The slot model is used for narrow apertures in a thin metal face. It is intended for apertures that are too narrow to be effectively modelled as features in the geometric model.
Slots are small in cross-sectional size, but with a relatively long length that can contribute to a significant electromagnetic field penetration. In the absence of a special slot model, a very fine mesh would be required to represent the strong variation of electromagnetic fields in the vicinity of the slot. Typically, at least five cells across the slot gap, five cells of similar size above and below the gap and five cells of similar size on each side of the metal panel would be required for an acceptable modelling accuracy. Obviously, such an approach would make the simulation of slots computationally very expensive.
The sub-cell slot model used in the 3D-TLM Simulator overcomes this difficulty by inserting a special lumped representation of the slot geometry into an otherwise coarse mesh. This approach vastly reduces the computational requirements for the simulation. A saving of the order of a factor of 100 in the computer run-time and a factor of 10 in the memory requirements can be achieved by using the slot model. This enables quick simulation times, even for models which were previously impractical to simulate due to excessive computer requirements.
A slot must be attached to a thin metal face and its width and depth must be defined as positive numbers. The slot model achieves the best accuracy if the slot width is relatively small compared to the cell dimension (across the slot) of the adjacent cells. For example, 10% of the cell dimension is a sensible measure for the slot width. The maximum slot width allowed in the model is 40% of the corresponding cell size. This means that if a slot is 4 mm wide, the minimum cell size across the slot would be 10 mm.
The slot depth is defined by the user and should correspond to the thickness of the supporting metal panel. However, the slot depth can be greater than the thickness of the supporting panel. This can be used to approximate seams created by two overlapping metal panels, in which case the slot depth represents the overlapping length of two panels. The slot depth is restricted to five times the cell dimensions normal to the slot plane.
In addition to simulating air-filled slots, the slot model can be used to simulate slots containing a conductive material (gasket) deliberately included to reduce the penetration of electromagnetic fields through the slot. The relative permittivity and conductivity of the gasket material are specified by the user and the gasket material is assumed to fill the volume of the slot.
The geometry of the slot is specified by the slot path. The slot path is defined using a set of points on the metal face. Generally, five or more cells are required to model the length of a straight slot section if the field description in the slot is to be modelled accurately.
More complicated slot structures can be obtained by joining two or more individual slots. For example, to create a four-way slot junction on a plane, the user can either create four individual straight slots with a common junction point, or create two individual slot bends overlapping at the bend point. Note, however, that this structure cannot be made from two straight slots overlapping in the middle. Slot can only be joined end to end.
Individual slots on the same metal face, which are not meant to be connected, should be kept apart by at least one cell. If this recommendation is not followed, slots which are too close to each other may be joined during the automatic meshing, thus creating unwanted slot connections. The 3D TLM simulator may trap this and an error will be issued - for example, when two parallel slots are too close to each other. However, if the erroneous slot connection occurs at the end of slots the 3D TLM simulator may accept the model, but it will not be correct.
In addition to the slot bends and junction made by slots joining on the same metal face, out-of-plane slot bends can be modelled. This kind of slot connections can be created by defining two slots running along orthogonal metal faces and ending at the edge of their respective metal faces. Out-of-plane slot junctions are not allowed.
Finally, the user should note that, after discretization, the slots are always positioned in the middle of their cell face patches. This should be observed when positioning slots running parallel with and very close to the edge of the metal face. In this case, the distance of slots from the metal edge is determined by the size of the adjacent cell patches and not by the geometrical distance.