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Re: [TowerTalk] Effective of ground snow on Antenna radiationpattern

To: Martin AA6E <>,Towertalk <>
Subject: Re: [TowerTalk] Effective of ground snow on Antenna radiationpattern
From: Jim Lux <>
Date: Sun, 14 Jan 2007 07:08:17 -0800
List-post: <>
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.
>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 
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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