Roy,

This is blowing my mind. What you said about ground loss peaking at some
certain conductivity makes perfect sense but I never thought about it before
as applied to a vertical antenna system like this.

Sure enough, I found a maximum. This is wild. We always think, "The better
the ground, the better the antenna system," but it's not that simple. It's
funny to think that really terrible ground can have an advantage over pretty
good ground.

Next I'm going to play with the radial length so I can see the other effect
you described for me.

Regards,

Al W6LX









"Roy Lewallen" wrote in message
news4CdnVoHG_g30pXVnZ2dnUVZ_vudnZ2d@easystreeton line...
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