Structure Modelling

The tasks required by the user are the following: The structure must first be divided into fairly flat polygonal surfaces, which are called major elements (see Fig.~#fg1#526>). WIREGRID can handle up to 160 such major elements. Each major element does not need to lie <#187#>completely<#187#> in a 2D plane, but severely ``twisted'' elements are really beyond the capability of the code. The frequent visual feedback available from the program will permit the user to identify unsuitable elements and rectify them. As mentioned in the Introduction, the code was developed for structures consisting mainly of flat surfaces; it is not really suited modelling curved surfaces, such as an aircraft fuselage. The coordinates of each major element's vertices, called major nodes, must be determined next (see Fig.~#fg1#527>). The simplest major element is defined by two major nodes, which would model a single straight wire. The most complex major element may be defined by a maximum of 10 major nodes.

When generating a mesh, it is important to ensure that meshes in adjacent elements are properly electrically connected across the boundary. (NEC2 users will be aware of a particularly insidious fault caused by two wires whose end-points that are closely located and should be physically connected, but are treated by the code as unconnected.) We will use the term conformality in this paper to describe a properly connected mesh. WIREGRID will ensure conformality provided that major nodes of adjacent major elements coincide on the boundary that the elements share. These requirements mean that when vertices on the boundary between two adjacent polygons are not coincident, extra major nodes for one or both of them must be defined. These extra nodes must coincide with the unmatched vertices (see Fig.~#fg2#528>). The same technique can be used for ensuring that a whip antenna is properly grounded to the vehicle frame. Superfluous nodes (for example, a node in the middle of an element side when there are no conformality problems) do not cause any problem within the program (providing of course that all elements sharing that side contain the extra node). However, the wire mesh generated may be distorted by their presence, and it is recommended that such unnecessary major nodes be avoided.

An additional geometric requirement for the major elements is that they must be convex, i.e. a straight line joining any two points in the polygon must still be enclosed by the polygon boundaries. The reason for this requirement is that it greatly simplifies the task of the mesh generator in WIREGRID. It can be adhered to easily, since any non-convex polygon can be divided into two or more convex parts.

A number tag starting from 0 is assigned by WIREGRID to each major element. It is often helpful to keep a rough sketch of the structure, indicating all the major elements with their number tags. In the NEC2 input file that WIREGRID generates, all wires belonging to a certain major element will be tagged with this number. When certain wires are to be loaded, excited or even altered by hand, this will help to locate them.