The reading material I’ll be referring to is L.B. Cebik’s QST article series ‘A Beginner’s Guide to Modeling with NEC’. I plan to discuss a number of antenna modeling programs here, so this will go on considerably longer than the original articles.
One of the best courses was originally written by L.B.Cebik W4RNL for the ARRL Online Continuing Education program. It’s pretty much the only game in town at this level. If you go to the right university, you can take courses that will cover it in far more detail, getting into the mathematics and physics much more deeply. The book seems to be out-of-print at the moment. Amazon.com lists it for sale and you may be able to find a used copy. The ARRL Antenna Book also has a very good chapter on Antenna Modeling which is worth referring to.
As we start, you can refer to the 1st Cebik article which you can find at http://www.arrl.org/members-only/tis/info/pdf/0011034.pdf. I’ll also be discussing this in the AntModel antenna modeling discussion list. Hopefully, some people will participate.
My plans are to work through the articles, doing the models and explaining why I’m doing certain things. I look forward to some level of discussion about modeling as we go along.
What is Antenna Modeling
I don’t need to explain to anyone here about the differences between antenna models and model boats and planes, but it IS worth spending some time talking about mathematical modeling and some of the issues relevant to antenna modeling.
If you really want to understand the mathematics and physics behind antenna modeling, one of the classic places to start learning is John D. Kraus’ book Antennas which is the basis for many graduate courses. I learned from an earlier edition of this book during a course in Radio Astronomy. The mathematics us far too complicated for our purposes here, but it’s fascinating to work with if you like building mathematical models.
Mathematical modeling is about creating a model of a system using mathematics as a language. I first got fascinated in mathematical models when we were still working them on slide rules. With my first exposure to computers, I fell in love and went right to work building models. It’s important to understand that models are never exact. Even the best models are limited in some respect.
Consider the simple antenna model for a dipole antenna.
length = 468/wavelength
Believe it or not, this is a REAL mathematical model of a 1/2-wave dipole antenna, but how exact is this. The actual speed of light is
186,282.397 0512 miles/second
AND we know that the wavelength is related to the frequency by the formula
wavelength = speed of light/frequency (Hz)
if we change frequency to MHz (1,000,000 Hz), then
wavelength (feet) = 983.282 960/frequency (Mhz)
and so a half-wavelength would be approximately (I’m using =~ to say ‘approximately equals’)
983/(2 * freq) =~ 491.5/freq =~ 492/freq
Huh? Why 492? Why didn’t we get 468 which is what the thumb-rule model gives us? The number 468 isn’t arbitrary. It has good reasons for being the choice, let’s see if we can come up with something like this working with an antenna model and in the process, learn something about building antenna models.