At 12:25 PM 2/28/2005, Bryan W5KFT wrote:
>The recent discussions on conductivity have talked about different soil
>types and saltwater. Is there any information available about
>conductivity for antennas mounted above "fresh" water (ie lakes)
>
>Is the additional conductivity worth the hassle for installation above a
>LARGE body of fresh water?
>Trying to figure out if its worth it to use Lake Buchanan, Texas (8 miles
>wide, 35 miles long)
>
>Bryan W5KFT
Part of the reason that antennas perform well over water is not so much
the conductivity, but the spectacularly high dielectric constant (80) which
means that any body of water is a really good reflector. Also, if you're
that close to a body of water, your soil is probably moist, which makes it
that much better a conductor.
And, of course, lake water is actually a fairly good conductor (compared to
rock or soil). I have seen values for Lake Tahoe (which is quite pure) of
9.2 millisiemen/m amd 85 mS/m for Lake Mead (near Las Vegas). (Atlantic
Ocean measures 4300 mS/m) Comparing to typical soils of 5 mS/m, even Lake
Tahoe is a better conductor.
(HFTA uses 1 mS/m for fresh water conductivity, which strikes me as a bit
low. Distilled water is usually around 0.05 to 0.3 mS/m. Tap water in my
area is typically around 50 mS/m (500 uS/cm))
You can probably call someone to find out what the conductivity is for Lake
Buchanan (or google for it... water conductivity is usually reported in
microSiemens/cm (or micromhos/cm)... divide by 10 to get millisiemens/meter).
Anyway, why not use a program like HFTA to take a look. Put 80 in for the
epsilon and 50 in for conductivity.
Jim Lux, W6RMK
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