INTRODUCTION
MODELING PROCESS
MODELING ACCURACY
REFERENCES
Because of the similarity in names, it has often been stated that MININEC is but a personal computer (PC) version of its big brother, NEC [Burke and Poggio, 1981]. Some of this confusion is described in Murray and Austin [Murry and Austin, 1994]. However, this could not be farther from the truth. There are significant differences between these two codes. Both codes use the method of moments to solve for currents on electrically thin wires. However, each code starts with a different version of the integral formulation for the currents and fields for wires. Then, each follows significantly different algorithms for implementation of the method of moments.
The Expert MININEC Series is a set of engineering tools for the design and analysis of wire antennas. The Expert MININEC Series user interface runs under the Microsoft Windows environment. Input data screens provide queued dialog boxes with spreadsheet like data displays. Output products include tabular and graphics displays. Input and output data screens are fully interfaced to Windows printer drivers as well as other window applications, such as word processors and spreadsheets.
The computational engines are written in FORTRAN for greater speed and make maximum use of available memory to set array sizes. The formulation uses triangular basis functions resulting in greater accuracy. As an example, the short segment limit is machine precision.
Because of the similarity in names, it has often been stated that MININEC is but a personal computer (PC) version of its big brother, Numerical Electromagnetics Code (NEC). However, this could not be farther from the truth. There are significant differences between these two codes. Both codes use the method of moments to solve for currents on electrically thin wires. However, each code starts with a different version of the integral formulation for the currents and fields for wires. Then, each follows significantly different algorithms for implementation of the method of moments.
The original MININEC was written by John Rockway with a little prodding and support from Jim Logan. The Rockway - Logan team has been responsible over the years for the development of this code into one of the best known and most useful method of moments antenna modeling codes available. A number of other individuals have also contributed small, but not necessarily insignificant, pieces to the MININEC capability, but it has been the dual efforts of the Rockway - Logan team that has made MININEC into the widely accepted antenna design and analysis tool it is today.
One day, back in 1980, John Rockway and Jim Logan were discussing the methods available to amateur radio enthusiasts for antenna design. The point was made that most amateurs at the time did not have access to large main frame computers and highly developed antenna modeling computer programs, such as the Numerical Electromagnetics Code (NEC). Personal computers (PCs) had not been on the market for very long, and they were relatively expensive and quite limited in performance. PCs were then generally regarded as mere novelties or toys. Jim Logan postured seriously,
How about trimming down NEC so that it would run on a PC?
This would be no small feat since, in those days, PCs were typically limited to 16K memory with 8 bit word length. There was no FORTRAN. The program would have to be written in BASIC.
The proposed computer program would necessarily be very limited. Several questions were raised.
Could so limited a code be used to do any serious design or analysis?
Would it ever be much more than a novelty?
Generally, the authors believed the code could be, at the very least, an educational tool. Even if it could model but a single wire, it should be of value to students, engineers and Hams in understanding some of the fundamental antenna principles.
The discussion turned to how the authors could accomplish the proposed computer program. It was agreed that to simply "strip down" NEC and "port" it to the PC would not be practical. In those days, to port a computer program to a PC implied tediously typing in the code line by line. NEC was (and still is) a very powerful computer code with tens of thousands of FORTRAN statements written for use on large main frame computers. The formulation would have to be changed to allow a simpler implementation of the method of moments to produce a more compact code. It would not be possible to include many of the powerful modeling options provided by NEC.
After some further discussion on how this might be accomplished, the authors placed a call to Professor Wilton at the University of Mississippi (now with the University of Houston) to solicit his advice. At first, Professor Wilton did not take the concept seriously. He enjoyed a good chuckle, and the conversation ended. But in a few days he called back to suggest a formulation and method of moments approach which might work for very simple wire antennas.
Armed with Professor Wilton's advice and a "portable" CDI computer (weighing over 50 pounds) with 32k of memory, John Rockway set off on his annual vacation, a pilgrimage to Washington State to visit his relatives. When he returned, he had implemented Professor Wilton's formulation for a single wire. The code could compute the current distribution and impedance for a single wire in free space. It was easily extended to multiple, but unconnected, wires.
