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?)
-- 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 . . .
What I use is Radio-Mobile using the USGS SRTM database maps. It's free. See: |
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the 2nd photo from the top at: |
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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: |
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calcs.
For a walk though on how to do link fade margin calculations, see: |
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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.
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