Over really good dirt there isn’t too much difference.
The other problem is that the NEC ground approximation coding usually
underestimates loss. This is because the ground approximation in the model
uses a monolithic ground. Same stuff 100 feet down as at the surface. Real
dirt never is. Water tables, etc, yada yada. Or farm dirt plowed, dug for
the first two or three feet down, below that undisturbed and thoroughly
packed for hundreds of thousands of years. That's not monolithic. Nor are
many properties that were leveled for construction, or in ancient lands,
built on construction rubble accumulated for millennia. Simple velocity
factor measurement of a 152' dipole on ground is monstrously variable,
sometimes just varying orientation in the same back yard.
That doesn't account for lots of trees, roots where the only way to reduce
the increased counterpoise loss from dielectric losses is to reduce the
fields from the counterpoise. That's where the FCP comes from.
NEC uses monolithic ground in its ground approximation because more
complexity results in monstrous increases in run time. And we have to
remember that NEC is from a period where mainframe run times cost a lot of
money, and some computational methods could result in jobs not completing
in a month.
Reality skeptics sure of underestimated NEC ground loss include W7EL, EZNEC
author.
One way to get a better idea is to set ground characteristics in EZNEC to
gawd awful (in all compared designs) and rerun. As in (.005,1). I do that
routinely on a design to stay away from designs that are more sensitive to
poor grounds.
E.g. set ground characteristics to (.005,1), then run a ground plane with 4
radials. Repeat with 8. Note the difference. Run at various radial heights.
Poor ground qualities happen and definitely appear to be the majority
rather than the exception. Designing for least sensitivity to poor ground
will protect the poor soul that has gawd awful ground in their backyard and
don't know it.
We spend thousands of dollars on transceivers and amplifiers and then go
cheep, cheep on radials? On 160, ground loss is the two ton elephant in the
room. Clean up after the elephant and send it to the zoo before you put
down new carpet.
73, Guy K2AV
On Fri, Dec 11, 2020 at 5:02 AM Artek Manuals <Manuals@artekmanuals.com>
wrote:
> Jim et all
>
> I agree with what you say on all points when it comes to radials on the
> ground and salt water affects.
>
> However I have always wondered about elevated radials . The NEC ( both 2
> and 4) models (not that those are to always be trusted) show very little
> (if any) improvement beyond four elevated radials, you have any theories
> on why that is? Intuitively (also not to be trusted) that is a lot of
> real estate in the spaces between
>
> Dave
> NR1DX
>
> On 12/11/2020 4:37 AM, Jim Brown wrote:
> > On 12/10/2020 11:14 PM, Raymond Benny wrote:
> >> With said, I believe my ground absorption is very high so I feel the
> >> higher
> >> numbers of radials helps greatly with my vertical efficiency and
> >> radiation.
> >
> > Hi Ray,
> >
> > The soil affects us in two important ways. First, poor soil burns
> > transmitter power underneath the antenna and it's near field. We use
> > radials to shield the soil from the field, and to supply a low
> > impedance path for return current. Magnetic fields produced around
> > each radial by virtue of current flow couples loss in the earth into
> > the radials in the form of series resistance. Loss in each radial is 1
> > squared R; each time we double the number of radials the current in
> > each is divided by two, so the power coupled to the earth by each
> > divides by four. So the more radials, the less power is coupled to the
> > earth. THAT'S why more is better. The result of all this is that loss
> > in the soil under the antenna reduces the total strength of our signal
> > by that amount.
> >
> > The second effect of soil is in the far field, where we field radiate
> > hits the earth and is reflected by it to form the vertical pattern.
> > The better the conductivity THERE, the our pattern will be both
> > stronger and at a lower angle. An antenna with its base just above sea
> > water is the extreme example of this -- the reflection is extremely
> > strong, and it is at a VERY low angle.
> >
> > We can help the first of these two effects with a good radial system,
> > but the only thing we can do about the second (the far field
> > reflection), is to move where there is better soil. Most of us live
> > where we do because we like living there for reasons other than radio.
> > And that includes me and my XYL.
> >
> > 73, Jim K9YC
> >
> > _________________
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>
> --
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>
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