Al Lorona wrote:
Hi, Everybody,

In the process of modeling a vertical antenna (specifically, I am using
EZNEC 5.0) I am noticing an effect I did not expect which could be the
result of a modeling error on my part.

The antenna is a 34-foot vertical above (12) 34-foot radials, making it a
1/4 wave on 40 and a 1/2 wave on 20.

On 40, the antenna works as I expected; as the ground conductivity goes up,
the gain and efficiency of the antenna both increase, too.

But on 20, if I increase the ground conductivity from, say, 0.005 to 0.008
S/m, the max gain and efficiency *decrease*! This is counter-intuitive to
me.

Can anyone point to something I'm doing wrong?

Thanks,

Al W6LX

Ground loss is a sort of impedance matching problem. If you have
perfectly conducting ground, there is no ground loss. If you have
perfectly insulating ground, there is no ground loss. There's always
some ground conductivity in between those extremes at which the loss is
maximum. This value depends on the frequency among other things. Try a
wider range of conductivities and you'll find this point.

You should also be aware that if you have radials which are above but
close to the ground, half wavelength ones can be considerably less
efficient than quarter wavelength ones. One reason is that the points of
maximum current are out near the centers of the radials, where they
induce current into the lossy ground. When the radials are a quarter
wavelength or shorter, the current maxima are near the center, so their
fields nearly cancel. Another often-overlooked fact is that radials very
close to the ground are electrically considerably longer than when more
elevated. So radials which are a quarter wavelength in free space can
have their current maxima well out from the center which results in
lower efficiency.

Roy Lewallen, W7EL