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.
Maybe
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!
Jeff
Smith