Hi Tom
You wrote:
> I recall a DXpedition to ZL8 used a wire vertical over salt water
> on either 80 or 160M. For ground, the submerged a galvanized
> bucket in the salt water. Would this greater surface area solve
> the current density problem mentioned by W8JI?
Well, looking at it on a model where a 1/4 wl monopole with 60 1/4
wl radials is compared with either average soil or seawater, at 15
degrees the difference is about 2 dB.
As for the elimination of radials, unless I'm missing something:
Seawater is 167 times more conductive than the better typical soils.
If the vertical antenna is tall and return currents are uniformly
distributed out for some distance we should, considering the
current density vs area, be able to reduce the size of what we know
we need for a radial system to about 12 times less than the radius
of a normal radial system and be somewhere around the same
point of the curve.
That would be about a 1/20th wl radius, or a 1/10th wl diameter
**surrounding** the vertical, for a "nearly perfect" ground.
Zone currents would be a big issue, if not uniformly distributed. If
the current doubles, we would need four times the area to maintain
the same power loss.
Everything in life is a trade off of how much work we do and what
we get back. A 30 foot radius screen or system of eight or ten
radials for 160 meters suspended above the water would be an
easy task.
A key factor is how much the impedance of the screen would
change with tides. That may limit how small it could be.
I would never use just two radials, unless they are high above the
water, because it would not help reduce zone currents except in
two narrow directions. The radials or screen could NEVER be more
than a fraction of a foot deep, or it would start to lose effectiveness.
With that in mind, I'd keep it above the water.
We all have impressions of how things work, and often it is far from
the truth. Take the little Hex Beam. Look at all the extreme
comments, and then look at an A-B test at one location done by
k1BQT or actual models of the antenna. While the latter show it to
be no better than any other small shortened antenna for gain, the
high F/B ratio fools people into thinking the gain is much higher
and they liken it to much bigger antennas.
Being on the beach is worth a few dB in our minds, having a DX
call is worth a few in everyone's mind who hears you, and operating
a contest guarantee's 599 reports and many callers. That would be
the worse possible case to tell how an antenna actually works.
73, Tom W8JI
w8ji@contesting.com
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