By Douglas Ruz, CO8DM
I live on a 75 × 30 foot city lot, which is very small for traditional 80 or 160 meter antennas. After 27 years in ham radio, I finally installed a tower, and mounted a tri-band Spiderbeam with two shortened elements on 40 meters. That combination works well from 40 to 10 meters, but I need something decent for 80 and 160 meters if I want good DX and contest results. I call my solution for an 80-meter antenna the “Lazy M.”
The 80-Meter Solution
I searched for a solution on the Internet, in textbooks and magazines, and by sending e-mails to various reflectors. In the end, I decided to experiment with MMANA, a free software for modeling antennas. After many days of head scratching, I found the solution for my 80 meter antenna. I called it the Lazy M because it looks like an M resting on its side . This antenna is a bent vertical dipole that is fed in the center. The lead photo shows it installed at my home station. I cut the antenna wire into 74 foot 8 inch dipole segments for each arm to obtain resonance on 3650 kHz. I bent each dipole segment in the middle to make the shape of an M on its side. The resistance at the feed point was about 15 W, predicted by the software. I did not model the effect of the 50-foot tower, but found by experimentation that, to obtain 50 W, I needed to connect a shunt coil across the feed point. The bandwidth was around 150 kHz for a 2:1 SWR. Figures show the feed-point details.
Construction and Installation
I don’t have a sophisticated antenna analyzer, so I tried several coils that I found in my junk box. Luckily, I got 1:1 SWR with one of those coils. That shunt-matching coil has eight closely spaced turns of #14 AWG insulated wire wound on a 1 inch diameter piece of PVC pipe. The exact dimensions are not very critical. Figure shows how the RG-223 coax from the shack is connected to the antenna wires. First, I fashioned a current choke made from 12 turns of the RG-223 coax feed line in a 12-inch diameter bundle, and connected the center and shield of the coax, each to an end of the dipole segment. Finally, I attached the shunt-matching coil across the feed point. To support the Lazy M antenna, I put a 20-foot aluminum mast on top of an existing 10 foot tall structure, about 50 feet from the 50 foot tall tower. I used four insulators to keep the wires suspended in the air. Two insulators are at the top and middle of the tower, and another two are at the top and middle of the aluminum mast, held tight by UV-resistant ropes. I mounted the feed point assembly using a pulley system so I could conveniently lower and raise it during adjustments.
I have completed many contacts with North America and Europe, and received good signal reports using a Yaesu FT-747 with just 100 W. Even in June with poor propagation, the skimmers reveal that this antenna is a good solution for the low bands; they show that my signals are heard all over Europe and North America with signal to noise ratios between 6 and 30 dB. As a future project, I intend to build the Lazy M for 160 meters. I hope this helps many fellow hams to work on the low bands. Any suggestions would be appreciated. (Published in QST)