1. Introduction

SuperNEC (SNEC) is an object-oriented version of the FORTRAN program NEC-2. This technical manual is therefore very similar to the NEC-2 manual. In fact, many sections have been copied verbatim from the NEC-2 manual. The reason this document has been produced (as opposed to referring the reader to any readily available NEC-2 manual) is because new theory will be added to SNEC and some features that exist in NEC-2 will not be implemented in SNEC. The easiest way of keeping an up to date reference of the theory implemented in SNEC is to start with the an electronic version of the NEC-2 manual and modify the document as the SNEC code evolves. Many extensions have been made to SNEC in prototype form. These features include hybridisation with UTD, fast iterative solvers, MBPE amongst other features. When these extensions are formally incorporated into the SNEC program, then the theory behind the extension will be added to this manual.

2. The Integral Equation for Free Space

The SNEC program uses an electric-field integral equation (EFIE) to model the electromagnetic response of general structures. The EFIE can be used to model both wire structure and surfaces. Surfaces are represented by wire grids and have had reasonable success for far-field quantities but with variable accuracy for surface fields. The EFIE and its derivation are outlined in the following sections.

2.1. The electric field integral equation (EFIE)

The form of the EFIE used in NEC follows from an integral representation for the electric field of a volume current distribution .

(1 )

where

and the time convention is .

is the identity dyad . When the current distribution is limited to the surface of a perfectly conducting body, equation (1) becomes :

(2)

where

is the surface current density.

The observation point is restricted to be off the surface so that .

If approaches as a limit, equation (2) becomes :

(3)

where the now represents a principal value integral. It is a principle value integral since is now unbounded.

An integral equation for the current induced on by an incident field can be obtained from equation (3) and the boundary condition for :

(4)

where

is the unit normal vector of the surface at

is the field due to the induced current.

Substituting equation (3) for yields the integral equation :

(5)

The vector integral in equation (5) can be reduced to a scalar integral equation when the conducting surface is that of a cylindrical thin wire, thereby making the solution much easier. The assumptions applied for a thin wire, known as the thin-wire approximation, are as follows:

1. Transverse currents can be neglected relative to axial currents on the wire.

2. The circumferential variation in the axial current can be neglected.

3. The current can be represented by a filament on the wire axis.