A Portable, Multiband Vertical Using the DK9SQ Mast
Special to The ARS Sojourner
DK9SQ's mast is a telescoping, fiberglass mast that extends to 33 feet, collapses to 46 inches, and weighs just 2.2 pounds. The sections each taper slightly, and twist together to lock. I've used it to support a number of wire antennas, but my favorite design is a full-size, quarter-wave vertical for 40 meters.
Now, I've never been a big fan of verticals, probably because most of my experience has been with commercial trap verticals. They never seem to cut it compared to dipoles, end fed wires, and loops. And for field work, I've always found radials to be inconvenient. They tangle, break, and require a lot of space. Two things convinced me to try a vertical design with the DK9SQ mast. First, I've been using Teflon insulated #24 wire for lightweight wire antennas, and found that the slippery insulation resists tangling. Also, I read in the ARRL "Antenna Book" and several other places that radials don't have to be a quarter wave in length. Instead, short radials would work if at least 16 were used. As an experiment, I hung a 33-foot Teflon wire from the mast, and cut 16 seven-foot radials. I set the antenna up as a ground-mounted vertical, the configuration that I would use in the field.
To meet my multiband requirement, I tried feeding the vertical with balanced line. I've used ladder line or transmit-type twin lead for wire antennas for many years. I've had good results, using an antenna tuner to let the antenna work multiple bands. I tried the same method on the vertical, and found that I could tune to 1.5:1 or better SWR on 80 through 10 meters, including the WARC bands. I haven't done any kind of technical analysis on the vertical, but on the air it compares well enough with my home antenna, a 40-meter horizontal loop up 40 feet, that I built a permanent version from aluminum tubing and mounted it on the roof. But that's another story.
K7BUG and I worked 57 stations during QRP To The Field 2000, using the DK9SQ supported vertical, and an 80-meter delta loop supported from a tree. Both antennas tuned up well using the automatic antenna tuner in the Elecraft K2. Both seemed to hear about the same, with some stations favored slightly by one antenna or the other. On the following Saturday, K7BUG, AE7I, and I set up a demonstration station at the local Scout Expo in Flagstaff. Unfortunately, the bands were nearly dead during the four-hour event, but the portable vertical made it possible to set up our entire station in fifteen minutes.
The mast itself is supported at the mid point with three guy lines made from 3/32" Dacron cord. To attach the guys, I used a nylon tie wrap around the mast, just above the highest section that has a metal reinforcing ring around its top. I leave the guy lines attached to the tie wrap, and slide it down the mast into place as I extend it. The lower ends of the guys are attached to plastic tent pegs, bushes, rocks, trees, or whatever's handy. I put just a slight tension on the guy lines- too much might collapse the mast. Occasionally, a wooden fence post or a tree offers an easier way- I just lash the bottom section of the mast to the support and don't mess with the guys. It takes very little to support the light, slender mast, even in strong wind.
I found that the top section tapers so gradually that the radiator tends to slip down several feet. I solved this problem by attaching the wire to a small loop made from a small nylon tie wrap. Then I pulled the tie wrap snug around the top inch of the mast, and cut off the excess tie wrap. Now I can slip the tiny loop over the top of the mast, and it only slips an inch or so. The radiator hangs down the side of the mast, and attaches to a loose tie wrap ring at the base. This serves to keep the feed line from pulling the radiator away from the mast.
All sixteen of the short radials attach to a loose ring made from #14 stranded wire. I stripped about 2 inches of insulation from each radial, and wrapped around the wire ring several times. I soldered just the end of the radial, leaving a few wraps loose so that the radial could flex without breaking at the solder joint. Both a short pigtail from the radial ring, and end of the radiator, are attached to a 2-conductor jack that I bought from Radio Shack years ago. Since the wires tend to pull out of its screw terminals, I will probably replace it with a small Molex connector.
I keep the radials rubber-banded together in a single bunch. To deploy them, I first slip the attachment ring over the base of the erected mast, and then lay the radials together on the ground. Since I can pull lightly against the base of the mast, it's easy to separate each radial and fan them out on the ground.
The final step is to plug in the twin lead feeder and attach it to the tuner. In the case of the K2's ATU, which doesn't have a balanced input, I use a homebrew 4:1 balun. The Emtech ZM-2 Z-match has a built-in balun, like most manual antenna tuners, so I can attach the twin lead directly to its balanced input. With either tuner, the vertical tunes easily from 40 meters up, and with a little more difficulty, on 80 meters. Operating with the automatic tuner is a joy. Once the tuner has memorized the setting for each band (which takes about 5 seconds per band), I can QSY instantly to any band.
To take the antenna down, first I detach and coil the feed line. Then I gather the radials at the base of the mast and fold the whole bunch back and forth into my hand. I fold the bunch about every 6 inches, in a sort of accordion fold, then rubber band it. I find this type of "overlay" coil tangles much less than a coil made by winding around my hand. It deploys almost instantly. Then I bring the mast down by unlocking each section from the base up. I fold the radiator using the same method I used on the radials, rubber band it, and slip both rings off the mast. Finally, I coil the guy lines, stow everything except the mast in a one gallon zipper plastic bag, and I'm done.
The DK9SQ mast is available from Kanga USA for $99 plus shipping.
****
Bruce Grubbs, N7CEE, has been active in QRP for the last fifteen years, and enjoys
homebrewing, kit building, and field contesting. His favorite wilderness activity is exploring the Grand Canyon on trips of a week or more, and he also enjoys long backpack trips in the mountains of the west.