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 significantly scaled back my original antenna plan. Now my plan is simply to get an antenna up for 20m that’s not too big or obvious to the neighbors. I have nothing from which I could hang a dipole, so I’m going with a shortened vertical. My design right now calls for a twelve-foot-tall vertical with a loading coil at the midway point and with a modest capacity hat at the top. The dimensions are chosen largely based on the available materials. The radiating element will be constructed using two six-foot-long, half-inch-diameter aluminum tubes, so it’s convenient to put the loading coil between them. The capacity hat is simply a two-foot-long brass rod that makes a tee with the top of the radiating element. The capacity hat is modest but it serves to reduce the amount of inductance needed in the loading coil by nearly half, and also gives me a convenient way to trim the antenna for minimum SWR when I actually build the thing. The ground plane for this antenna will be 16 0.1-wavelength radials laying on or buried slightly under the ground.
Antenna modeling software is entertaining, to say the least. It didn’t take too long to whip up the whole model and run it, optimizing the inductance of the loading coil for minimum SWR in the middle of the 20m band. 4Nec2 tells me that I need 4.2 uH of inductance in the coil to reach a minimum SWR of about 1.3:1. 4Nec2 allows me to do a frequency sweep calculation so I can see the SWR across the entire band, and this antenna design has an SWR below 2:1 across the band. This is actually consistent with what I’ve experienced using AD5X‘s Ultimate Portable Vertical. I suspect that this is not so much due to how great the design is but rather the fact that verticals suffer ground losses.
Anyway, 4Nec2 was also able to show me the effect of changing the size of the capacity hat, to convince me that I could trim that to fine-tune the antenna for low SWR. Cool. And I was also able to the far-field patterns to see that the gain was -3 db compared to an isotropic radiator, and that the field strength was maximum at a take-off angle of 30 degrees above the horizon. Finally, I was able to compare the far-field patterns with those of a full-sized vertical for 20m (with the same ground plane), and the differences were well below what I’d be able to hear in my receiver. Excellent.
4Nec2 has a great editor for defining your antenna model parameters (actually several editors) and a geometry display window that updates as you build your model (so you can see whether you’ve made any obvious mistakes). It’ll produce all the kinds of charts and graphs you’d expect to see from a full-featured modeling package. It’s simply a marvelous package, especially considering it’s free.
I know a picture is worth a thousand words. I’ll get some pictures up in a future post. You’ll be impressed.