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These past few months, I’ve been spending a lot of my Saturdays volunteering at the California Historical Radio Society. This is a great little museum in Alameda I would recommend for any ham or radio enthusiast. I have spent a lot of time operating in the W6CF ham shack, mostly on phone modes but also introducing the museum to modern digital modes such as FT-8. It has been a fun and rewarding experience. I’ve also gotten some pointers on antenna design from some of the other volunteers. On my free days when I’ve not been at the museum, I’ve been touring various Parks on the Air (POTA) parks in the Bay Area, tweaking my antenna design as I go.
Now that I’ve upgraded to the General License class, I wanted to build an antenna that could work well on multiple HF bands. My old monoband 10m dipole has served me well, but I needed something more versatile. My requirements are as follows:
1. It needs to have a relatively small footprint and work with my existing 13 foot mast. Ideally less than 20 feet for the radiating element.
2. It needs to work on multiple bands, ideally 10m, 15m, and 20m.
I also have a built in tuner on my rig, so I was drawn towards a random wire antenna (https://en.wikipedia.org/wiki/Random_wir...) design with a 9:1 unun. Ideally a random wire antenna would be longer than 20 feet, but I wanted to see what I can get done on a shorter length of wire. For my first design, I simply cut a 17 foot length of speaker wire, which gave me two wires of equal length. I used one for the radiating element and one for the radial. I deployed this iteration at The Presidio (US-7889) (fig. 2) to decent success. It performed somewhat well on 20 and 15 meters. Achieving a 2:1 SWR for 15m and a 1.5:1 SWR for 20m. Not awful, but not great. All in all not bad for the shorter random wire length, but I knew I could do better.
After my first attempt, I reported back to the museum with the results of this initial design. The recommendation I received was to add more radials. Unfortunately I was out of wire at home. Fortunately, the museum has an excess of random wire and other junk, so someone sent me home with a spool of wire. Coming home, I twisted together three 8.5 foot segments and crimped them with a fork terminal. At this point my design featured the one 17 foot radial and 3 8.5 foot radials (fig. 3). My second attempt I went to McLaughlin Eastshore State Park (US-3427) for a POTA activation, and had great success, with little change in SWR readings. I managed to make a contact from California to Sweden (SM3NRY) on this day. After this, I was curious what more radials would do.
The design I've settled on now additionally includes three 11.5 foot radials. The final tally of radials includes one 17 foot radial, three 8.5 foot radials, and three 11.5 radials. (fig. 4) I've tried this design out for POTA activations at the Albany State Marine Reserve (US-3394) as well as Rosie the Riveter National Park (US-0754). Taking SWR readings once more I found it to be little changed on 15m, but on 20m I have achieved a segment of resonance just around the general class privileges on 20m (fig. 5)! Although it doesn't perfectly line up with general class privileges, it is pretty close. I've settled on this design for now. Here's hoping it serves me well for many future POTA activations to come!