Here is the 4Nec2 input file for my shortened antenna model:

Short vertical NEC input file

I welcome any comments on the validity of the model.

Note that I make no claims that this is a super antenna. It works. It appears to suit my needs, given the constraints under which I designed it. I’ve documented my antenna design and building efforts here in my blog mainly so anyone who might be interested in learning about and using antenna modeling software like 4Nec2 might find the information useful. In my case, the model appears to predict behavior that’s consistent with the actual performance of the antenna. This is good–and cool.

It’s high time I posted a few pics of my new antenna. First, the loading coil. This was made from a stock MFJ inductor (3″ dia, 10 turns/inch). the center rod is a nylon rod I purchased from U.S. Plastic, and the brass fittings are 1/8″ NPT couplers and a nipple from McMaster-Carr.

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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

1. I used a single 6-ft aluminum tubing section on the bottom instead of constructing multiple smaller sections.
2. I used another single 6-ft aluminum tubing section above the coil instead of the telescoping antenna.
3. 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.
4. My antenna has a 30″ brass rod forming a tee at the top of the radiating element, as a capacity hat.

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This is my model (so far) for the shortened 20m vertical I’ve been designing, as visualized by 4Nec2.

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Anyone who’s been reading this blog (and I can probably count that number on one hand) knows that I’ve been thinking about a new (to me) antenna design. Being in a covenant-restricted neighborhood, I can’t erect a permanent obvious-looking antenna. But the neighborhood covenants are not extremely restrictive, I have nice neighbors, and there doesn’t appear to be any actual HOA that actively searches for covenant violations, so I figure I can get away with an antenna with a reasonably small visual footprint that I only put up when I want to play radio. To top things off, I have no trees or other readily available antenna supports, so whatever I end up using must be free-standing. The obvious choice, at least to me, is a vertical antenna.

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