On Fri, 20 Apr 2007 07:44:52 +0100, "Richard"
wrote:

Before you dig up the earth, you might try a quickie fix, a 1/4 wave
elevated counterpoise, run from your tuner ground nut, out the window,
hung from the side of the house or whatever. That's all it took to
provide a 1:1 swr for my last "long wire," a 3/4 wave wire for 20
meters.

bob
k5qwg


"Jim Lux" wrote in message
...
Richard wrote:
I want to put up a "long wire" with the feedpoint at one of the back end
corners of the garden. I need a good RF ground.

If I got a length of 6 foot wide copper sheet and made out of it a
cylinder,
say 6 feet in diameter, and then placed that cylinder in an upwards
position
in the ground, the top level with the surface, would that be a good idea
for
trying to acheive a decent RF ground? Also, is there anything I could
place
around the cylinder to better the conduction between the cyinder and the
earth around it? Should I drill holes in the cylinder so I can pour water
into the cylinder to keep the ground around it moist?

Would I still benefit from some radials in the ground?

My garden's about 33 foot long, 21 foot wide. Ground gets rather rocky
and a bit sandyish at about 9 inches or so from the surface.TIA.

That might be overkill.. Why not spend the money on a spool of copper
wire and lay radials. For RF purposes, large area of coverage is more
important than low impedance at a particular point. You want to improve
the apparent conductivity of the soil over as much volume as you can.
Think of your grounding system as (partially) a big leaky capacitor to
"the earth".. you want to spread the RF current out over as large an area
as possible, and radials are probably the easiest way to do it. There's
nothing special about how you lay the radials or their length (they're in
dirt, so they're not tuned or resonant.. just wires).

Maybe a 3 foot wde cylinder would be adequate.

One thing: Why are radials so effective? They are not resonant
counterpoises, and from the aspect of surface area in contact with ground,
radials have hardly any surface area that "connects" with "the earth". I can
understand if I put in a 20 foot by 20 foot copper plate flat in the earth
that would "connect" to earth quite well, I mean the measured ohmic
resistance to "the earth" could be quite low. But also capacitive coupling
would be low too. But radials have no real capacitive coupling to "the
earth" neither much surface area in contact with "the earth". So it confuses
me as to why they are so effective.

BTW, since it looks like many wires in the earth are as good as a plate,
perhaps I could put in a wire mesh grid about 1 foot under the soil over the
whole garden . I'm digging up the garden anyway. Perhaps with some ground
stakes here and there connected to the grid, although maybe not necessary.

Richard Clark wrote:
On Fri, 20 Apr 2007 07:58:16 +0100, "Richard"
wrote:


More like the radials make the ground below your feet more like say
a pool of salt water than the high resistivity ground that it normally is.
What do you think?


Hi Richard,

Actually salt water sucks as a local ground - it is as poor a "good"
conductor as you could imagine. Carbon is a far better conductor than
salt water, but no one yet has suggested building on top of a coal
seam.

You would be better off filling your yard with sand to the depth of 30
feet or so (yeah, sure). The testimonials attributed to salt water
comes with its far field qualities of a tremendous mismatch to air and
offering spectacularly low radiation launch angles.

mostly due to that epsilon of 80, more than the conductivity


So, copper replaces a very poor conductor (as a first pass
approximation). Invest your copper in close proximity to the base of
the antenna. That is, a lot of short radials, and a fair number of
medium size ones, and a few long ones.

Exactly...

In fact, one can do some numerical analysis to figure out an optimum
strategy, based on minimizing IR losses in the soil. The current
density is higher close to the base of a vertical, so, at first glance,
it would appear that improvements in conductivity there would have more
"value". I imagine there's some nice integration that covers it all.

All that analysis and spreadsheets out there all make the assumption
that you want all radials the same length, which isn't necessarily so.

What also throws a "wrench into the gear train" is that if you start
looking at verticals with nonuniform current distributions, especially
if they aren't representable by a "simple" form such as linear or
cos(h), an analytical approach gets tricky (hence the suggestion for
numerical methods). Consider, for instance, a half wave dipole with the
center, say, 3/8 wavelength above the ground. Or some sort of asymmetric
vertical with an elevated feedpoint, or loading coils. The optimum
radial layout gets a bit trickier to figure out.


