Super-sizing the “cheap Yagi” (Part 1)
One of the frustrations of doing VHF on the cheap is getting enough gain to make your low-power signal loud (or simply being heard) at the other end. I had a couple of options with my 6-element WA5VJB “cheap Yagi” on 2 meters:
- Increase the height of the antenna. This is impractical at the present QTH without installing a tower. Actually, the tower would have been possible but I wasn’t ready shoot first and ask questions later with it. Nor was I ready to have my folks spring some Rohn 25G out of storage in their garage for the trip here when I had the opportunity (a truck bringing some furniture from them).
- Run lower-loss cable. I have regular old RG-8 (PE dielectric) running up to the antenna. It’s only about a 50-ft run. So, I’d be hard-pressed to do a lot better. I did figure out how to recycle improperly-installed N-connectors for LMR-600 from a dumpster-diving excursion. Although I have twenty-some connectors, I haven’t yet secured any scraps of LMR-600 to use. This is a future consideration. At $1.50/ft, LMR-600 would still cost $75. No deal.
- Stack multiple 6-element antennas. This is actually a good idea that I’m keeping in the back of my head for the future. It would be nice to do something like this. Maybe some day.
- Launch a rocket to do a chemical release whenever/wherever I needed a sporadic-E layer. Unfortunately, you can’t launch rockets over land. (Update: I was reminded later that this is not 100% correct.) Furthermore, at a megabuck per shot, it’s not cost-effective.
- Dispense with the 6-element design and go for something bigger.
I elected option #5.
The first step was to consider suitable designs. I tried scaling the 11-element 432-MHz cheap Yagi to 144-MHz. Fail. A NEC model showed that the pattern stunk and the input impedance was pretty far from 50 ohms. Knowing that W5UN had built an array of wood-boom antennas for his EME setup, I looked into readily-designed options.
The ARRL Handbook (1993 edition for reference) and ARRL Antenna book (18th edition) have the K1FO optimized Yagi designs in them. This antenna has been around for a number of years (clearly) and is available commercially from Directive Systems. It seemed like a relatively good choice. So, I moved forward with it…