Intermediate Antenna Modeling
A Hands-On Tutorial

L. B. Cebik

Analytical Table of Contents

Introduction
In these preliminary notes, you will become familiar with the basic purpose and design of this book on antenna modeling with NEC. You will come to understand why you should go through the exercises carefully with your software as well as reading the principles and techniques portions of each chapter. In addition, you will learn something about the author.

Part A The Geometry Commands

1. A Review of NEC Fundamentals and Limitations

This chapter reviews in a very compressed way the fundamental aspects of NEC models. The exercises proceed from basic modeling formats and conventions to an enumeration of necessary and recommended modeling procedures. As well, you will review NEC limitations for both NEC-2 and NEC-4 and review basic tests of model adequacy.

2. GW, GM, GS, and GE: Wire Entry Alternatives

After becoming clear on the distinctions among the concepts of a wire, a segment, and a tag, we shall examine the three most fundamental geometry commands: GW, GS, and GE. Although we can create most antenna structures using only these commands, we shall also observe how the GM command can let us effect significant economies of model formation as well as offer alternative means to the same modeling goals.

3. GC, CW, and GA: Special Wire Shapes

In this chapter, we shall examine 3 commands that yield special structural shapes. GC, a continuation of the GW command, allows both wire length and radius tapering, and has a special format. CW, a NEC-4-only command, permits modeling catenary wire curves. Finally, GA provides the ability to model arcs ranging from a few degrees to a complete circle.

4. GH in NEC-2 and NEC-4

This chapter will take a long look at the formation of helices within NEC-2 and NEC-4. In the process, you will learn how the command structures used by each core differ from each other. As well, you will meet not only what the two cores have in common with respect to helix formation, but as well, what unique potentials each version of the command can offer.

5. GX and GR: Symmetry

The seemingly simple symmetry commands require considerable care in order to achieve good results. In this chapter, you will become acquainted with the use of symmetry, first as an alternative means to producing modeled antenna structures, and second as a method of reducing the required run time for the core and reducing the requirements for memory for structures of considerable size.

6. WG and GF: Numerical Green's Functions and Wire Grids

This set of exercises will introduce some methods of handling complex geometries, ranging from the importation of externally synthesized files to wire-grid fabrication. Related to such geometries are methods of saving core-run time, particularly the use of Numerical Green's files. You will learn how to both write and then recall such files.

7. Finding Geometry Limitations in a Model

In the final exercise on model geometry, you will meet in a concentrated dose many of the limitations associated with setting up a model using those commands. In addition, you will become acquainted with some of the tools available both in the NEC cores and in the overall program for detecting geometry error and warning conditions, along with ways of correcting them.
Part B Far-Field and General Control Commands

8. Output Tables and Graphics

The first step in mastering the control commands is to become familiar with the structure and content of the NEC output file contents under various command conditions. Then, we may proceed to understand how implementing programs translate the tabular calculation results into a large variety of graphic presentations to enhance data comprehension.

9. Far-Field Radiation Pattern Requests

The radiation-pattern request command is central to NEC calculations and is more flexible than most modelers imagine. In this first set of RP exercises, you will examine the space wave options available and what they can tell you about the performance of different types of antennas that you may model. As well, you will learn several cautions to observe when using the command.

10. Voltage and Current Sources and Frequency Specifications

We shall explore two commands in these pages. Excitation, especially the voltage source, is necessary to any model and we shall learn how to place and control sources, and even how to convert a voltage source into a current source. The second command necessary to all models is the request for one or more frequencies, and we shall look at both linear and multiplicative sweeps.

11. Grounds: Types, Applications, and Specifications

You will encounter in these exercises 4 types of ground options, a range of ground quality specifications, and two ground description commands (GN, GD) that allow you to set up one or two media. The second medium can use a circular or linear edge. You will also understand the differences between NEC-2 and NEC-4 treatments and the interaction of ground specifications with RP commands. 

12. Mathematical and Wire Transmission Lines

These notes will examine both the special TL command, with its ability to model 2-wire transmission lines operated in the transverse electromagnetic mode (TEM), and modeling parallel transmission lines using wires, that is, GW commands. We shall learn of unique potentials for each type of transmission-line model as well as limitations for each type.

13. Impedance Loads and Their Limitations

You will examine in this chapter the LD command and the many types of loads that you may create with it. Besides mastering the command's structure, you will also look at a number of basic applications ranging from setting wire conductivity to using antenna "traps." You will also discover some of the limitations associated with impedance loads.

14. Networks and Some of Their Applications

The much-neglected NT command offers you another non-radiating function that has many potential applications, once you master at least a few practical steps in implementing 2-terminal port values. In addition to seeing some of the potential for NT, you will also learn a way of implementing some kinds of networks that are frequency nimble.
Part C Special Outputs, Control Commands, and Techniques

15. Power Information: Power Efficiency vs. Radiation Efficiency

In these exercises, you will become very familiar with what the power budget output report can and cannot tell us about antenna performance. In the process, you will learn how to distinguish power efficiency from radiation efficiency. You will learn how to obtain a radiation efficiency report by using the RP command to derive something other than a radiation pattern.

16. Material Loads, Wire Insulation, and Work-Arounds

This set of exercises will introduce the modeling of insulated and other wires exhibiting a velocity factor. We shall meet the NEC-4 insulated sheath (IS) command and discover a way to simulate the command within NEC-2. In addition, we shall explore some situations that require us to accumulate data from a series of related models and use external methods of coalescing the information.

17. Special Radiation Pattern Data

Our return to the RP command will focus on three types of outputs. One is the full free-space sphere or the hemisphere over ground, useful for surface and gain-averaging plots. Second is the RP1 command for ground waves. The last is a sample of data that we can develop in post-core-run calculations to yield special pattern information.

18. Near-Field Analysis in NEC-2 and NEC-4

Near-field analysis requires us to master Cartesian and spherical techniques of arranging input data in both the NE and NH commands. We shall also examine the nature of the output data, including the calculation of peak values. Finally, we shall look at some unique features of NEC-4's treatment of near fields, including the introduction of the LE and LH commands.

19. Receive and Scattering Data: Excitation and Data Requests

Receive and scattering data acquisition will acquaint us with three plane-wave excitation options available within the EX or excitation command: linear, right-hand elliptical, and left-hand elliptical sources external to the antenna. Conjointly, obtaining this data will require that we examine the options of the PT command that controls printing the wire-segment currents.

20. Miscellaneous Special Commands

In this potpourri of control commands that we have so far by-passed, we shall briefly note the entry requirements and major reasons for using them. Some commands, such as EK within NEC-2, have sufficient general importance for more extended treatment. Other commands that are common to both cores, such as PQ and CP, will call for more work than those that appear in only 1 of the cores.

21. Modeling by Equation

The exercises in this chapter will use the NEC-Win Plus insert of NEC-Win Pro and GNEC to introduce some rudimentary aspects of modeling by equation. Beginning with the use of simple variables, the exercises will progress to more complex equations that will define the antenna geometry and permit automated scaling of a design to other frequencies.

Appendix: Some Useful Data for Antenna Modelers

1. Conductivity and Permittivity of Common Ground Conditions
2. Conductivity and Resistivity of Common Materials Used in Antenna Construction
3. Relative Permittivity of Some Common Wire (and Other) Insulating Materials
4. Common Wire Gauges and Associated Diameters in Inches and in Millimeters
5. Some Common Frequency-Wavelength Relationships
6. Frequency Domains
7. Some Common Transmission Lines Values
8. Metric-English Conversion

Index