Two things to consider. The ground closest to the antenna is
responsible for efficiency in loading. The ground further out
(between 5 and 10 wavelengths, or more) is responsible for launch
efficiency (offering lower launch angles).

73's
Richard Clark, KB7QHC

On Apr 20, 12:44 am, "Richard" wrote:


One thing: Why are radials so effective? They are not resonant
counterpoises, and from the aspect of surface area in contact with ground,
radials have hardly any surface area that "connects" with "the earth". I can
understand if I put in a 20 foot by 20 foot copper plate flat in the earth
that would "connect" to earth quite well, I mean the measured ohmic
resistance to "the earth" could be quite low. But also capacitive coupling
would be low too. But radials have no real capacitive coupling to "the
earth" neither much surface area in contact with "the earth". So it confuses
me as to why they are so effective.

To put it simply, rather than couple with the earth, they help shield
the antenna
currents from the lossy earth..
IE: Richard Clarks comment..
"The alternative view of radials is that they operate as a shield
against loss."

Thats about what it boils down to...
In fact, I think 60 radials one inch off the ground should be slightly
better
than 60 radials touching the ground, both using bare wire.
But radials that are elevated to any real height should be resonant
for
best results.

BTW, since it looks like many wires in the earth are as good as a plate,
perhaps I could put in a wire mesh grid about 1 foot under the soil over the
whole garden . I'm digging up the garden anyway. Perhaps with some ground
stakes here and there connected to the grid, although maybe not necessary.

Would be ok, but actual radials are a better use of wire, than mesh.
Also
mesh can develop connection problems on down the line.
Mesh has an equal density over it's whole area.
But radials place a higher density of wire at the feedpoint, which is
what you want. The ground area at the feedpoint is the most critical.
But that doesn't mean you want to ignore the area a bit farther out.
Normal radials spoked out from a center give you a good appx overall
density of wire in the areas you want to have it.
In general, for a given amount of wire on the ground, many shorter
radials are better than a few long ones. That places more of the wire
at the feedpoint.
A cylinder or tube, etc makes a lousy rf ground. The lightning ground
is
the one that needs a super good connection to earth.
The RF ground requires *no* connection to earth to function properly.
MK

One Question
Do you have a chain link fence?

They make great RF grounds...


"John Ferrell" wrote in message
...
On Thu, 19 Apr 2007 17:55:03 +0100, "Richard"
wrote:

I want to put up a "long wire" with the feedpoint at one of the back end
corners of the garden. I need a good RF ground.

If I got a length of 6 foot wide copper sheet and made out of it a
cylinder,
say 6 feet in diameter, and then placed that cylinder in an upwards
position
in the ground, the top level with the surface, would that be a good idea
for
trying to acheive a decent RF ground? Also, is there anything I could
place
around the cylinder to better the conduction between the cyinder and the
earth around it? Should I drill holes in the cylinder so I can pour water
into the cylinder to keep the ground around it moist?

Would I still benefit from some radials in the ground?

My garden's about 33 foot long, 21 foot wide. Ground gets rather rocky
and a
bit sandyish at about 9 inches or so from the surface.TIA.

Check out http://www.bencher.com/pdfs/00361ZZV.pdf
and http://www.bencher.com/pdfs/00803ZZV.pdf

We usually associate an RF ground with verticals. I believe what you
are referring to as a "long wire" is usually called an :inverted L".

I think your proposed depth of 1 foot or more may be too deep for best
performance.

If you are planning cultivate your garden on a regular basis and use
it for a radial field you have some unique problems. Insulated wire on
top of the ground works as well as buried bare wire.

The consensus seems to be that each time you double your number of
radials you gain a db up to about 128 of them. essentially, more is
better. Popular thinking is that 16 are the minimum.

I experimented with using electrical extension cords of various length
for a radial system and found that all efforts would work but more is
always better! One radial will get you on the air but just barely! I
suspect a quarter wave radiator in any configuration and 4 radials
would beat any mobile installation.

If I were you I would invest in a few orange extension cords and run
them on top of the ground IN the garden rows. Orange will be easy to
see and avoid when you cultivate your garden. They will be easy to
remove and re deploy for garden cleanup.
Plan on assuring the curious neighbors that you are electrically
stimulating your garden. That will probably subdue any objections.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"