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[TowerTalk] 1

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Subject: [TowerTalk] 1
From: (Tom Rauch)
Date: Sun, 8 Aug 1999 14:13:13 -0400
> In a heavily vegetated region, the reflection plane (for HF) is
> actually above ground by approximately the amount of the height
> of the vegetation.  You should figure the "effective height" of
> your horizontal antennas as the distance above the plane where
> the effective dielectric constant begins to differ significantly
> from 1.0.  In the horizontal plane, the important part of this
> region begins at about one wavelength from your tower and extends
> out to 20 (or more depending on antenna height) wavelengths
> radius.

Can you point me to some reference where I can find the effective 
d.c. or loss of a stand of trees at HF?? 

Trees are not like a lake, where it 100% saturated with the lossy 
media. There is one heck of a lot of air mixed in with the trees, and 
below the canopy there is often a large air-gap with only the 
occasional trunk or bush.

It seems to me if the woods was thick enough to look like a solid 
wall of dielectric at HF, neither light nor people nor animals could 
wade through the mess. I'd wager there is NOT an abrupt change in 
anything at the tree level at HF, since even 144 MHz signals are 
barely affected.

73, Tom W8JI

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