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.
Up to now I’ve been using XLog for logging ham radio QSOs in Ubuntu, but I gotta say I’ve never really liked it much. The user interface leaves a lot to be desired in terms of being unambiguous. I’d been using XMLog for doing my general logging under Windows–it’s a great free program that allows me to easily import contacts from my contest logger and exports to ADIF for sending to Logbook of the World. So, this morning I decided to see if I could get XMLog to run under Wine in Ubuntu 7.10.
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.
This is my model (so far) for the shortened 20m vertical I’ve been designing, as visualized by 4Nec2.
Last week I told you about how I’d gotten a nice free antenna modeling package for Windows called 4Nec2 to run in Linux using Wine. Since then, I’ve been able to create a few models and run them, and I gotta say I’m really impressed with the capabilities of 4Nec2. I doubt I’ve even scratched the surface of what’s possible, but it’s worked flawlessly for me so far. I have experienced a few minor glitches that are the result of running under Wine, but those are minor and easily worked around. Mostly, those glitches are associated with trying to run the help file, and I haven’t spent any time trying to fix those things. No big deal, as far as I’m concerned, and certainly not a knock on 4Nec2 itself.
I’ve already written a couple of times about my efforts to use antenna modeling software to design a new antenna for the back yard. My original plan for a four-band vertical is perhaps overly-ambitious, so I may scale back to designing some sort of short vertical for 20m that’s both sturdy enough and inconspicuous enough to leave erected in the back yard for more than a few hours at a time. In the mean time, I continue to use Phil Salas’ ultimate portable vertical when I get the chance. But I digress.
Since I launched this little project, I managed to sidetrack myself by converting completely over from Windows XP to Ubuntu Linux. If you know me (or perhaps you can detect this from my blog entries), you know that I tend to be thinking about several different ideas at the same time, and this often results in my making little or no progress on any of them because I can’t focus on any single idea for very long. Maybe it’s an attention span deficit or something. And such appears to be the case with my antenna project. But while I continue to enjoy learning about the wonders of Linux (and I confess I’m thoroughly enjoying Ubuntu so far), I figured it was time to get back to antenna designing.
This weekend I took the plunge big-time, completely replacing my Windows XP desktop installation with Ubuntu 7.10. Being a ham, my computer setup is a little more complex than normal because I use my computer to control my ham radio (an ICOM IC706MKIIG) for contesting and for making contacts in digital modes. I use a West Mountain Radio Rigblaster Plug & Play USB interface to control my radio and for sound card and keying interfacing. So, it was with a little trepidation that I abandoned the world of Windows for Ubuntu.
Well, in a moment of possible lunacy this weekend, I blew away the remaining vestiges of Windows XP on my two PCs and went over to the Ubuntu side. I’d already been running Ubuntu exclusively on my notebook and using that as my day-to-day system for a couple of weeks now. But I had still hung on to my Windows XP desktop system (nothing special–a 1.5GHz AMD Athlon with about 750 MB of RAM and a 40GB HD), in case I needed to go back.
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.