On 18 abr, 19:02, "Al Lorona" wrote:
Very cool, Roy. Thank you for this, this is good information that makes
sense to me. There is a lot in there to think about and apply to the
application for which I'm analyzing this antenna.

When you talk about the current maximum and loss of a radial, you bring up
another question. My 1/2 wave vertical is going to have a much higher
feedpoint impedance than a standard 1/4 wave vertical. Let's say it's
somewhere around 2000 ohms as compared to 40 or so ohms for a 1/4 wave. What
effect does that higher feedpoint impedance have on the radial system? Does
it relax the requirements for the number or radials or the length of them?
Does it mask the loss that you'd have normally? Or do the "rules" of adding
radials apply no matter what the vertical's input impedance?

Thanks very much.

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

Hi Al,

From my experience, and simulation, a halve wave vertical has less
stringent radial requirements. The current reduces with a factor of
about sqrt(2000/50)= 6 (16 dB).

The result is less loss in the vicinity (near field) zone of the
antenna. Far field maximum of radiation (elevation) will virtually not
change because of canceling effect of reradiated field from ground
below the (pseudo) Brewster angle. Because of the somewhat higher
radiation center with respect to a 1/4 wave, the elevation of maximum
radiation will be somewhat less. Off course when you have high ground
conductivity (for example fertilized wet soil), you will have maximum
radiation under lower elevation.

It is likely that in the end the produced field under relative low
elevation will be higher than that from a horizontal dipole.

Elevated radials give less ground loss. At high frequency, the
relative epsilon becomes important. A High dielectric constant and low
conductivity may result in less loss. At high frequency a larger part
of the current goes to the capacitance of the ground instead of the
loss resistance (in a parallel equivalent circuit). See it as having a
low loss dielectric under your antenna.

Hope this help you a bit.

Best regards,

Wim
PA3DJS
www.tetech.nl
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