> I ran a quick NEC4 model of a 40m dipole at 50cm above
"average earth"
> (ideal conductor for the antenna though)
>
> As one might expect, it has a fair amount of gain straight
up (9 dBi)
> Out to 45 degrees elevation the gain ranges from 2 to 6
dBi
> At 30 degrees, it's dropped off to -4.4 to +3 dBi
> At 15 degrees, it's -16 to -2.7
If you look at Hagn's actual measured data you'll see over
30 mS or 50 mS/m soil (very good earth) FS drops several dB
as antenna heights move below .05 wl.
.05 wl would be about six feet on 40 meters and about a
dozen feet on 80 meters.
I know from my own measured A-B comparisons if I add a good
screen below a dipole at 35 feet on 80 meters signal levels
increase about 5-6 dB.
So how are you getting 9 dBi straight up? That makes no
sense unless the model handles earth losses poorly. With an
error or disagreement like that, I'd toss the models out.
> OK.. So, if we assume that for NVIS, you're interested in
angles ABOVE 45
> degrees, going above 1/4 wavelength off the ground really
starts to
> hurt. And, the low level radiation starts to come up.
So what? Groundwave is zero degrees, not 15 and certainly
not 45 degrees. A horizontally polarized signal at 3.5 or 7
MHz has virtually no ground wave signal at all. I measure
over 40 dB of isolation between a vertical and a dipole
spaced less than 1000 feet, and nearly twice that
attenuation between two dipoles. The maximum vertical
component is off the ENDS of the dipole, and the level of
that vertical component is very low at the zero degrees
necessary for ground wave. As a matter of fact, most of the
groundwave radiation comes from the feedline, not the
antenna!
> And, for those above 45 degree angles, the height of the
antenna makes
> almost no difference, all the way from 50 centimeters
(pretty close to
> 18") up to 10 meters.
Then the model is behaving poorly because that makes no
sense at all.
When I run the model in EZnec4 over average soil using high
accuracy ground, I get 7dBi (straight up) at 30 feet high,
and -3.63 dBi at 3 feet high. That's 7.2% antenna efficiency
at 3 feet of height.
The EZnec numbers are very realistic. The gain values you
listed that show no change as the induction fields are
brought very close to a lossy media make no sense at all,
and disagree with every measurement I've seen including the
data from Thailand.
73 Tom
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