The length should be one half wavelength plus approximately
12 inches. To find the length for one- half wavelength in feet,
divide 468by the desired frequency in MHz.
You can do the winding by hand, with help of another person, or
by improvising a jig like that in Figure
4. If you have access to a 0 to 120- V Variac,
you can use a 3/8-inch or larger electric drill (see Figure 4). If necessary, use a handle
with 3/8- inch or larger drill chuck mounted on it, instead of
the electric drill setup shown.
With the shaft turning slowly, wind on about one-
half or a little more of the wire allotted, starting with spacing
of about 1 inch.
As you reach the loading coil position, gradually
reduce until the turns are barely separated. The bottom of the
loading coil should be roughly 12 inches from the top of the pole.
Loading Coil
The loading coil offers several variations. You
are, of course, dealing with low power here- in the area of 100
watts or less.
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This is definitely not a high powered linear amplifier.
If you are using a "fat" pole (5/16- inch
or larger diameter at the loading coil position), you can wind
the loading coil directly on the pole- especially for 10 or 12
meter antenna. However, VSWR usually seems to be better with larger
diameter. A loading coil of ½- inch PVC pipe works well,
but if fiber glass poles with larger ends are available (say1/2
inch or slightly larger) a short section cut from the larger end
would be lighter and better.
You will have to improvise when mounting the coil form.
Corks with a hole in the center and some epoxy will do the trick.
Cut or grind off the excess part. The small mushroom
shaped wooden buttons used for covering screw holes in furniture
work nicely for ½- inch PVC by enlarging the hole in the
end of the pipe. These buttons usually cost a dime or less at
most hardware stores.
If the loading coil is approximately
½- inch in diameter, it should be about 3 inches long for
10 and 12 meters, 4 inches long for 15 and 17 meters, and about
5 inches long for 20 meters.
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