Antenna Modeling with Jeff , VE1ZAC
Why are we interested in modeling, and what exactly does it do ? Modeling is a way to describe an antenna that we are thinking about, place it in an environment we desire and see what it does when presented with an RF drive signal. The results can be quite accurate, depending on:
The modeler works by breaking our physical antenna up into smaller elements, so that the antenna becomes a series of connected small elements. Remember when we talked about L, C and mutual inductance? The modeler then describes each of these elements with a matrix of elements describing it’s L and C properties, and he accompanying reactance at some frequency ( or frequencies) of choice. Next, the modeler calculates all of the mutual inductances between each element and each of the other elements in the antenna, and the ground (as a very special element). Obviously, this takes s a lot of computing, and the near instant results we get from our PC’s belie the fact that this is a very calculation intensive business.
ten years ago, antenna models that take seconds to compute now would take a
desktop PC 8 or 10 hours to run! The engine used to run most of the popular
programs comes from NEC (Numerical Electrical Calculator) that was originally
developed by the
Small programs typically allow 500 elements to be used to describe an antenna. Larger programs may use 1500 to many thousands of elements.
EZNEC which I am going to demonstrate was developed by Roy Lewellen who just retired from Tektronix as a senior RF engineer. EZNEC was a hobby project that now occupies his time and is arguably the most common modeler used by hams and some RF designers. EZNEC version 3 allows 500 elements and uses NEC2 to calculate the basic parameters. There is a lot of articles in ham magazines about using it, and also on the ARRL web site. Many articles on antennas also include the basic model in EZNEC so the reader can experiment with the antenna from the comfort of his/her desk.
After all the calculating, EZNEC presents the user a visual representation of the transmit field around his antenna in the far field. You can specify the transmitter source and power and the output will give you a field representation or a tabulated list of values for the combined electric and magnetic field around the antenna. You can use this program to predict the field strength at some location away from the antenna, ignoring any propagation effects.
There is skill required to use modelers and obtain meaningful results! However, it is a superb tool to find answers on antenna problems, design new antennas, compare antennas and predict performance. You can learn more about antennas with a modeler in a few weeks than most hams have learned in a lifetime. But real skill with predictions comes from understanding the basics and sensible descriptions of the antenna and ground being studied. The output should be considered”Advice” rather than hard and fast information, especially at first.
You should investigate “modeling antennas with EZNEC” on Google, or a similar search engine. EZNEC, even older versions , is recommended. It will provide you with everything you need to model nearly any ham antenna you can envision. The best approach is to start with a simple antenna and gain confidence before you tackle the ultimate Yagi-Uda. As you will discover, you still have to know an awful lot about antennas before you can go “All the way”
Have fun with it!