Getting a feel for antennas

When readers e-mail me, I often turn around and ask them what topics they would like me to write about here on KB6NU.Com. Well, yesterday, I got this reply:

I think it would definitely be useful to cover antennas and RF transmission theory.  I have a small bit of experience in electronics…completed an AAS EET degree years ago (actually 2 decades – YIKES), but looking at my old textbooks I see that we really did very little with radio. Once basic AC/DC theory was done, it was all about computers.

I think antennas are such foreign things to us nowadays, that I’m probably not in the minority when I say that’s the thing I struggled with the most. Dipoles are pretty easy, but once you get beyond that, it gets a lot more complicated. For example, E9C02: What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 90 degrees out of phase? I know that it’s a cardioid, but I’m not entirely sure I understand why. I can visualize the patterns, but I don’t feel like I GET it.

I replied:

Antenna theory is my weakest area, too. Some people just seem to get it, but I struggle like you do. I think one way to overcome this is to play around with antenna modeling software. I think that by modeling some simple antennas, you’ll see that patterns that result, and after a while, get a more intuitive feel for them.

One program that many use is called MMANA-GAL. It’s free, and apparently, a very good program. If you do download it and play around with it, I’d like to get your impressions of it. Heck, I’d even post that to my blog. I think a review like that from a new user would be very valuable to  other new users.

So, now, I have a couple of questions for the rest of you:

  • Do you use antenna modeling software?
  • If so, which program?
  • Do you find it easy to use or hard to use?
  • Do you find it useful to you in learning how antennas work?

Comments

  1. Dave AD9DP says:

    I’ve used 4NEC2 (http://home.ict.nl/~arivoors/) a lot because it’s free. You get a lot more than you pay for. Modelling is half the thrill of putting up a new antenna – seeing how close you can get to the actual performance. It might save some money if you want to push something or see if an antenna manufacturer’s claims are realistic.

    I used it to model each of my past wire and tape measure dipoles. I needed to fold my attic dipoles to make them fit, first a G5RV jr, later an Alpha-Delta DX-CC trap/parallel dipole. I wanted to know before I bought them if folding would work. 4NEC2 predicted that it would and taught me what not to do, like fold at acute angles. The results were ALMOST good enough to cut and install, but I started with the wires long as shipped and trimmed them to tune. 4NEC2 was dead-on as to how many inches/Khz to trim, so I was able to nail multiple bands with only two trips into the attic.

    4NEC2 is a collection of separate text and graphical editors to create, read, and edit input files for the NEC2 calculation engine, the engine itself, and different apps that display or plot the output files.

    Entering a model requires patience because the UI of some of those apps behave differently from the Windows standard, e.g. you need to make sure a user entry field is completely empty before you type into it or it may append. The graphical editor’s inference engine is limited so graphical editing is very tricky except in XY, XZ, and YZ projections. It very easy to make an accidental, undesired change and many functions have no “undo.” The solution is to experiment with one of the the sample half-wave dipole models that comes with it until you have a feel for the UI, then start by modifying and re-saving one of those models into what you want. Leave “bread crumbs” in the form of frequent file saves as your model develops. If you make an accidental edit and everything goes down the drain, just back up to your last save.

    The original LANL documentation (scanned pages of courier 10) was written for its electromagneticists and will discourage anybody who tries to start there, although does explains the NEC2 engine’s input and output file formats that were originally submitted on punched cards.

    To get started in modeling, I suggest Cebik’s 4-Part “Beginners Guide” from the 11/2000 through 2/2001 QSTs. They should be available to ARRL members from the QST Archive try http://wireless.ictp.it/school_2005 /download/nec2/nec_part1.pdf, …part2.pdf, …part3.pdf. …part4.pdf These articles are exemplary in teaching you how to model.

    ‘Hard to argue with free. I’ll have to try MMANA-GAL on the next one.

    73s
    Dave

  2. Dave, N8SBE says:

    EZNEC, and mainly because it was the one referred to in the ARRL online Antenna Modeling course I took a while back. Since then, I haven’t played with it much. Not especially easy or intuitive, but I’m not sure that such an animal exists, even at the high-bucks end of the spectrum.

    What I would envision (pun intended, I suppose) is an interactive 3D-type of modeling, where you could draw and size antenna elements on screen, see and rotate/roll/tilt the radiation plots with your mouse, and be able to drag element sizes and locations while seeing the radiation plots change in real time on the screen.

    Maybe too much horsepower needed out of today’s laptops to make this a reality?

  3. Elwood Downey, WB0OEW says:

    Well, this is a huge topic. I have taken graduate level antenna courses and built many but I still find antennas fascinating and mysterious.

    My current favorite modeling program is CocoaNEC. It’s just for Macs but with it you describe your antenna geometry by writing C-like programs. This allows you to parameterize the location of the antenna structure and its dimensions in order to more easily explore such effects and height and shapes. This allows you to make changes and watch for overarching effects to build up intuition. For example, by varying the height and watching the input impedance one can see how the input resistance goes down because there is less ground loss; you can watch a dipole bandwidth increase as one adds end caps or makes elements thicker and so on and on. The program includes the NEC2 engine but is also compatible with the improved NEC4 engine if you have the license from LLNL. I also have the PRO version of EZNEC for Windows. It uses the same underlying engine and so the accuracy is identical but I find the spreadsheet geometry entry mechanism much more cumbersome for exploration.

    Another program I’ve used is AN-SOF (see http://www.antennasoftware.com.ar). The full program is not free but there’s a demo that gives you a good idea. It’s getting a little long in the tooth but the support is still active. It allows interactive creation and manipulation of the 3D structure along the lines of what Dave has described. While it’s true this is easier at first than writing spreadsheets or code for simple structures, in my view it simultaneously becomes much more awkward to make changes to more complex structures. It uses MOM theory like NEC but is an independent implementation that removes some of its limitations such as small acute angles and very close segments. I find accuracy is comparable to NEC4.

    L. B. Cebik W4RNL (SK) has written extensively on antenna modeling strategies and made detailed comparisons of all the modern MOM implementations, including MMANA. His work clearly shows there is no one implementation that is superior in all cases. All of his work is available at http://www.cebik.com, although some requires a subscription.

    A good way to dive into learning more about antennas is to get on the free antennex.com discussion mailing list. There are some heavy hitters on there to keep everyone on their toes but a lot of hams also that keep the discussion well grounded. I also like the web site http://www.antenna-theory.com.

    Well, this could go on for ever. I think the short answer could be just pick any of the excellent free programs and start using it. Start with a simple dipole and start changing the dimensions and excitation and see what happens to the pattern, near field, input impedance, etc. You’ll soon see trends that improve your intuition so you can go on from there limited only by your time and imagination. Good luck.

  4. Elwood Downey, WB0OEW says:

    I forgot to mention a book I would recommend: “Small Antenna Design” by Dr. Doug Miron. He starts by covering basic antenna theory in a very approachable way. He uses some light calculus but the text describes what is going on in plain English and many results are shown as simple formulas that can be applied with worked examples. Next he discusses many practical types of antennas and matching techniques and discusses the effect of making changes, again with the intent of building good intuition about behavior. Dr. Miron writes knowing full well the reader may well be an amateur radio operator and many examples are directly applicable to practical ham situations. He then goes on to show many modeling examples, explaining the approach and pitfalls along the way.

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