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

Roy:

[snip]
"Roy Lewallen" wrote in message
...
..
..
..
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
[snip]

That's exactly what I thought.

Since; (a) I don't know my soil characteristics and (b) because of property
limitations that dictate a non-uniform radial field, I felt that all I can
do is to lay down radials in the property area I have avaliable until I see
the change in Zin due to adding more radials become insignificant, then...
I'm done!

I don't really care what the exact value of Zin = Rin + jXin ends up to be,
since there is nothing I can do about it anyway. When I reach the point in
laying down radials to where I can't reduce Zin much by adding more radials,
I will then have the most efficient radial field with the lowest ground
resistance Rg that I can get for my money and effort [smile].

Heh, heh... Money is only money, the effort, hmmm... well that's me crawling
on my hands and knees for hours digging in the dirt (sand) in the blazing
hot Florida sunshine, heat and humidity!

When I arrive at that point in burying radials, I believe that I can then
tune out any reactive part of Zin with my 'tuner' and and end up feeding
power into the remaining resistance Rin which then should be the sum of the
vertical element radiation resistance Rr and whatever value I have ended up
with for ground resistance Rg.

I just won't know what the value of Rr and Rg is, but I will know that I
have achieved the most efficient radial field I could put down here at my
place.

Is this right?

-- Pete k1po
-- Indialantic By-the-Sea, FL

PS: I took your advice and did some reading on simple impedance bridges,
and I saw one that you had designed in one of my ARRL pubs. Cool!

Another idea I came upon that allow the use of those antenna analyzers in a
situation where they can be somewhat immune to BC interference involved a
two step process that went as follows...

(1) Hook up a transmitter through an antenna tuner to the antenna and ground
systems under test and tune the tuner for zero reflected power with a 50 Ohm
reflectometer (VSWR meter). Then (2) disconnect the tuner from the antenna,
and without disturbing the tuner settings, hook a 50 Ohm load to the input
side where the transmitter was connected, and then use the antenna analyzer
to measure the impedance looking back into the output, or antenna terminals,
of the tuner. This impedance should be the conjugate of Zin. Here the
analyzer may not be as affected by potentially strong BC RF signals picked
up by the lengthy antenna element.

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