[TowerTalk] Tree attenuation
jimlux at earthlink.net
Mon Nov 7 17:37:19 EST 2005
At 01:24 PM 11/7/2005, Jim Brown wrote:
>On Mon, 07 Nov 2005 12:15:59 -0800, Jim Lux wrote:
> >As far as HF goes, it gets quite a bit more complicated(!), for two reasons:
> >1) The forest scale (height of trees, etc.) is on the same order as the
> >wavelength (at VHF, trees are MUCH bigger than a wavelength, and leaves
> >are MUCH smaller).
> >2) Most practical applications of a model have the antenna in the forest,
> >where the forest is in the near field of the antenna.
>I skimmed both the Power Point and the longer document. The analysis that
>into the Power Point make it considerably more valuable in guestimating what
>might happen at VHF and low UHF, but neither gives much insight into HF.
>The obvious question is, is anyone aware of anything equally good for the HF
>spectrum? My future antenna supports are likely to be redwoods, 25-30 meters
The paper you want is by Theodore Tamir, "On Radio-Wave Propagation in
Forest Environments", IEEE Trans on Ant and Prop, Vol AP-15, #6, Nov 1967,
He covers 1-100 MHz, and lots of experimental and theoretical analysis..
1) For constant antenna height, received field varies inversely as distance
squared [RMK note: oddly, irrespective of the terrain when forested]
2) The presence of vegetation produces a constant loss, which seems to be
independent of the distance between the communication terminals.
3) The transmission loss is reduced by raising either receiving or
transmitting antenna. This produces a "height gain" effect which, when
measured in dB various roughly logarithmically or linearly with antenna
height. [RMK note: This isn't the same thing as the Longley-Rice height
effect familiar to VHF/UHF predictions]
4) The received field may be considerably depolarized, relative to the
orientation of the transmitting antenna.
Some more notes.. below about 12 MHz, the height gain effect was too small
to be detected.
You can model the forest as a dielectric with epsilon=1.1 and sigma=0.1
There's a factor you apply that depends on the distance from the antenna to
the "top" of the forest, and it runs around 0.25 dB/m up to 1.1 dB/m,
depending on the the dielectric properties.
The middle of the curve starts at around 0.2 dB/m for 1 MHz, rises steadily
to 0.5 dB/m for about 15 MhHz then sort of flattens out to 0.6 dB/m up to
400 MHz. (Figure 8 in the paper)
(that distance is the distance from antenna to TOP of the forest... so if
your antenna is up 10m and the forest is 15m tall, you'd have about 2.5 dB
Note that this for both receiver and transmitter in forest. If only one is
in the forest, cut those in half. (Tamir's analysis assumes skywave
propagation, by the way).
>Jim Brown K9YC
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