Here is some more food for thought
w.r.t. modeling of ground by NEC.
I got interested in NVIS by the
recent article in QST. I put up
five 40 meter antennas to check
this out: a ground mounted vertical,
a dipole at 18 feet, an inverted vee
at 30 feet, an inverted vee at 60 feet,
and a dipole at 18 feet over a 230 foot
diameter ground screen. Beyond 300 miles,
these antennas were all too close to call.
Closer than 300 miles, the vertical was
terrible (no surprise). The 60 foot vee
was generally 3 dB worse than the lower
antennas. The 18 foot antennas were
the same as the 30 foot antennas except
within 100 miles, when the 18 foot
antennas were 3 dB better than the 30
footer. What was interesting was that
the two 18 foot high dipoles were too
close to call, even though one was over
bare dirt, and the other was over a
ground screen with much higher conductivity
than even salt water.
According to NEC, the bare dirt 18 footer
should have lost 3 dB due to ground losses.
I just didn't see this 3 dB, even though
I could see it clearly in the other cases
mentioned above. Go figure.
Rick Karlquist N6RK
> > W8JI wrote:
> > >In my experience models tend to fall in line more on 40
> meters and up
> > >with real-world results, while on 160 results vary the most.
> For example, our antenna modelling programs assume the earth is a
> perfectly homogeneous media and represent the earth as a single
> boundary layer with certain characteristics. The programs we often
> use do not even consider groundwave, and so show zero radiation at
> zero angles and blend the pattern to that point.
> This would especially be true with things heavily dependent on the
> ionosphere, real earth, or any other "soup" of varying
> characteristics, where we might be lucky to get within ten
> dB. Models
> give you direction, not absolute answers.73, Tom W8JI