## Section I - Introduction

The Numerical Electromagnetics Code (NEC-2) is a user-oriented computer: code for the analysis of the electromagnetic response of antennas and other metal structures It is built around the numerical solution of integral equations for the currents induced on the structure by sources or incident fields. This approach avoids many of the simplifying assumptions required by other solution methods and provides a highly accurate and versatile tool for electromagnetic analysis.

The code combines an integral equation for smooth surfaces with one specialized to wires to provide for convenient and accurate modeling of a wide range of structures. A model may include nonradiating networks and transmission lines connecting parts of the structure, perfect or imperfect conductors, and lumped-element loading. A structure may also be modeled over a ground plane that may be either a perfect or imperfect conductor.

The excitation may be either voltage sources on the structure or an incident plane wave of linear or elliptic polarization. The out put may include induced currents and charges, near electric or magnetic fields, and radiated fields. Hence, the program is suited to either antenna analysis or scattering and EMP studies. NEC and its predecessor AMP have been used successfully to model a wide range of antennas including complex environments such as ships. Results from modeling several antennas with NEC are shown in refs. 36, 37, and 38 with measured data for comparison,

The integral-equation approach is best suited to structures with dimensions u3 to Several wavelengths. Although there is no theoretical size limit, the numerical solution requires a matrix equation of increasing order as the structure size is increased relative to wavelength, Hence, modeling very large structures may require more computer time and file storage than practical on a particular machine. In such cases standard high-frequency. approximations such as geometrical or physical optics, or geometric theory of diffraction may be more suitable than the integral equation approach used In NEC.

The code NEC-2 is the latest in a series of electromagnetics codes, each of which has built upon the previous one. The first in the series was the code BRACT which was developed at MBAssociates in San Ramon, California, under the funding of the Air Force Space and Missiles Systems Organization (refs. 1 and 2). BRACT was specialized to scattering by arbitrary thin-wire configurations.

The code AMP followed BRACT and was developed at MBAssociates with funding from the Naval Research Laboratory, Naval, Ship Engineering Center, U.S. Army Strategic Communications Systems, U.S. Army Strategic Communications Command, and Rome Air Development Center under Office of Naval Research Contract N00014-71-C-0287. AMP uses the same numerical solution method as BRACT with the addition of the capability of modeling a structure over a ground plane and an option to use file storage to greatly increase the maximum structure sire that may be modeled. The program input and output were extensively revised for AMP se that the code could be used with a minimum of learning and computer programming experience. AMP includes extensive documentation to aid in understanding, using, and modifying the code (refs. 3, 4 and 5).

A modeling option specialized to surfaces was added to the wire modeling capabilities of AMP in the AMP2 code (ref. 6). A simplified approximation for large interaction distances was also included in AMP2 to reduce running time for large structures.

The code NEC-1 added to AMP2 a more accurate current expansion along wires and at multiple wire junctions, and an option in the wire modeling technique fat greater accuracy on thick wires. A new model for a voltage source was added and several other modifications made for increased accuracy and efficiency.

NEC-2 retains all features of NEC-1 except for a restart option. Major additions in NEC-2 are the Numerical Green's Function for partitioned-matrix solution and a treatment for Lossy grounds that is accurate for antennas very close to the ground surface. NEC-2 also includes an option to compute maximum coupling between antennas and new options for structure input.

Part I of this document describes the equations and numerical methods used in NEC. Part III: NEC User's Guide (ref. 7) contains instructions for using the code, including preparation of input and interpretation of output. Part II: NEC Program Description -- Code (ref. 8) describes the coding in detail. The user encountering the code for the first time should begin with the User's Guide and try modeling same simple antennas. Part II vill be of interest mainly to someone attempting to modify the code. Reading part I will be useful to the new user of NEC-2, however, since an understanding of the theory and solution method will assist in the proper application of the code.