The Helical Whip for RV and Mobile Use

Figure 3

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.

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.

www.antentop.org

Page- 47