The Helical Whip for RV and Mobile Use

The Helical Whip for RV and Mobile Use

By: J. T. McCullough, W0BHG

When I retired I found I needed to reduce the complexity of my antennas for several bands, and simplify their mounting and connections. I needed to cut down on the time I spent setting up and disassembling antennas for recreational vehicle (RV) and motor home use.

Advantages

The antennas presented here will also work well for ordinary mobile operations. They can be adapted easily for home use in tight space situation in an attic, on a porch roof, or on a small lot. While it is quite likely that the helical vertical will perform any better that a full quarter-wave vertical in the same situation, the difference can be rather negligible. Helicals can also be used back to back for a one half wave dipole or horizontal dipole. They will work as radials for a ground plane antenna too.

For temporary or permanent use on one WARC band, attach the helical vertically to the framework of a metal beam assembly near feedpoint. Then connect the base directly to the coax feed. The framework and elements of the beam provide the ground plane. By mounting the helicals at slight angle from each other, you could use two or three new bands in the same way.

It appears that at certain times of day, especially in the early evening, stronger DX signals may be obtained from 10, 15, and 20-meter helicals atop a metal-covered RV than from much more sophisticated antennas in the same location. This may be because the metal covering of an RV makes an almost perfect ground plane.

Development

I started using helical whips with a large magnet mount about two years ago. My first was a CB antenna, modified for 20 meters. It was fantastic! I used it atop my pickup truck while traveling, and on my trailer or motor home at campsites.

Photo A Five-band helical antenna on single spring mount

I worked a lot of DX, including all continents. (More about modifying the CB antenna later)

I wanted to work five bands: 10, 12, 15, 17, and 20 meters. To do this I tied all five helicals together near the feedpoint. I started with a heavy 4- inch spring mount. Then I made a 2-1/4 inch square plate out of 1/16 inch thick aluminum and drilled five 3/8-inch holes- one in the middle and one in each corner.. I bent each corner down about 15 degrees. I installed on antenna in the center hole to hold the plate in place on the top of a spring mount. Next I put one antenna in each of the other four holes, using lock washers and 3/8-24 nuts. See Photo A and Figure 1 for details.

Performance Comments

There appears to be no significant interaction between the five antennas, with the positive exception of the center unit.

Each was developed individually using a method I will be describe later, so that VSWR was lowest in the center of the desired band.

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