in the model.
I recall also doing some comparisons of predicted and measured strengths
involving scattering (over quite long distances) in the VHF range with good
correlation.
IONCAP, and its predecessors and successors, I have used to good effect
for almost as many years.
In short, the developed propagation methods have been proven by me, and
many others, to provide reasonably small uncertainties. Of course, the
critical element is knowing which tool to use. That, I believe, is part of
the point brought forward by Richard Fry and others. But put yet another
way, any dam fool can (now) put numbers into a computer and get numbers back
out of the computer - experience and judgment is needed to have significance
accrue to the results of such calculations.
Central to all of the propagation models is the need to understand what
the antenna and its environment actually does. I am also delighted that
several of you are providing the education to the silent so that they do not
fall into the traps that are always present.
Warm regards and season's greetings, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:
"Richard Fry" wrote in message
...
On Dec 22, 11:13 am, "Frank" wrote:
In this example the vertical half wave dipole, with the base 30 ft above
an average ground, on 147.3 MHz, shows a field strength at ground
level of: 0.418 uV/m from 30 W into the antenna.
And, obviously, at 50 km.
________________
Here is another method (Longley-Rice) for calculating the field
intensity produced at the receive site by your model. But the NEC
approach is less accurate than L-R for long path lengths (due to earth
curvature), and for specific terrain contours.
In your model the path loss calculated using L-R is about 68.8 dB more
than the free space loss. The peak, free space field produced by a
1/2-wave, linear dipole radiating 30 watts over a 50 km path is about
770 uV/m. This voltage reduction of 68.8 dB is a field multiplier of
about 0.00036, so the 770 uV/m field is reduced to about 0.28 uV/m --
a bit less than your NEC model predicts. Agreement probably would be
better over shorter paths (as long as no specific terrain profile
needed to be applied), and worse for longer paths.
In the L-R example I set the path over the middle of Lake Michigan in
order to get a smooth earth contour, such as used in NEC models.
This all just illustrates that analyses made using NEC and any other
method need to consider the limits inherent in their algorithms with
respect to the physical reality being analyzed.
http://i62.photobucket.com/albums/h8...strialPath.gif
RF