Whip Antenna

A Continuously Loaded Helically Wound Vertical Whip Antenna

During September 2001, I purchased an Autek Research VA1 -- an outstandingly versatile instrument, by the way! In order to test it out and learn how to use it, I decided to tackle a compact antenna project. My original idea was to make an antenna suitable for Pedestrian Mobile operation. So far I have only used the antenna for fixed station operation, however. (The antenna has held up well to severe winter WX and high winds, I might mention.) Anyway, I thought that a discussion of what I did might be helpful to others interested in building a compact vertical antenna. I started with a one-half inch diameter, eight-foot long surplus fiberglass rod that I bought at a garden supply place many years ago. Then I took 45 feet of wire (stranded, insulated, #22 if I recall correctly) from my "every day is Field Day" dipole that I finally took down (about three months after Field Day). I wound the wire on the rod so that it covered about six and one-half feet of the length. I never got around to measuring the pitch, but it was probably about three-sixteenths inch per turn. I attached the vertical to a metal fence post so that the feed point was about five feet above ground. Using the VA1 I tuned for resonance (zero phase angle) which I found to be around 13.5 MHz. So far so good. I checked the feedpoint impedance and found it to be about 150 ohms. Next I clipped on some temporary 12 foot radials that I used for an earlier experiment with my Hustler mobile antenna. With six radials the feedpoint impedance was about 56 ohms, and with eight radials it was 52 ohms.

I made up six permanent radials, seven and one-half feet long each, using the other 45 feet of wire from the dipole and installed these. Three are horizontal or slightly drooping running parallel to the top of the fence, and the other three slope down to the top of some three-foot tall fiberglass electric fence posts. With this setup the feedpoint impedance was about 55 ohms. Next I added a top hat made from four one-foot pieces of aluminum wire. This lowered the resonant frequency to about 10.5 MHz. I started uncoiling wire from the rod, spacing out the remaining turns to keep the full six and one-half foot length. I checked for resonance every few feet. After removing a total of 14 feet of wire I ended up with a resonant frequency of 14.140 MHz and a feedpoint impedance of about 54 ohms. The top two feet of the antenna are still wound with a pitch of about three-sixteenths of an inch, and the bottom four and one-half feet have a winding pitch of about three-eigths to one-half inch. I wrapped black vinyl tape around the rod every foot to keep the windings tight. I connected the antenna to my coax (about 100 feet of RG-8, previously used for a Butternut HF2V) and went in the shack to run some tests. I made a few SSB contacts with my Ten-Tec Argosy and a couple with my FT-817. Later that night with the FT-817 I worked an OH on 20m CW. (He was running 500w to a Yagi and was 589. He gave me a 559, and seemed impressed to learn that I was running 5w to a 2m-tall vertical!) The antenna appears to perform acceptably and is small enough to carry into the field, although you might want to remove the top hat for transportation. According to articles that I have read on short verticals the feedpoint impedance over a good ground should be quite a bit lower, so I no doubt still have significant ground losses.

Here is the picture:

A terrible picture! Overall mast length is eight feet, with the antenna wound on the top 6.5 feet. The fence post is at a "Y" junction of three sections of chain link fence, one tall section (visible) and two short sections.