At 06:12 AM 1/14/2007, Martin AA6E wrote:
>There is snow and there is snow. (Ask a cross country skier.) Dry powder
>has lots of air in it, which gets partly compacted out after a while.
>Wet snow has liquid water mixed in, etc.
>
>Ken,
>
>There are two effects to worry about. The biggest problem would be RF
>absorption (loss), which would be worse for higher liquid water content
>IMO. The other issue is dielectric constant. Ice and water have high
>"epsilon" - which means that your antenna will be detuned.
Ice doesn't have all that high an epsilon (3.2 according to this reference
http://snobear.colorado.edu/Markw/SnowHydro/dielectric_const.html)...
Liquid water is really high (being a liquid and polar molecules...)
but once those molecules are locked into a crystalline lattice after
being frozen....
The absorbtion/loss also varies too (hence the problem with thawing
frozen meat in a microwave.. thawed meat is MUCH more lossy than frozen meat)
Still... 3.2 is enough to screw up the tuning of an
antenna. Fortunately most snow is mostly air, so the epsilon of snow
is probably down in the 1.3 area (10:1 air/ice ratio.. I guessed at
this because 12 inches of snow = 1 inch or rain, give or
take). Resonant frequency generally goes as the square root of
epsilon (it's tied to propagation velocity), so one might see a
10%-15% change in resonant frequency.
> (Apparently
>not an issue if your SWR is OK.) It also means that you would get some
>change in antenna patterns because you have a layer of (somewhat) lossy
>dielectric (snow) over your normal ground. Ground is still ground, but
>the snow will dissipate some of your power, and your antenna pattern
>will change somewhat. It could be modeled, if you know the properties
>of your snow.
And therein lies the rub... know the properties of your snow. Of
course it is just such things that allow the satellite measurements
previously mentioned... Measure the difference in RF propagation,
and figure out the snow properties ("retrieve" is the term of art in
the remote sensing biz).
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