Chapter 1
Familiarization with Antenna Modeling
The first step in mastering the art and science of antenna modeling is
to become familiar with the basic idea of computerized modeling antennas and
the software that is available to do the job---at least within the boundaries
of the typical radio amateur budget. The first two Lessons in this course are
devoted to these topics.
In the first Lesson we shall examine what a computer antenna model is, at least
in terms of modern NEC calculation software. The second Lesson will examine the
calculating engine
that we call NEC-2. As well, we shall survey the
sources of software that implement the calculating core in commercial packages
that are user-friendly and that provide a variety of convenient output tables
and graphics.
Lesson 1
The Concept of Antenna Modeling by Computer
OBJECTIVES
In our first exercise, we shall become acquainted with the proper idea of mathematical modeling that applies to NEC-2 antenna models. NEC stands for Numerical Electromagnetics Code. We shall look at a specific example of an antenna to see how refined modeling can be. Then, we shall examine the basic elements of a model using a sample model file (MOD1-1) to illustrate those elements. In the process, you may grow a little more familiar with the particular software that you use for modeling antennas.
GETTING STARTED
In this course on antenna modeling by computer, we shall master the basic techniques of constructing good models using only one of the numerous modeling software cores: NEC-2. Although many (but not all) of the techniques would carry over to other cores, we shall confine ourselves to the most available and used core of all. Trying to cover all available modeling cores would open the door to confusion among them. "Raw" NEC-2 is public domain software that is available from various sources.
Covering all of the implementations of NEC-2 would be impossible. Therefore, we shall illustrate the elements of modeling antennas using two of the most popular packages available at a cost that most hams can afford. One package is EZNEC 3.0 by Roy Lewallen, W7EL (http://www.EZNEC.com/). The other is NEC-Win Plus by Nittany-Scientific (http://www.nittany-scientific.com/). Since each commercial software package has its own proprietary file system, models used as examples will be available in the EZNEC (*.EZ), NEC-Win Plus (*.NWP), and basic ASCII (*.NEC) format. Be sure to access the model collection that suits your software.
This course presumes that you have one of these packages or that you are willing to adapt the ideas that we shall explore to the package available to you.
FROM RUDIMENTARY TO COMPLETE ANTENNA MODELING
We have all done a little bit of mathematical antenna model, even if only with a hand calculator. Consider the antenna in Fig 1-1.
 Fig 1-1--A simple 10-meter dipole, 35' high. |
The sketch represents a 10-meter (28.5 MHz) #12 copper-wire dipole, raised to 35' above ordinary soil on a level field. The support structure is made up of non-conductive materials; that is, insulators, rope and posts. This is truly a basic ham antenna.
The antenna itself is our chief concern. We are interested in its length and its height above ground. Suppose that we wanted to know in advance of looking at the sketch how long to make the antenna in order to make it resonant at 28.5 MHz. For wire antennas in the high frequency (HF) range, we would go back to a very familiar formula:
where the length, L, in feet is determined from the frequency in MHz, f, and a constant (468) that takes into account the so-called "end-effect" of the wire. We have done a piece of rudimentary mathematical antenna modeling.
Eq 1 tells us that the wire should be 16.42' long. But there is much
that the formula does not tell us. Consider Table 1-1, which places the
antenna at different heights from 3/8 of a wavelength to l
 |
The dipole at 35' is the only case in which it is resonant, where
resonance means a condition of having no reactance at the feed point or source
of the antenna. The 70-W
Although the differences may not make much of a difference to operating with a real-world dipole, they are interesting facts about the antenna. And the more facts we have about how antennas perform, the better we can understand them.
WHAT GOES INTO THE MODERN MATHEMATICAL ANTENNA MODEL?
The easiest way to discover what goes into a modern mathematical antenna model is to use an example. In your modeling program, locate file MOD1-1, with the proper extension for your software. Open this model.
Basically, an antenna model can be divided into three categories of interest: the geometry of the antenna, the antenna's environment, and the data requests that we make for the calculated outputs. Let's sample these areas of interest, one at a time.
ANTENNA GEOMETRY
Although we shall later spend an entire lesson mastering how to describe an antenna for accurate calculation of its properties, we can here note some of the fundamental terms used in such descriptions and see how the geometry looks on the screen.
 Fig 1-2--The NEC-Win Plus main data input screen for dipole in Fig 1-1. This is laid out in a typical spreadsheet fashion. |
The NEC-Win Plus main page, shown in Fig 1-2 for our 10-meter wire dipole, shows the basic elements of the antenna geometry. It is located in the lower portion of the figure, in the area with the heading "Wire." A NEC model of an antenna is a collection of straight wires. We ordinarily specify the end points of each wire as a set of Cartesian coordinates, that is, coordinates heading along three axes called the X, Y and Z-axes. When we have a ground surface, as we do in our dipole model, X and Y are considered horizontal axes, while Z is the vertical axis that records the wire height.
 Fig 1-3--EZNEC 3.0 main screen for the dipole in Fig 1-1. |
Some programs, like EZNEC, provide only a summary of the antenna model data on the main screen, as in Fig 1-3, which is still our 10-meter wire dipole. Programs like EZNEC require you to click on the appropriate line of main screen data to call up a more detailed and specific data window. See Fig 1-4 to see the wire geometry that we have been reviewing. Note that the end dimensions (in the Y columns), the antenna height (in the Z columns), the diameter (#12), and the segmentation (11) are the same as in the NEC-Win Plus model. The distance between the end points in the Y columns adds up to 16.75', the overall length of the antenna.
1. An antenna is resonant when
2. An NEC-2 antenna model is an example of
3. The Take-Off (TO) angle of an antenna is
4. Which of the following model facets is not part of the antenna's environment?
This is intended only to provide a taste of what the course offers. Please take the "WebMentor" Course System Sample CTDLC Course to familiarize you with the course template and tools used for all our online courses. Logon with "Guest" (for both StudentID and Password) and take the WebMentor Sample Course.