On 11/3/13 6:47 PM, Michael Tope wrote:
I think that would be true for a fresh water lake, John. OTOH, I would
expect the the conductivity of water saturated soil to depend heavily on
what materials dissolve into the water from the dry soil.
Actually, it's the size of the soil particles that seems to be the
biggest effect, RF-wise. Fine Clay is very different than coarse sand,
for instance. Partly it's also that clay can hold a lot more water
without being "waterlogged", because the particles are smaller.
A soil that is 10% water is quite wet.. Get up to 20% water and it's
positively soggy.
So, if you think about using a simple mixing rule.. at 20% water, the
other 80% is "rock" and "air", both of which are fairly non-conductive,
and low epsilon. Most Rock is in the 4-5 range for epsilon.. Most dry
soil has an epsilon around 2-3 (half air, half "stuff")..
There's also huge frequency dependencies when it comes to wet/damp soil,
and the finer the particles, the bigger the variation
If anyone is interested, there's two papers that are sort of the "go-to"
reference
M. C. Dobson, F. T. Ulaby, M. T. Hallikainen, H. A. El-Rayes, “Microwave
Dielectric Behavior of Wet Soil – Part II: Dielectric Mixing Models”,
IEEE Transactions on Geoscience and Remote Sensing, Vol. GE-23, No. 1,
January 1985
M. T. Hallikainen, F. T. Ulaby, M. C. Dobson, M. A. El-Rayes, L-K Wu,
“Microwave Dielectric Behavior of Wet Soil – Part 1: Empirical Models
and Experimental Observations”, IEEE Transactions on Geoscience and
Remote Sensing, Vol. GE-23, No. 1, January 1985
as well as
W. A. Wensink, “Dielectric Properties of Wet Soils in the Frequency
Range 1-3000 MHz”, Geophysical Prospecting, Vol. 41, Issue 6, 27 April 2006
73, Mike W4EF...........
On 11/3/2013 3:37 PM, john@kk9a.com wrote:
I do not think fresh water has much effect on verticals and even less on
horizontal antennas.
John KK9A
To:"towertalk@contesting.com" <towertalk@contesting.com>
Subject:[TowerTalk] Impact of high water table levels on antennas
From:Rudy Bakalov <r_bakalov@yahoo.com>
Ever since I have put up two inv-Vs, one for 80m and one for 160m, with
their
apexes at roughly 90', I have been puzzled by their exceptional
performance.
Ditto for my vertical on 40m. I have done tons of comparisons using
skimmer
data and my signal seems to be pretty darn close to the big stations I am
using
as my benchmark. The performance is so good that I have been wondering
if I
should bother with building 4SQs. I have read tons of books on
antennas and
the
performance of these two antennas simply does not match what the books
describe.
I shared my thoughts with a friend of mine (a WRTC2014 participant)
and he
shared a similar experience with his station. He recently relocated to
a new
place, about 30 miles from his old place, and his antennas at the new
place
perform significantly better than the old location. Same antennas, tower,
feed
line, and FLAT terrain. His only explanation is that the new place had a
very
high water table that somehow impacted antenna performance.
This is when I realized that I also have a very high water table. Even in
the
driest months of summer, the area around my tower is damp and the
grass is
very
green, growing like crazy. This was the obvious common element between
his
and
my situations.
I have not seen anything on high water tables in my antenna books.
The soil
itself is mostly sandy. The impact I believe I am seeing is mostly on the
lower
bands, but I am not sure if this is also the case on the upper bands
as at
105'
my antennas are a bit too high.
Is there any rationale in our thinking? Can high water table explain
better
than expected performance from low band antennas? If so, what is the
theory
behind it and how do I take advantage of it? If not, any other
suggestions
for
why the antennas work so well?
Rudy N2WQ
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