Final result : Momobeam Mono 4-20 and the Optibeam OB1-30
What was originally planned for the IARU HF World Championship in mid-July, I ended up completing a few months earlier with the CQ WPX Contest at the end of May in mind. This time, it’s a 4-element for 20m from Momobeam, so it can be used in more conditions and over more years than a 10 or 15 meter antenna.
This builds further on the 10m monoband project from a few years ago, with the same foundation: easy ground-level assembly of a large antenna without needing additional people.
The added challenge here is the mandatory suspension of the antenna. In my 10m project, I had already incorporated the option of an extra tube acting as support for suspending the yagi, but I could omit it if another antenna was mounted higher.
This time, installing a suspension was a must—the boom length is 8.6 meters—and with a dipole above the yagi, this creates issues: the extra tube needs to tilt over the motor’s top tube, otherwise the suspension ropes would get blocked against it.
I found the solution by installing two suspension points, high enough above the yagi to provide a sufficient pulling angle, but just low enough to avoid touching the dipole. This way, both ropes can be tensioned properly.
This antenna, weighing 33 kg with a wind load of 0.92 m², is pushing the limits of my mini tilt plate (rated at 35 kg and 0.93 m², albeit at 160 km/h wind). The plan is to lower the mast at wind speeds of 90 km/h or more.
It’s still quite something to see these dimensions laid out on the ground—this is no small antenna.
The installation went flawlessly. The instructions and labeling from Momobeam are truly excellent. The only calculated risk was raising it in 45 km/h wind. It’s hard to predict in advance how the mast will behave during tilting when a gust hits.
As a quick test, I played around on FT8 with the antenna at just 9 meters height. The results look quite good, and the SWR is perfectly flat in the CW segment, exactly as intended.
Stations that received my signal, yagi directed to the USA.
