A VERTICAL HELIX DIPOLE FOR 20 METERS
Next to their rigs (transceivers, receivers, and transmitters), amateur radio operators love their antennas–be they commercially made or “homebrewed” on site from available materials. This weekend I found myself exploring an antenna design used in such devices as the “Ham Stick” , The Outbacker”, and other spiral-wound antennas on a fiberglass support. While these antennas have their drawbacks, they are useful for mobile operations and for severely restricted antenna areas. Equipped with a suitable dipole mount from MFJ, two “Hamsticks” can often be used as an emergency or backup antenna. I keep a 20 and 40 meter “Hamstick” and a mag mount in my van for such emergencies.
What would happen if I used the spiral-wound or helix type of antenna for a backyard vertical to replace the inverted vee and “L” shaped vertical near my house? Most antenna books have something about these antennas. It was worth a try.
According to some older ARRL antenna books I had on my shelf, one could get a 1/4 vertical on a small mast if 1/2 wavelength of wire for your chosen frequency could be spirally wound on a suitable form, such as a fiberglass or pvc mast.
I decided to take the process a step further. I had an old MFJ 33-foot fiberglass mast in the garage (it collapses to ablut 3-feet) that was just begging for my misplaced attention. So, I drew a line mid-way up the mast, wrapped 66-feet of #14 gauge housewire in a helix up and down from the center point. At the center, I attached 50 feet of 450-ohm ladder line, which was connected to a 4:1 balun, and then, through another 15-feet of RG-8 coaxial cable, to the Drake MN-4 tuner. Two 18-inch RG-8X patch cords connected the tuner and low pass filter to the trusty Swan 100MX transceiver.
In effect, what I had was two small vertical helices connected together via each lead of the ladder line. I added four-18-inch brass rods to each end of the helix to provide a bit of capacitive loading.
Before I raised the old MFJ mast and attached it to a hombrewed stake and swivel mount, I weatherproofed the ladder line connection to the helix dipole with clear nail polish and several layers of vinyl electrical tape.
Once I secured the mast and added three dacron guy ropes, I had a two-part vertical helix cut for the 40 meter band. Each half of the helix contained 66-feet of #14 gauge wire, which roughly translated into a quarter wavelength for each element.
Unlike some of my past vertical antenna experiments, this vertical helix dipole did not require a mass of radial wires or a tuned counterpoise. I had a dipole erected in a vertical configuration. I used a similar scheme for a 2 meter window mounted antenna in the shack. In that case, I used a 4-foot wooden dowel to support two, 19-inch elements fed by a short length of RG-8X coax.
PRELIMINARY RESULTS AND CAVEATS:
Most of my antennas are designed for very cramped quarters. My backyard is small, but is large enough to accommodate a vertical with a few radials or a tuned counterpoise. An inverted vee has worked well. My under the house 40 meter loop is a basic NVIS (near vertical incident skywave) antenna that provides excellent state wide coverage from the Ka’u District on Hawaii Island to Princeville on the Island of Kauai. What I wanted with this vertical helix dipole was an antenna that would perform like a normal dipole with out taking too much space. The absence of an extensive radial field was a definite plus.
Initial results show that performance matches that of my 40 meter inverted vee that is fed with 450 ohm ladder/window line. Like the vee, the vertical helix covers all bands between 40 and 10 meters. However, unlike an earlier full-sized 40 meter dipole I “shoe horned” into my small lot, the bandwidth was quite narrow, requiring me to adjust the Drake MN-4 every 15 to 20 Khz. Performance on 15 meters was about the same as on the inverted vee. Twenty and 10 meters were very touchy, but I could use those bands with careful tuning. I could adjust the SWR on all bands from 40 to 10 meters to 1.7 to 1. Although these figures are not impressive, the old Swan 100 MX ran cool and the Drake MN-4 didn’t seem to complain. I ran power levels from 10 to 50 watts. Overall, my cw contacts gave me anywhere from 559 to 579 on 40, 20 and 15 meters. I received one 559 RST reading on 10 meters.
For now, I’m satisfied that the vertical helix works. Of course, more work needs to be done to improve performance. Like my other vertical antenna experiments, this antenna can be lowered to ground level when the operating day is done. This keeps the antenna out of sight most of the time and affords some protection from lightning and high winds. Of course, unused antenna feed lines are disconnected and grounded.
Perhaps you can design a vertical helix of your own. It’s a fun project and requires little time or money to build. I just happened to have some left over wire and an old mast around the garage that seemed to fit the requirements. Let me know how your antenna experiments are turning out.
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Aloha es 73 de, Russ, KH6JRM–BK29jx15–along the beautiful Hamakua Coast of Hawaii Island.
Filed under: Amateur Radio