Snipping the Yagi
Earlier I mentioned the McARCS coastal meeting where we built cheap Yagi-Uda antennas (many thanks to Steve, KJ6EIF.) The design was based on WA5VJB’s cheap yagi, and by “cheap” I mean about $5. The antennas were made from lengths of ordinary house wiring attached to wooden garden stakes. The feed method was a simple half-folded dipole (no gamma match or balun needed.)
Even though we closely followed the instructions, the antennas didn’t tune as well as we expected. They resonated lower than we wanted, and the SWR wasn’t the best. After some experiments we suspected the problem to be the insulation left on the directors (to make the elements stronger in the wind.) That insulation affects the velocity factor, making those elements electrically longer than they should be, and on a Yagi, that creates major problems.
Last weekend I decided it was time to see if the antenna could be fixed, or whether the boom should return to the tomato garden. I discovered that it only took a few clips here and there to make the antenna just about perfect. Here’s what I did:
- Mounted the antenna on a mast with the elements vertical (and mounted it from the end, not the balance point.)
- Connected Steve’s antenna analyzer to the feedpoint. The SWR was about 1.5:1 and resonated at 143.5 MHz.
- Wanting to fix the SWR problem first, I snipped 1 cm from one side of the 1st director. The SWR fell to 1.3!
- Then, trimmed the other side of the 1st director another 1 cm, and the SWR dropped to 1.1:1. Even better!
- Clipped 1 cm from both sides of the 2nd director. No effect (on the analyzer that is, but maybe in the gain and pattern.)
- Thinking I was on a roll, I snipped 1 cm from both sides of the reflector. Whoops… the SWR climbed just a hair.
- Now the big decision was how to deal with the driven element. It’s not easy to cut because half of it is a loop. Throwing caution to the wind, I just snipped 1 cm from the non-loop side and bingo! The resonant frequency jumped to 146.23 with an SWR around 1.1:1.
- Finally, I clipped 0.5 cm from each side of the 1st director. The SWR landed at 1:1!
Hurray! The antenna was right where I wanted it.
I measured the SWR as 1.32 at 145, 1.02 at 146, and 1.3 at 147. That’s just about right for the FM part of the 2M band.
Now, I just need to figure out where to bolt it outside, and then… do I want to point it toward the Mendocino coast, toward the Bay Area, Eureka, or Clear Lake? Decisions, decisions. Hey Steve, could next year’s workshop be about building our own rotators for $5?