Borrowing from ideas in his Master's Thesis at Syracuse University, Jim Logan suggested an approach of over lapping segments to treat wire junctions. The user would have to designate which segments overlapped onto other segments, but multiple wires with junctions could be modeled. Later Jim Logan would implement an automatic scheme for determining wire connections. The new code could compute currents and impedance for antennas having up to 10 wires, either connected or unconnected, and in free space or over a ground plane, all with surprising accuracy. The code was written in 500 lines of BASIC and would run on an Apple computer with 16k memory. The first successful MININEC program had been developed.
The first public release of MININEC occurred in 1982. The code was 550 lines of BASIC and would run on an APPLE II computer with 64 kilobytes of memory. It could compute the current distribution, impedance, and far field pattern of an arbitrarily oriented set of wires in free space or over a perfectly conducting ground plane. Lumped impedance loads were allowed at segment junctions except for segments intersecting with the ground plane. Also, wires intersecting the ground plane were restricted to right angles. In interpreter BASIC (there were no BASIC compilers at that time) the problem size was limited to 10 wires and 50 currents (or 70 segments with junctions). To use the code, a user would have to type in the number of segments, the wire end coordinates, wire radius, and wire connection data in response to serial prompts. The latter was a little tedious because the user would have to determine the connection information before running the code. The integer magnitude indicated the wire number onto which the wire connected or overlapped. The sign of the integer indicated which wire end. There was no means for saving or editing the geometry. This led to much frustration using this initial MININEC program. However, the results proved to be very accurate and quite useful for a wide variety of simple wire antennas.
MININEC was an instant success. Almost immediately, a small user group developed and began to grow. In 1984, partly to meet demand and, as well as, share other computer algorithms, the authors teamed up with two colleagues, S. T. Li and D. W. Tam, to publish a book which contained an improved version of MININEC. MININEC2, as it became known, was not significantly different from its predecessor, but the limitation for wires intersecting the ground plane was removed. Wires could intersect the ground at any angle.
In the mean time, the power of PCs began to grow. Computers were getting faster, had more memory, and utilized math coprocessors. BASIC compilers also became available. These factors opened up new vistas for MININEC. In 1986, the authors released MININEC3. This code featured a new user interface which automatically determined wire connections from the user inputs for wire end coordinates. It could also read and interpret a limited NEC input data set. However, again, there was no way to save and edit geometry data. MININEC3 included near fields, an expanded lumped parameter loading option, and a Fresnel reflection coefficient correction to the patterns for real ground which was similar to the far field corrections used in NEC. MININEC had grown to just over 1600 lines of BASIC. With a math coprocessor and a BASIC compiler, MININEC3 could solve antenna problems up to 50 wires and 50 current unknowns.
The next MININEC effort by the authors produced the MININEC SYSTEM in 1988. This was an effort by the authors to provide improved problem definition, save features, and on-line graphics. The release of the MININEC SYSTEM happened to coincide with the introduction of Microsoft Windows which took the PC world by storm. The authors were too close to publication to backtrack and implement a Windows system. However, there were many worthwhile innovations represented in this code. This was the first version of MININEC which required a compiler, a BASIC compiler. Previous versions could be run in interpreter BASIC. The solution time and storage requirements for rotationally symmetric antennas were greatly reduced. The transpose elimination algorithm was available as a user select option to allow computation of larger problems, up to 50 wires and 90 current samples or 190 segments were permitted without recompiling. There was also an option for the optimization of the feed system of an array to synthesis a pattern. Another option could calculate the antenna-to-antenna coupling for a co-site antenna environment. In addition there was an option for the design of an antenna matching network from MININEC computed antenna impedance.
Many others have also attempted to improve on MININEC. Most notable are the innovative user interfaces and graphics displays offered by Roy Lewallen in 1991 and Brian Beezley in 1992. The authors have also heard numerous reports from different sources of attempts to rewrite MININEC into machine language, ADA, PASCAL, C, and FORTRAN. As an example, in a column published by E. K. Miller in 1989, a FORTRAN version of MININEC was reported. In fact, the authors have tried PASCAL, C, and FORTRAN at various times over the years, but have always returned to BASIC for various reasons including the shortcomings of the available PC FORTRANs. However, excellent FORTRAN compilers are now available for the PC. The authors have chosen it as the language of choice for the computational intensive portions of MININEC.
