The Helical Whip for RV and Mobile Use

Figure 4

Wind all the remaining wire, except for the last 12 or 14 inches, close spaced on the loading coil form. The remaining 12 inches or so of the wire can be loosely spiraled up the pole to hold the wire steady for testing. You can use tape or epoxy to hold turns in place where needed, like the beginning and end of the loading coil.

Noe you are ready to screw the assembly onto the spring magnet mount. Place the antenna on the ground plane in the yard or on top of the vehicle. Use a grid- dip meter and VSWR test equipment as described in the section on testing.

It may be necessary to cut off half or more of the last 12 inches of wire. If, after cutting it all off, the resonant frequency is too low, remove a few turns from the top of the loading coil and extend the last 12 inches as before. Then recheck the resonance with a grid- dip meter. This may all sounds like a lot guesswork, but it seems that you have used a prescribed amount of wire, the antenna will almost always work out when vary last 12 inches.

All the bands can have nearly the same pole length of about 5 feet, if you desire. The big variable is loading coil. Removing 1 inch from the tip will make a rather large difference in resonant frequency, but you have need remove several inches from the loading coil to make a similar difference.

Tunable Tip

You can make the tip adjustable if you wish by using telescoping sections from TV rabbit ears, old auto or portable radio antennas, and so forth. Use only two sections; choose that that will fit the rod above the loading coil after cutting off excess fiber glass rod Epoxy the tip onto the rod after sanding of spot of chrome to allow for easy soldering of the wire end. The total midadjustment length of the telescoping section will be a little shorter than the length of wire it replaces because of greater capacitance of the tubing. Recheck the VSWR, and if the midrange setting hits the desired VSWR at the frequency selected, the section is ready for completion- after you plug the hole in the end of the tip.

With the exception of the sliding tip, you can use heat- shrink tubing over the length of the antenna. This can be a bit difficult because of the larger diameter of the loading coil, and possibly the base. It may be expedient to shrink tubing over the steam area before sliding on the loading coil form. Have two or three sizes of heat- shrink tubing on hand. If places remain where the tubing does not fit closely, fill the gap with epoxy and sand smooth.

To shrink the tubing on long items like antenna, try a toaster oven. This is another easy to find garage sale item, you can often find one for $ 3 to $ 5. You can dedicate your oven to this type of service by cutting a 3 or 4 inch diameter hole in the back.

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