Structure of the Model

A model is constructed by breaking conducting surfaces into flat polygonal elements. These — called the major elements — are entered by specifying the co-ordinates of the nodes or corners of the polygon. The co-ordinates of the nodes may be in any order. However, elements with corner angles larger than 180o are not allowed. The elements do not need to be perfectly flat, but the discretisation procedure may fail if some of the nodes are too far from the plane of the element. Straight wires can be entered as elements with only two nodes. (Multiple node elements can have more than two nodes lying in a line, provided not all the nodes lie in line.)

The surface is discretised according to a specified ``maximum segment length''. By changing only this parameter, the density of the mesh can be changed throughout the structure. The discretisation procedure first divides the element sides into segments of equal length — while also ensuring that the maximum segment length is not exceeded. Next primary lines are spanned between sub-nodes on opposite sides of the element and finally the secondary wires are spanned orthogonal to the primary lines. The primary lines are divided into segments of equal length, with the number of segments determined by the number of secondary lines crossing that particular line. The secondary lines link the sub-nodes on the primary lines to each other and the nodes on the element sides — thus they are not straight and their segments may also differ in length. Each secondary line segment is written to NEC in a separate GW card. If the length of a segment on a secondary line is less than 30% of the specified ``maximum segment length'' it is not added. (This usually occurs only for the first or last segment on a primary line in the vicinity of a sharp corner where the short segment is likely to have its centre inside one of the two wires it is connecting.)

If the structure is too big to be discretised within the limits of the code (a maximum of 50 for either primary or secondary lines) that element will not be discretised and a warning will be given. Apart from the warning (which will be repeated when the NEC input file is created) the program will continue as usual. It is possible to ignore the warning and continue with both the model creation and writing the NEC file. This will allow the user to determine if the problem occurs on only a few segments which can then be split up, or if the maximum segment length must be increased (lengthened).

The maximum number of segments allowed on any wire or major element side is 255. Thus any element side or wire longer than 255 times the specified ``maximum segment length'' will have segments of unacceptable length — the user will not be warned about this, but an option in the Verify menu allows the user to check the complete model for such elements. The user is advised to divide these elements into a number of smaller elements. (This limit is likely to affect only two-node wires as the limit of 50 lines in the discretising process will, in most cases, be the first to be exceeded.)