The Draw menu allows you to draw a new structure or to edit an existing structure. It contains all the elements needed to draw a discretized model and to specify its electromagnetic properties. The TLM grid is displayed when Draw is active.

Select an item by pointing to it and clicking the left mouse button once. Upon its release the Draw menu disappears and the selected feature can be drawn into the mesh. Different types of elements have different colors and can thus be easily distinguished on the screen.

• The color of the element changes to dark purple.
• The name of the element appears in the Status Bar.
• You can then delete the element by clicking on Delete Element or hitting the Delete key.

• Note:

  If the element you have selected has properties that were specified in a dialog box (such as a Reflection Wall or a Computation Region), they can be inspected again by using the Property feature in the right mouse button menu.

• Delete Element This item has the same function as the Delete key. It deletes an item that has been "selected".

• Tip:	Another way of deleting an element is to draw the same element over it.

A dialog box prompts you to enter the number of turns on both sides. If they are identical, the Interface Wall has no effect on the simulation.

A dialog box displays a value for the relative dielectric constant. The properties of the Reflection Wall will be computed with respect to that relative dielectric constant. The default value is the relative dielectric constant of the most recent Computation Region that has been drawn (see that item below). Changing it will not affect previously entered items.

One of the following wall properties must be entered (select the appropriate radio button):

• the TEM wave reflection coefficient of the wall, or
• the absolute wall impedance in Ohms, or
• the normalized wall impedance, or 
• the local TLM impulse reflection coefficient. 

The remaining three are computed automatically. The default is a TEM absorbing wall in the dielectric medium specified above. 

MEFiSTo-2D can use only local impulse responses and modal responses for frequency-independent cross-sectional field distributions. Global Johns Matrices are not implemented since they would require massive computational resources. Up to 2 different Johns Matrices are allowed. The boundary response during a simulation is obtained by numerical convolution. The convolution options are:

• Separate convolution at all branches cut by the boundary,
• Convolution at the center branch only; the signal reflected into the other branches is identical (uniform transverse field distribution).
• Convolution at the center branch only; the signal reflected into the other branches is weighted sinusoidally (dominant mode in rectangular waveguide applications).

A Johns Matrix file must be entered from a dialog box.  Up to 2 different Johns Matrices can be used in one simulation, but several boundaries associated with the same Johns Matrix file can be used in a structure.

• TR = top right; 
• TL = top right; 
• BR = bottom right; 
• BL = bottom left. 

For air-filled structures (e_r=1) the following P-values are recommended:

• P-value = 0.123 for 90-degree corners.
• P-value = 0.105 for 180-degree corners (knife edges).

When you click OK in the dialog box, the little corner branch will appear on the screen. Make sure it points from the corner node to the edge. To remove a Corner Node, click on it with the Corner Node item selected, or use the select-and-delete procedure. 

If the corner lies in a medium other than air, or close to an interface between two different dielectrics, the P-value must be determined by a numerical experiment as described at the end of Chapter 7 of the "MEFiSTo-2D Theory" booklet.

Important — Note that TLM computation is performed only inside a Computation Region. All other nodes in the TLM grid are dead. If a structure has a complex shape, it can be divided into an arbitrary number of Computation Regions that touch each other. Thus, unnecessary computation can be avoided by placing Computation Regions only inside the domain in which the field will be computed, rather than using a single big box over the entire mesh area. Computation Regions must be contiguous but cannot overlap.  A Computation Region must be at least 1Dl wide and 2 Dl long. 

Each Computation Region can have different properties. If the node voltage Vy models the electric field in y-direction, interface conditions between touching Computation Regions with different permittivity and conductivity values are automatically satisfied. No interface wall needs to be drawn. However, if network and field quantities are dual, corrective interfaces must be placed be-tween the boxes using the Interface Wall function (See Chapter 3 of the "MEFiSTo-2D Theory" booklet).

A Varactor Diode box must be at least 1 Dl wide and 2 Dl long. 

To enter an Active Diode into the mesh, draw a box that has approximately the size and shape of the actual physical element and enter the diode parameters in the dialog box. Note that you enter the global parameters of the diode. The properties of each individual cell in its cross-section are computed automatically. Consult Chapter 5 of the "MEFiSTo-2D Theory" booklet..

An Active Diode box must be at least 1 Dl wide and 2 Dl long.

There are some important restrictions that limit the functionality of this option. They are:  

• The Variable Link should only be implemented in a subregion that has a relative dielectric constant of unity (free space).
• The Variable Link should only be implemented as a vertical boundary on the right-hand side of a structure,
• The Variable Link box should only be one mesh parameter wide,
• The Variable Link box should be bounded on the right-hand side by an Electric Wall.

A dialog box requests the following input:

Initial Alpha Value

This is the coefficient that determines the initial position of the electric wall with respect to the nodes inside the Variable Link box.

• Alpha = 0 places the Electric Wall right into the nodes, 
• Alpha = 1 places it at 1/2 Dl to the right of the nodes (equivalent to the position of a regular Electric Wall),
• Alpha = 2 places it one Dl away to the right of these nodes. 
• A negative value of Alpha causes instability, and a value larger than 2 introduces an additional dispersion error.

Increment per time step

This number is added to the present value of Alpha after each time step. A positive increment moves the Electric Wall to the right and a negative increment moves it to the left. A zero increment keeps it in the position specified by the initial Alpha Value.

Note that Alpha in the Dialog Box changes during a simulation. It must thus be reset if the same simulation is to be repeated.

Number of time steps at which the wall starts moving.

This specifies the number of time steps after which Alpha will be incremented. It allows keeping the wall stationary until a signal actually reaches the boundary.

Tip:  The comment box can be used to store the initial values.

Note Only one of the three types of special elements (Varactor Diode, Active Diode, or Variable Link) can be used in a given structure, but several elements of the same type may be implemented. In this case, all elements have identical properties, but their size can be different.

Tip: To create a source point or a probe, place the cursor on the desired node and click the left mouse button once. To draw a line or a box, put the cursor at a starting position, then press and hold the left mouse button, drag the cursor across the mesh to a new position and release the button. Elements created by the above functions can be deleted from the mesh by simply drawing the same elements over them, or by using the Select-and-Delete procedure. The program performs syntax checking on the elements created, so that partial overlapping among elements of the same type is not allowed, and total overlapping implies a deletion request.