AN ALTIMETER FOR YOUR TRAILSIDE ANTENNA
The ARS Sojourner
Using sophisticated terrain modeling software, Carpenter explained why a 40 meter dipole is a much better performer at 70 feet than at the more common 30-35 feet elevation.
Even before reading the article I'd often wondered how high my portable antennas really were. Just because my dipole "looked pretty high" didn't necessarily make it so. So I set out make a simple device to measure antenna height. My goal was to create something cheap, light weight, easy to use and accurate.
My first thought was to just add a few strips of tape, spaced at regular intervals, to my antenna feedline. That proved unsatisfactory for several reasons. First, I don't always use the same feedline with a given antenna, and I really didn't want to mark and measure every piece of coaxial cable and ladder line I own.
A second problem arose when I realized that I rarely erect my portable antennas over flat terrain. During the 1997 Flight of the Bumblebees sprint, one end of my 40 meter dipole was only about 35 feet above the ground while the other end was a hundred or more feet high. Yet, the antenna itself was almost perfectly level with the horizon.
So, how high was the antenna?
After considerable thought I realized that what I really needed was the dry land equivalent of a nautical sounding line. A sounding line is simply a length of rope with calibration marks every few feet.
A lead weight is attached to one end of the rope. In use, the weighted end of the rope is tossed overboard and allowed to spool off a storage reel until the weight hits bottom. Then the calibration marks are read to determine how deep the water is at that point. The same principle was applied to a homebrew antenna altimeter.
The heart of my antenna altimeter is a 100 foot length of nylon mason line with calibration marks placed every ten feet. The marks are made with a felt-tipped permanent marker.
One end of the nylon line is formed into a loop which is fed onto an oval shaped, splitring key chain as shown in Figure 1 (Figure 1 is a photograph of the key chain, nylon line and antenna wire). The knot that forms the loop is coated with a one or two drops of Super Glue to keep it from unraveling.
In use, the key ring is fed onto one leg of a dipole antenna to be measured. Raise the antenna while allowing the nylon line to spool off the storage reel. With the antenna at its final height, subtract the number of ten foot increments of line laying on the ground from the overall length. That is the actual height of the antenna above ground.
If the terrain is sloping, you can move the key ring along the dipole leg to any point between the center and the end.
When not in use, the nylon mason line is wrapped around a plastic reel taken from a broken video cassette. The idea of salvaging parts from VHS cassettes was suggested by Walter Dufrain, AG5P, in an e-mail post to the QRP-L Internet Mail Group on May 4th.
While the antenna altimeter certainly doesn't qualify as a high-tech device it is nonetheless a useful addition to my field antenna storage bag. If you hear me on the air and I tell you my antenna is 80 feet high, believe it.
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Bill Jones, KD7S, is a technical editor for The ARS Sojourner, an avid builder and outdoorsman living in Sanger, CA.