Basic NEC with Broadcast Applications

J.L. Smith holds a B.S. in Physics from the University of Houston and an M.S. in Engineering from Southern Methodist University. Through his career, Mr. Smith held the usual positions in research and development beginning his career at KTRH in Houston, Texas in 1946, joining Collins Radio Company in 1956 where he was Department Head, Research and Development, then Manager, Broadcast Systems Engineering. Mr. Smith has been active in FCC matters having filed the first petition advocating automatic unattended operation of FM broadcast transmitters. He participated in the coordination of international broadcasting through his service on CCIR Study Group 10, authored over 50 technical papers and has published Basic Mathematics with Electronics Applications (Macmillan Company 1972) and (Interference Control Technologies 1993.) J.L. Smith is now retired in Covington, LA where he devotes much of his time to analytical research pertaining to AM directional antennas.

Chapter 1 - The Array Adjustment Process (11pp)
This chapter uses the concept of local and global minima to explain why the results of NEC2 computer modeling need not be exact to be beneficial. It also gives a preview of the analysis process.
1.1 The Nature of NEC2
1.2 The Directional Antenna Adjustment Process
1.3 Local and Global Minima
1.4 The Role of NEC2
1.5 Analysis Overview
1.6 Additional NEC2 Benefits
1.7 Software Requirements

Chapter 2 ? NEC2 Fundamentals (27pp)
This chapter covers the basic use of NEC2. It teaches how to write the input file, the types of input files, naming conventions, how to run bnec.exe and how to read and interpret the output file.
2.1 The NEC2 Engine
2.2 NEC2 Operation
2.3 Creating the Input File
2.3.1 Naming the files
2.3.2 Data Commands
2.3.3 Data Command Types
2.3.4 An Input File Illustration
2.3.5 Comment Commands
2.3.6 Geometry Commands
2.3.7 Program Control Commands
2.4 Reading the Output File
2.4.1 The Header
2.4.2 Structure Specification
2.4.3 Segmentation Data
2.4.4 Data Lines, Frequency, Loading and Environment Data
2.4.5 Antenna Input Parameters
2.4.6 Currents and Locations
2.4.7 Current Moments
2.4.8 Power Budget
2.4.9 Radiation Pattern
2.5 Exercise 2

Chapter 3 - Modeling the Radiator (20pp)
This chapter covers the various NEC2 model configurations that can be used to represent the radiating tower, the rules that apply to modeling and exciting the tower. It also introduces a viewing program to verify tower coding.
3.1 Modeling Guidelines
3.2 Guideline Summary
3.2.1 Modeling the Radiator
3.2.2 Modeling the Voltage Source
3.3 Tower Configurations
3.3.1 Single Wire Configuration
3.3.2 4-wire Configuration
3.3.3 2-wire Configuration
3.3.4 Lattice Configuration
3.3.4.1 Coding the Lattice Configuration
3.3.4.2 Simplified Lattice Model
3.4 Viewing the Tower Configuration
3.5 Exercise 3

Chapter 4 ? Array Geometry (15pp)
This chapter converts the coordinate system used by NEC2 to a system consistent with the coordinate system normally used by the broadcast community. In addition it teaches how a multi-tower array is modelled and establishes a notation for multi-tower arrays.
4.1 The Coordinate System
4.2 Array Geometry ? An Example
4.3 The Array Input File
4.4 Exercise 4

Chapter 5 ? Loads, Networks and Transmission Lines (15pp)
This chapter tells how to create a lumped impedance load in a NEC2 wire segment. It also shows how to use non-radiating networks in NEC2 and how to calculate the Y-parameters for those networks plus it shows typical uses for networks. A similar treatment is given for transmission lines.
5.1 Modeling Impedance Loads
5.2 Modeling non-radiating networks
5.2.1 Typical Networks
5.2.2 Typical Network Applications
5.2.3 General Guidelines for Networks
5.3 Modeling Transmission Lines
5.4 Network Output File Listing
5.4.1 Network Description
5.4.2 Network Source and Load Impedance
5.4.3 Network Input Parameters
5.5 Exercise 5

Chapter 6 ? Calculating Base Drive Voltages (25pp)
This chapter teaches how to use post processing software to convert the broadcaster's field ratios to NEC2's base drive voltages to initiate the computer modeling process.
6.1 Direct and Induced Currents
6.2 Current Moments
6.3 The Development Concept
6.3.1 Normalized Drive
6.3.2 Full Power Drive
6.3.3 Shunt Reactance and Networks
6.4 Example ? A Three Tower Array
6.4.1 Create a Unity Drive File
6.4.2 Calculate Unity Current Moments
6.4.2.1 Tower #1 Moments
6.4.2.2 Tower #2 Moments
6.4 2 3 Tower #3 Moments
6.4.3 Solve for Normalized drive Voltages
6.4.4 Determine Full Power drive Voltages
6.5 Exercise 6

