How much antenna height to get Line of Sight to Poor Mountain

I live in montgomery county in Virginia. There's a hill between me and
poor mountain. I'm considering building an antenna to get line of
sight to poor mountain (between montgomery county and roanoke), because
there are a number of Wireless internet providers with transmitters
there.

How can I determine how high I have to build in order to get line of
sight? (what software will do it, if I give it GPS coordinates?)



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Of course, if someone were to build a repeater on the hill across from
the Truck Stop on the IRONTO 128 exit, that would solve my problem. Or
on the crest separating Ellett Valley and Resedale . . .

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http://marriage.jamessmall.com

On 14 Mar 2006 18:29:31 -0800, (E-Mail Removed) wrote:

>I live in montgomery county in Virginia. There's a hill between me and
>poor mountain. I'm considering building an antenna to get line of
>sight to poor mountain (between montgomery county and roanoke), because
>there are a number of Wireless internet providers with transmitters
>there.
>
>How can I determine how high I have to build in order to get line of
>sight? (what software will do it, if I give it GPS coordinates?)

What I use is Radio-Mobile using the USGS SRTM database maps. It's
free. See:
| http://www.cplus.org/rmw/english1.html
See the 2nd photo from the top at:
| http://www.cplus.org/rmw/rme.html
for sample output. (Hint: Use SRTM and not DEM/DTED data).

However, if you assume the earth is flat, you can use simple geometry
to do the antenna height calcs. The sloppy way is to avoid
trigonometry and just use proportional parts.

For example: Assume end #1 of the link is on the ground. The hill in
between 100ft high. The distance from end #1 to the hill is 2 miles.
The distance between the hill and end #2 is 3 miles. How high does
the antenna at end have to be?
100ft / 2 miles = x ft / (2 + 3) miles
x ft = 100ft * 5 miles / 2 miles = 250ft.
You can juggle the proportional parts to make this work with any pair
of towers seperated by a hill. If end #1 is some finite height, just
add it to the calculated height of end #2 as above. If the clearance
to the hill is close, be sure to include Fresnel Zone clearance and
earth curvature.

You also need to be able to see the other end of the link. Worse, if
it's any substantial distance, you'll need to insure that you have at
least 0.8 times the Fresnel Zone clearance. See:
| http://www.terabeam.com/support/calc...esnel-zone.php
for calcs.

For a walk though on how to do link fade margin calculations, see:
| http://en.wikibooks.org/wiki/FAQ_for...k_Calculations

I'm not sure it's such a great idea shooting at or through a heavily
occupied hill/mountain commerical site as you will probably find
interference of some sorts.

Also see:
| http://my.athenet.net/~multiplx/cgi-...eless.main.cgi

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