I was able to steal some time this weekend to construct the 20m shortened vertical I’d been designing using 4Nec2. It didn’t turn out quite like I was planning, but it’s good nevertheless. I used half-inch-dia aluminum tubing and a section of an MFJ prewound coil to build it, and I used construction techniques stolen largely from AD5X’s portable vertical design. The main differences between his design and my implementation are
- I used a single 6-ft aluminum tubing section on the bottom instead of constructing multiple smaller sections.
- I used another single 6-ft aluminum tubing section above the coil instead of the telescoping antenna.
- My coil used only 3″ or so of the MFJ prewound coil, and I used a length of nylon rod instead of wooden dowel. Mine is also tappable with an alligator clip.
- My antenna has a 30″ brass rod forming a tee at the top of the radiating element, as a capacity hat.
In tuning-up yesterday, I was able to find taps on the coil for both 20m and 40m (and I’m sure I can tap it for 30m, too–I just haven’t done so yet). I didn’t get a chance to actually get it on the air, but I found coil taps that yielded SWR of 1.3:1 or so in 20m, and always less than 2:1 on 40m.
So, seeing as how I initially set out to create an antenna for 20m, it’s a little ironic that the first QSO I made with it was a PSK31 QSO with K7SU in Twin Falls, ID running about 30W on 40m. We had solid copy this morning for a decent chat. So, this antenna works better than I would have expected on 40m.
Last night after I finished finding the coil taps for 20m and 40m, I set up 4Nec2 and plugged in an approximate value for the coil inductance for 40m, and I found that I wasn’t too far off from the model. My estimate of 28 uH (made by counting coil turns and using the classic formula for solenoid inductance) was only slightly off the value of 32 uH that 4Nec2 gave me. It’s always cool to see some agreement between the model and real life, eh?