The newest MININEC effort by the Rockway-Logan team is the Expert MININEC Series for Windows. The formulation has been changed to use triangular basis functions resulting in greater accuracy. The limitations of previous versions of MININEC have been eliminated. The computational engines are written in FORTRAN for greater speed and make maximum use of available memory to set array sizes. The user interface runs under the Microsoft Windows environment. This represents a significant improvement in the MININEC antenna modeling capability. The user interface provides many convenient options for defining antenna geometry. Built in graphics, on-line context sensitive help, and problem diagnostics aid the user in problem definition. Output graphics and linkage to spreadsheets and word processors of other Windows applications greatly enhance the interpretation and analysis of MININEC computations.
The Expert MININEC is a series of capabilities that include:
The principal advancements include programs dimensioned for larger problems and faster computations. The computational codes have been compiled with newer FORTRAN 90 compilers that have resulted in faster computations. An improved new problem development has been added. This significantly improves the ability of the new users and expert users to develop problem descriptions. Finally, significant use of the MININEC Professional Series has provided feedback on suggested improvements. These improvements have been implemented in Expert MININEC.
The Expert MININEC Series are computer programs for the analysis of wire antennas using the Microsoft Windows that is available on compatible IBM personal computers (PCs). This book provides the theory and examples for the use of Expert MININEC to design or analyze antennas that are characterized by an arbitrary collection of thin wires in free space or over a ground plane. A method of moments approach for analysis of thin wire antennas is the basis for Expert MININEC. A Galerkin procedure is applied to an electric field integral equation to solve for the wire currents. This formulation results in an unusually compact and efficient computer algorithm.
The user interface to Expert MININEC is through Microsoft Windows. Input data screens provide format sensitive entry boxes in individual windows with tabular data displays.
Expert MININEC modeling geometry constructs include
Cartesian, cylindrical and geographic coordinate systems
Meters, centimeters, feet or inches selection
Straight, helix, spiral, catenary and arced wires
Wire meshes
Automated canonical structure meshing
Node coordinate stepping
Symmetry options
Rotational and linear transformations
Numerical Green's Function
Automated convergence testing
Electrical description options include
Free space, perfect ground, and imperfect ground environments
Frequency stepping
Loaded wires
Lumped loads
Passive circuits
Transmission line bridle
Two-port networks
Two-port transmission lines
Voltage and current sources
Plane wave source excitation
Solution description options include
Near fields
Radiation pattern
Two-port coupling
Medium wave array synthesis
In addition the Expert MININEC includes a user-oriented capability to analyze finite arrays within the limits of Expert MININEC capabilities. Output products are displayed in both tabular and graphics forms.
The integrated graphics of Expert MININEC include
3-D geometry displays with rotation, zoom and mouse support.
3-D currents, charges and pattern displays.
Linear, semilog and log-log plots of currents, coupling, near fields, impedance and admittance.
Smith Chart plots of impedance and admittance.
Linear and polar pattern plots.
Input and output data screens are fully interfaced to Windows printer drivers as well as other window applications, such as word processors and spread sheets. On-line, context sensitive help is also provided.
The computational intensive algorithms are implemented in FORTRAN for greater speed and make maximum use of available memory to set array sizes. The formulation has been changed from earlier versions of the MININEC to use triangular basis functions. This results in greater accuracy. The short segment limit is a function of machine accuracy. Square loops and Yagi antennas may be solved with confidence. In addition, a Fresnel reflection coefficient approximation improves the calculation of currents in the vicinity of real ground. As a summary Expert MININEC solves
Currents and charges on wires (peak or RMS)
Impedance, admittance, S11 and S12
Effective height and current moments
Power and voltage losses
Multi-port (antenna-to-antenna) coupling
Near electric and magnetic fields
Radiation patterns (dBi or electric fields, power or directive gain)
Medium wave array design
Auxiliary calculations of ground wave, stub matching, and tower footing impedance
Not all of the capabilities of the Expert MININEC are available in all of the series options. A list of Expert MININEC capabilities is provided. The availability of these capabilities in each of the Expert MININEC series options are indicated. Expert MININEC Professional is suitable for the experienced student, hobbyist and the professional engineer. Expert MININEC Broadcast Professional is a tool for the advanced student and the professional broadcast engineer.
There are a few minimum requirements to run Expert MININEC . The computer must be an IBM PC or compatible. A 486 processor or better is recommended. The minimum internal memory requirement is four megabytes. Eight megabytes or higher is recommended. Approximately 15 megabytes of disk drive is required to store the program. Problem definition and results files can accumulate quickly requiring further hard disk space. In the use of any Microsoft Windows should be useable. In the use of any Microsoft Windows program, a mouse is recommended.