Our Friend Pythagoras

Pop quiz: Who can state the Pythagorean Theorem?

Peter A, Bergman N0BLX 3517 Estate Dr. SW Brainerd MN 56401

Because I am an amateur radio operator as well as a member of the Civil Air Palrol, I am often involved in public service and emergency communications. Frequently during these events, we have need for an antenna support more reliable than motor vehicles or Mother Nature.

With this in mind, 1 started experimenting to de\clop the best means for setting up—and keeping up—the portable PVC masts we often use. Bui the method had to be easy for one person to implement, since helpers aren't always there when they'd be most helpful.

There is nothing magical about what I am about to explain, Those of you using a similar method know that it can save a lot of random running around.

it is common practice when erecting a mast to start by attaching the guys and halyard and then anchoring the base to some kind of pi vol After that you pick up the top of the mast, lift it o\er your head and start walking toward the pivot. If you can. Meanwhile, two or three assistants fan out w ith ihe guys to keep ihe mast front falling beyond center Finally, ihe guy anchors are placed and the business of adjusting the lenglhs lo hold the rnast vertical starts, i hen the antenna is hoisted on the halyard.

There's got to be

If you would like to try an easier way. here it is (sec Fig. I), We are going to replace Brownian movement with some planning and just a bit of engineering.

L Select the location for ihe antenna's feed point. This uiil probably he near a spot where you can set up your cornm station. Mark that spot with a small slake. Back off a reasonable distance, set up your camp stove and stan some coffee,

2. If you arc installing a dipole, unroll it and lay it in the desired orientation, with the center insulator at the stake you drove earlier. Attach and unroll the feed line,

3. Place the side gu\ line stakes for the center support mast Measure this distance with a piece of cord the same length as the mast and either drive the stakes now or mark their positions with a puff of builders' chalk; The cord is stored by wrapping it around the chalk buttle. The stake positions can be "eyeballed" or you purists can be more precise by checking the "constructions" chapter of any basic geometry text for information on constructing a perpendicular bisector to a line.

4. At each end insulator, drive another slake.

5. Using the cord and chalk, "puff a circle around each end stake with a radius equal to the height of the mast. You don't need to use a lot of chalk—just enough to tell where the circle is (see Fig. 2).

At the point where the antenna uirc crosses each circle, drive a guy line stake.

7. On the circle you have drawn around each end of ihe dipole, eyeball a point on the side opposite from the antenna and measure one radius each way around die circle. Place anchors at those points,

8. Attaching the precut guy lines is when the plan starts to come together. Remember the Pythagorean Theorem? A-+ B:- C:. What it really says is that the square of the height of the attachment point on the mast plus the square of the distance from the mast to the anchor stake equals the square of the length of the guy line. So, it the mast is 30 feet high and the anchor stake is 30 feel away, then the length of the guy line equals ihe square root of 900 plus 900, which equals 42.5 feel plus a little for good luck.

A 30-loot mast made of three-inch schedule 40 PVC seems to work pretty well with just top guys. If you plan lo use more height or smaller diameter pipe, plan on more guy lines.

V. Attach the antenna insulators lo the masts,

10. Go to one end of ihe antenna and while facing the bottom end of the mast, piek up the lop and place it on your shoulder with the antenna and guys hanging behind you The whole works is going to look pretty sloppy and floppy—especially if you are usin^ one-and-a-half-inch pipe —but have faith and start pulling ihe base toward you. As >uu do so, ihe top will rise and the base will move toward Amateur Radio Today • November 1997 19

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