"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

Interesting comparison between methods at VHF frequencies. For curiosity
I had done a comparison between the FCC predicted curves, for an AM
broadcast station on 1655 kHz, and NEC. It seems that at the lower
frequencies NEC has greater accuracy. Of course NEC was never intended
as a propagation tool, but still appears to be reasonably useful. I had cut
and pasted an Excel spread sheet below, so not sure if it will retain the
formatting when posted.

Frank


Field Strength Comparison at 1655 kHz..

Antenna Description: 45.3 m ground mounted monopole. 120 X 45.3 m
radials, 15 cm below ground. All conductors copper.
Input power 100 W

Source: http://www.fcc.gov/mb/audio/73184/index.html per 47 CFR
Sections 73.183 and 73.184
Nittany Scientific GNEC Version 1.1a. Ground parameters: Conductivity
5 mS/m, permittivity 13 (Average ground)
Field strength RMS V/m.

Distance FCC GNEC Difference Difference
(km) (mV/m) (mV/m) (%) (db)

0.10 950.000 960.000 1.0 -0.09
0.50 170.000 168.000 1.2 0.10
1.00 77.000 75.000 2.6 0.23
5.00 8.500 8.110 4.7 0.41
10.00 2.400 2.270 5.6 0.48
50.00 0.068 0.067 2.1 0.18
100.00 0.014 0.015 7.0 -0.61
200.00 0.002 0.004 62.1 -5.58