On Aug 23, 8:09*pm, Roy Lewallen wrote:
On 8/23/2010 5:38 PM, Gary wrote:



Hi Pete, *If you are evaluating a radial field for a vertical, then
the value you should be shooting for is 30-35 ohms, with no
reactance. *As you know this is 1/2 the impedance of a 1/2 wave
dipole-70 ohms in free space. *I have never done this, but an
impedance below 40 ohms with little reactance should provide you with
an efficient vertical. *Once you get around this value, I guess there
is nothing to be gained by adding more radials. *What your analyzer
tells you, as others have mentioned, depends on the rf in the area.

Gary N4AST

The actual value you get when the radial system loss is minimal depends
on a number of factors, including the height and diameter of the
vertical. I've also seen convergence to other resistance values when the
ground was dry on the surface but apparently wet at some depth below. In
that case, radial current can be significant at quite a distance from
the antenna (as opposed to the exponential-looking decay you see in the
current on radials buried in moist ground), making the system act more
like a system of elevated radials. In those systems, radial length also
plays a role in determining the feedpoint resistance value.

The bottom line is that I don't trust a single value or its comparison
to 36 or 40 ohms as being a reliable indication of efficiency. You
either need to look for convergence of the feedpoint resistance as Peter
proposed, or even better yet, look for convergence of field strength
values at a fixed location as you increase the number of radials.

Roy Lewallen, W7EL

Hi Roy, I agree that field strength would be the best indicator of
efficiency as additional radials are laid down. However, for the
average Joe Ham trying to figure out how many radials is enough, and
all he has is an antenna anlyzer, what do you recommend?

Seems to me, with a 1/4 wave vertical of diameter large enough to
minimize I^2*R losses over average ground, if you add enough radials
to get the impedance to 35+/-j0 ohms as measured by the antenna
analyzer, you should be good to go. An additional check with the
analyzer could be the SWR bandwidth. If the swr was 1.7 across a
broad range then the 35 ohms is all ground losses. If the 1.7 swr
bandwidth is very narrow then you have a high Q antenna, and very
efficient at the operating frequency. Does this make sense?

Gary N4AST