DX RXThe Doctor of DX prescribes the |  |
DX RXThe Doctor of DX prescribes the | |
I apologize for the quality of the diagram. I had to squash it for this column. The original is far easier to read and understand, but the accompanying figure gives you an idea of the antenna layout. Thanks, Don!!
Rather than get into the theory of why this antenna works, I will simply describe it here and let results speak for themselves. If there’s a demand, I’ll do a follow-up article on the relevant theory. The antenna is a center-fed half-wave vertical with about 70 feet of vertical length with the remainder of the top and bottom of the antenna bent horizontal and parallel to each other. The antenna looks like a squared-off letter "C" fed in the middle of the vertical part.
Thus, the 160-meter antenna is a 270-foot dipole fed in the center with the bottom antenna wire bent parallel to the ground about 10 feet off the ground and the top at 80 feet off the ground. The horizontal parts are 100 feet long and parallel to each other. The 80-meter antenna is a 130-foot dipole, fed in the center with 70 feet vertical and 30 feet horizontal 10 feet off the ground and 80 feet off the ground.
Think of it as an inverted L fed against an L. The two Ls are balanced with respect to each other and because currents in the horizontal sections are out of phase, the antenna has a minimum of horizontal radiation.
The 80 meter and 160 meter antennas are separate, fed from a common coaxial feed line. In my case, the 80-meter and 160- meter horizontal sections are about 30 degrees apart. The 160 horizontal wires run east while the 80 wires run east-northeast.
On both bands, the bulk of the current flows
symmetrically in the center of the antenna,
with the current peak about 45 feet off the
ground at the feed point. On 80, the current
loop peak is about 0.16 wavelengths above
ground and on 160, the current loop is about
0.08 wavelength above ground. The
accompanying figure illustrates the antenna.
You can adjust the resonance of the antenna by adjusting the lengths of lower horizontal sections. The small asymmetry doesn’t bother anything. The center impedance of mine at resonance is very close to 50 ohms on 160 and close to 70 ohms on 80. The 160 antenna presents high impedance at 80 meters and the 80-meter antenna looks like a parallel capacitor across the 50-ohm 160 antenna. The 160 antenna covers 1800-1860 with under 2:1 SWR. I needed to take about 2 feet off the horizontal section to get mine resonant at 1830. If 80-meter current flows in the 160 antenna, it tends to flatten the current loop in the vertical section.
The 80 meter antenna is resonant at 3750 with the 130-foot length shown. The VSWR is under 2:1 over the DX part of the ‘phone band. It needs to have some length added to cover the CW portion. I haven’t tried to bring it to resonance in the CW band, but have chosen to feed it through a tuner.
I originally had this antenna hung from trees. This year I put up an 80-foot Rohn- 25G with three sets of guys. I hung the wires from ropes attached to the tower so they are separated from it by about 3 feet. The coaxial feed line comes off perpendicular to the antenna and is then taped to the tower. The center conductor goes to the top and the shield goes to the lower part. Before the coax turns on to the tower, I’ve wrapped some 30 feet of it into a coil. I expected to see a lot of interaction, but the tower and guys seem well off resonance at the operating frequencies and I didn’t see any to worry about.
The first QSO on 80 was VK6LK, long path on SSB at sunset on September 12. I’ve worked a few ZSs, HF0POL, LU, and the usual Europeans. On 160, the first QSO was KP4SN on September 15. In the couple of weeks since hanging it from the tower, I’ve worked ZS6UT, TU2MA, TL5A, VK6VZ, VK6LK, VK3ZL and NL7Z and the usual horde of Europeans on 160 with no fuss. I run 800 watts out from a Ten-Tec Centurion.
Last January, I had about 200,000 points with 750 QSOs in CQWW CW 160 using the predecessor hung from trees.
(Reprinted with permission of Don Toman, K2KQ)
“I don't think you will have much difficulty with 60 feet of vertical. The key to this antenna is symmetry. It needs to have the bent ends equal (more or less) length and (more or less) parallel. You can actually end feed one of the antennas (Zepp style) as an alternative to center feed. The disadvantage is that you don't get to feed 80 and 160 from the same feedline.
My first implementation was hung from a big ash tree. The Feedline was 300-ohm TV twinlead, which worked just fine, attached to an MFJ-989C tuner. The center impedance is close to 50 ohms on 160, so it's easy to feed without a tuner if that's the way you want to do it. Trim to resonance by adjusting the lower horizontal portion length.
I used this antenna from J3A during the 1998 CQ WW CW contest. There, it was coax-fed and about 60 feet vertical. We made over 400 QSOs on 160 with it driven by a barefoot IC-706. Most guys wouldn't believe that it was only 100W.
This antenna will work as well as any grounded quarter wave. The thing that makes for superb performance for a vertical is good ground conductivity, such as in a salt marsh. You can't get good performance with good conductivity only within a quarter wave of the antenna. All that does is minimize return loss. This antenna, because it is balanced, has a minimum of return loss. It will work as well as anything else over whatever ground you have. It will work better at W1KM's place than in the rocky hills of Westchester County, but there's nothing you can do about that!
I have a 30-foot vertical version of this (50 feet horizontal) hung from a pine tree and strung to another at my place on Martha's Vineyard. It's only for 80. It works well.
You certainly may reprint the article and encourage people to contact me if they like. 73, Don...K2KQ (ex-W2UFT from 1947 to 1975...DXCC #206).”
Given the real estate situation that many of us must work with, Don’s solution is an extremely exciting way to have a great deal of fun, and be very competitive, on 80/160 Meters. Many thanks to Don for sharing with us, and I’m looking forward to a couple weeks after Field Day, when I know that a certain “Tiny Elephant” (K2TE) will be more than happy to assist me in ‘shooting’ the tump and support lines for this project.
73 es Good DX, de K7CTW