Roger,
From the link Dan AC6LA posted there are some long standing different
views of near and far fields from vertical antennas. A discussion above
my pay grade as to whether NEC 4.2 analysis is correct for these models,
but it is validated in my experience. I can offer an intuitive
explanation to part of your question.
So why does a vertical at the edge of the sea radiate more energy
seaward than landward? The relative conductivity is different by a
factor of 1000, 4 S/m for salt water vs 0.005 S/m for "average" earth.
So in that situation the return currents flow in the low resistance side
to a much higher value than the high resistance side. Further the
losses from a radiated field over salt water ground resistance
approaches that of copper. I think that accounts for the directivity
gain. Perhaps the more important factor is that the pattern starts to
look like a vertical over "perfect" ground which shows the elevation
lobe at a maximum value at the horizon, which is great for long distance
DX propagation if you look at the HFTA statistics re arrival angles.
N6BT offers some interesting observations about periodicity of gain as
the antenna is moved back from salt water. I did not see that in my
modeling. However, it is certain that the ground properties of coral
atoll or sand spit DXpedition locations are probably highly variable
over small distances from salt water intrusion effects. Published
estimates indicate wet sand is about 30% salt water content so a 1 S/m
conductivity value is reasonable. There are other dynamic effects
through the day on propagation from tide cycles. For a number of top
100 DX locations, near mountains are high enough to interfere with the
<10 deg radiation angle in some directions, frustrating amateurs in
those directions, otherwise the sea is totally "flat". see USA vs the
current VP8 DXpedition.
I was able to real time A/B a vertical at the sea edge vs one about a
1.5 wavelengths back and the difference was significant, as much as 2 S
units over a 8000 km path. (typo in last post 10KM should be 10,000km).
Grant KZ1W
On 2/3/2016 23:54 PM, Roger (K8RI) on TT wrote:
I ask the following as a serious question.
Given the verticals appear to work well in these instances, yet salt
water is not a low a resistivity as copper radials, but it is "flat"
compared to most land, much lower resistivity compared to wet dirt and
it is the equivalent of an infinite number of radials that extend
through effectively an infinite far field. Is it the conductivity
over a wide area and how does this relate or react in the near field
and far field ? How does this compare with a copper disk (flat plate)
with a radius matching typical radial lengths in the simulations?
We've all heard the claim, "I had the vertical over a 150 ft deep well
and it really played". Yes it was a great safety ground, but the RF
saw very little of it/
In this case the verticals work, but why?
73
Roger (K8RI)
On 2/4/2016 Thursday 1:20 AM, Grant Saviers wrote:
Having used several "verticals on the beach" for three DXpeditions,
we verified they work extremely well. This is because the pattern in
the direction of the salt water goes to near the horizon, at least as
close as EZNEC can model it. In "take off angle" parlance, think
less than 10 degrees. Thus, for the 10KM paths or longer from the
South Pacific to ROW, they were the best choice on a north facing
beach, ie they are directive antennas. All were with 1 or 2 raised
radials, tuned for the band of interest. Since then I've extensively
modeled verticals on the beach and found that 2 radials parallel to
the tide line that are elevated at least 0.05 and preferably up to
0.15 wavelengths maximize the seaward gain as long as the vertical is
less than 0.4 wavelength or so from the water. The verticals on the
beach with elevated radials have a resonance Z around 35 ohms, which
indicates very low ground losses.
To get an omni pattern with your vertical on a dock, it needs to be
about 1.5 wavelengths from land and a single radial is ok. The
directivity gain is lost, but the very low angle pattern is preserved.
Using an "electrical" connection directly to salt water is a very
interesting question which I only have anecdotal information about
from the sailing community. From a DC resistance perspective it
takes very little surface area to make a low resistance connection.
From an RF perspective what I've seen recommended is in the several
sq ft of surface area and then within the top few inches of the water
as that is the skin depth at multi MHz. If any towertalkians have
data on this, I'd sure appreciate getting a copy.
What is proven, as other DXpeditioners have documented, is that
elevated radials for a vertical close to the tide line work extremely
well. Also, Al Christman K3LC modeled this extensively in a NCJ 3
part series published in 2005.
Grant KZ1W
<snip>
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