Chapter 7 ? Applying the Output File Data (24pp)
This chapter shows how to translate the information in the NEC2 output file to useful design parameters. It teaches how to confirm the validity of the calculations and how to apply the results. It also shows how to use the output file data to set the start-up parameters for a new array.
7.0 Overview
7.1 Verify the Field Ratio
7.2 Plot Far Field Radiation Pattern
7.3 Detune Unused Towers
7.3.1 Detuning by Base Loadiung
7.3.1.1 Example ? Detuning by Base Loading
7.3.1.2 Base Loading Input File Listing
7.3.2 Detuning by Skirting
7.3.2.1 Example ? Detuning by Skirting
7.3.2.2 Input File Listing ? Detuning by Skirting
7.4 Antenna Monitor Readings
7.4.1 Optimum Height for Sample Loops
7.4.2 Arbitrary Height for Sample Loops
7.4.3 Base Current Samples
7.5 Drive Point Impedance
7.5.1 Drive Point Impedance When Using a Network
7.6 Exercise 7

Chapter 8 - Model by Measurement (10pp)
This chapter helps the reader match the self impedance of a tower as calculated by NEC2 to a measured reference by showing how changes in the various tower parameters tend to influence the calculated self impedance.
8.1 Objective
8.2 Adjusting the Model
8.2.1 Number of segments
8.2.2 Tower Diameter
8.2.3 Segment and Radius Taper
8.2.4 Base Capacity
8.2.5 Drive Segment Radius
8.3 Exercise 8

Chapter 9 ? Top Loaded and Skirted Towers (13pp)
This chapter applies NEC2 to the design and use of a tower skirt when applied to top loading, current distribution control and folded monopole applications.
9.0 General Considerations
9.1 Top Loading
9.1.1 Estimating the Size of a Top Hat
9.1.2 Determining the Degree of Top Loading
9.2 Skirted Towers
9.3 Folded Monopoles

Chapter 10 ? System Bandwidth Analysis (18pp)
This chapter extends the application of NEC2 to include the system networks. The methods described view the various functional interfaces within the signal path and displays their impedance bandwidth characteristics.
10.1 Introduction
10.2 System definition
10.2.1 Tower Models
10.2.2 Tower Base Drive Voltages
10.3 Bandwidth Analysis
10.3.1 Source Impedance of the Drive Voltage
10.3.2 Intermediate Data
10.3.2.1 Procedure for Calculating Intermediate Data
10.3.2.2 Tower Base Impedance
10.3.2.3 ATU Input Impedance
10.3.2.4 Transmission Line Input Impedance
10.3.2.5 Phasing Network Input Impedance
10.3 2.6 Power Divider Input Impedance
10.3.3 Total System Bandwidth
10.3.3.1 The Common Point Network
10.3.3.2 Radiation Bandwidth
10.4 Bandwidth Conclusions

Chapter 11 ? Case Studies (24pp)
This chapter is used to show the assets and liabilities of a NEC2 analysis. It uses measured data to confirm that the results of a NEC2 analysis need not be exact to be beneficial.
11.0 Comparative Data
11.1 Case Study #1 3-tower Array
11.1.1 Array description 3-tower Array
11.1.2 Self Impedance 3-tower Array
11.1.3 Antenna Monitor Reading 3-tower Array
11.1.4 Array Data 3-tower Array
11.1.5 Discussion 3-tower Array
11.1.6 Nec2 Input File 3-tower Array
11.2 Case Study #2 6-tower Array
11.2.1 Array description 6-tower Array
11.2.2 Self Impedance 6-tower Array
11.2.3 Antenna Monitor Reading 6-tower Array
11.2.4 Array Data 6-tower Array
11.2.5 Discussion 6-tower Array
11.2.6 Nec2 Input File 6-tower Array
11.3 Case Study #3 6-tower Array Night Time
11.3.1 Array description 6-tower Array Night Time
11.3.2 Self Impedance 6-tower Array Night Time
11.3.3 Antenna Monitor Reading 6-tower Array Night Time
11.3.4 Array Data 6-tower Array Night Time
11.3.5 Discussion 6-tower Array Night Time
11.3.6 Nec2 Input File 6-tower Array Night Time
11.4 Case Study #4 Tall Tower Array
11.4.1 Array description Tall Tower
11.4.2 Self Impedance Tall Tower
11.4.3 Antenna Monitor Reading Tall Tower
11.4.4 Array Data Tall Tower
11.4.5 Discussion Tall Tower
11.4.6 Nec2 Input File Tall Tower

Chapter 12 ? Supplemental Topics (16pp)
This chapter adds a few topics that may not be absolutely necessary but in any event they are interesting and add some insight into the analysis. Moreover, they are topics that are not well documented elsewhere thus their presentation here fulfils an existing need.
12.0 Introduction
12.1 Parallel Feeds ? Network Combiners
12.2 New Structures ? The NX Command
12.3 Numerical Green's Function
12.4 Ground Screens
12.4.1 The GN Command
12.4.2 The GR Command
12.5 Finite Ground
12.5.1 Reflection Coefficient Approximation
12.5.2 Sommerfeld/Norton Procedure

Appendix A ? NEC2 Input File Statements (71pp)
This appendix is a full listing of all the NEC2 command statements and serves as a valuable reference to the beginner and professional alike.

Appendix B ? Error Messages (9pp)
This appendix references error number versus an explanation of the error along with suggestions for remedy.

Appendix C - Software (11pp)
This appendix is essentially a user's manual for the software included on the associated disc. It includes instructions for the NEC2 program (bnec.exe), the geometry viewing program (NVCOMP.exe), the pattern plotting program (WJGRAPS.EXE), the