Copyright © 1999-2011 by Harold Melton KV5R. All Rights Reserved.
Understanding Shortwave Antennas: Page 16
Support Planning And Erection Safety
It’s a pretty good idea to plan and graph any proposed erection which could fall into power lines — as opposed to killing yourself like several people per year do. This is easy to do using graph paper or a computer drawing program with a grid.
In the following example, a 40-foot aluminum tower will be anchored to a tilting base plate, bolted to a concrete pad, then attached to the house gable at 13 feet, then guyed near the top (39 feet) with one set of three 3/16″ braided steel cables with turnbuckles. Up to 20 feet of 1½″ heavy-wall steel mast pipe (commercial fence-post stock) will stand atop the tower, giving a height of 60 feet. The upper pipe mast will be guyed with three regular 7-strand galvanized guy wires.
Positioning the power line at the proper height and distance, then drawing an arc, will show where the tower must be placed, and the maximum height at that location, that will clear the lines.
To further eliminate the possibility of a line strike, the tower may be guyed with an additional safety cable extending away from the power lines. This will ensure that the top pole would topple before the tower, thus reducing the arc of fall to 40 feet. This failure mode assumes tornadic wind conditions. If it would take tornadic winds to topple the tower, the same would topple the power lines and poles! Thus, well-anchored steel cables, rated at over 2500 pounds working tensile, will totally prevent a power line strike.
Most electrocutions occur during tower erection. When pulling up an assembled tower or pole, at the very least a safety rope should be tied to the top, extending perpendicular, away from the power lines, tied securely to a tree trunk. If the tower falls during erection, the safety line will swing it down parallel to the power lines. Never try to man-handle a tower into position without safety lines !! One extra helper is ten times better!
No antenna, pole, mast, or tower should ever be raised alone, nor by unsupervised children.
Well, that’s all for now, folks! I hope this article has been helpful. If you wish to continue learning antenna theory and construction, obtain a few good books on the subject. I recommend you start with The ARRL Antenna Book.
73, de KV5R
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