Figure 1: [[Propagation Module]]'s Impenetrable Surface dialog.
Under an impenetrable block group, you can draw any of EM.Cube's native solid or [[Surface Objects|surface objects ]] or you can import external model files like STEP, IGES or STL. You can change the properties of an impenetrable surface. In the property dialog of the surface group, click on the table that list the properties to select and highlight a row. Then, click the '''Add/Edit''' button to open up the "Edit Layer" dialog. In this dialog, you can change the name of the material and its permittivity and electric conductivity. The box labeled "Specify Loss Tangent" is unchecked by default. If you check it, you can specify the '''Loss Tangent''' of the material, which, in turn, updates the value of electric conductivity at the center frequency of the project. You can also use EM.Cube's Material List, which will be explained later.
[[File:PROP23.png]]
Figure 1: [[Propagation Module]]'s Penetrable Surface dialog.
Under a penetrable surface group, you can draw any of EM.Cube's native solid or [[Surface Objects|surface objects ]] or you can import external model files like STEP, IGES or STL. You can change the properties of a penetrable surface group including its default thickness. In the property dialog of the surface group, click on the table that list the properties to select and highlight a row. Then, click the '''Add/Edit''' button to open up the "Edit Layer" dialog. Similar to the case of impenetrable surfaces, from this dialog, you can change the material properties (permittivity and electric conductivity) as well as '''Thickness''', which is expressed in the project units. You can also use EM.Cube's Material List, which will be explained later.
[[File:PROP25.png]]
# While impenetrable blocks can be created using any of EM.Cube's solid or surface CAD object creation tools, terrain objects are created either using EM.Cube's '''Terrain Generator''' or by importing an external terrain file.
# Terrain objects belong to a special type of CAD objects called "Tessellated Objects", which differ from other regular CAD [[Surface Objects|surface objects ]] or EM.Cube's polymesh surfaces.
# Terrain surfaces do not diffract impinging rays at their many small edges.
# Terrain objects affect the elevation of other objects or transmitters or receivers that are located above them.
=== Using Terrain Generator ===
[[File:PROP18.png|thumb|250px|[[Propagation Module]]'s Terrain Generator dialog]]
EM.Cube provides a convenient and powerful Terrain Generator for creating a variety of terrain [[Surface Objects|surface objects]]. EM.Cube's Terrain Generator looks very similar to [[CubeCAD]]'s Surface Generator. However, whereas the Surface Generator creates a generic or polymesh surface object, Terrain Generator always creates another special type of object known as a '''Tessellated Object'''. A terrain object is much simpler than EM.Cube's polymesh objects and is usually made up of triangular or quadrilateral facets. As such, terrain objects have limited editing capabilities. For example, you can cut, copy, paste, translate or rotate terrain objects. But operations like scaling, mirroring, grouping (composite), arraying, exploding, linking or Boolean operations do not work on terrain objects.
To create a new terrain object using Terrain Generator, first you need to define a terrain group in the Navigation Tree. Right click on the name of the terrain node and select '''Terrain Generator...''' from the contextual menu. This opens up the Terrain Generator Dialog. Using Terrain Generator, you can build a single terrain surface or an array of surfaces patched together. Some of the available terrain models include:
Every time you create a new terrain object using Terrain Generator, an ASCII data file named "GeneratedTerrain" with a "'''.TRN'''" file extension is created and placed in your project folder. This is EM.Cube's simple native terrain file format that basically lists all the (x, y, z) coordinates of the generated surface points on a horizontal, rectangular XY grid. Terrain Generator simply takes your custom function definition or one of the selected catalog surface types and generates the digital elevation data on the specified grid.
Another type of terrain model that the terrain generator provides is '''XY Grid Data'''. In this case, you define a rectangular XY grid with a uniform grid cell size along the X and Y directions and manually define the Z-elevation for each grid point. This is similar to the surface generator's "2D Uniform Grid" model type in [[CubeCAD]]. Based on your input to '''Range Start''', '''Range Stop''' and '''Range Step''' along X and Y, a 2D grid is set up and displayed in a table at the bottom of the terrain generator dialog. By default, all the Z-elevations are set to zero initially. You can click on each table cell and overwrite it with a new value. At the end, click the '''Create''' button of the dialog to add the new grid-based terrain object to the Navigation Tree.
[[File:terrain10_tn.png]]
When you start a new project in EM.Cube's [[Propagation Module]] and draw a solid object like a box in the project workspace without having defined any surface groups, it is assumed to be of the impenetrable surface type. A default impenetrable surface group called Block_1 is automatically added to the Navigation Tree, which holds your newly drawn object. The default group has the material properties of "Brick" (e<sub>r</sub> = 4.4 and s = 0.001 S/m.) with a dark brown color. You can continue drawing new objects in the project workspace and adding them under this block node. Or you can define a new surface type with different properties. By default, the last surface group that was defined is '''Active'''. The current active surface group is always listed in bold letters in the Navigation Tree. When you draw a new object, it is always inserted under the current active surface group. Any surface group can be activated by right clicking its name in the Navigation Tree and selecting the '''Activate''' item of the contextual menu.
You can move any object from its current surface group into any other available surface group. First select the object, then right click on its surface and select '''MoveTo > Propagation >'''. A submenu appears which lists all the available surface groups where you can transfer the selected object. You can also move objects among surface groups by selecting their names in the Navigation Tree and using the contextual menu. In a similar way, you can transfer objects from [[Propagation Module]] to EM.Cube's other modules or vice versa. '''Keep in mind that all the external model files such as STEP, IGES, STL, etc. are first imported to EM.Cube's [[CubeCAD]], from which you can transfer them to other modules.''' First select the object, then right click and select '''MoveTo >'''. In the submenu you will see a list of all the EM.Cube modules that have at least one available group where you can transfer your selected object. You can select multiple objects for transfer. When using the keyboard's '''Shift Key''' or '''Ctrl Key''' for multiple selection, make sure that those keys are held down, when you right click to access the contextual menu.
== Defining Sources & Observables ==
In a typical SBR simulation, a ray is traced from the location of the source until it hits a scatterer. The SBR method assumes that the ray hits either a flat facet of the scatterer or one of its edges. In the case of hitting a flat facet, the specular point is used to launch new reflected and transmitted rays. The surface of the facet is treated as an infinite dielectric medium interface, at which the reflection and transmission coefficients are calculated. In the case of hitting an edge, new diffracted rays are generated in the scene. However, only those who reach a nearby receiver in their line of sight are ever taken into account. In other words, diffractions are treated locally.
EM.Cube's [[Propagation Module]] allows you to draw any type of surface or solid CAD objects under impenetrable and penetrable surface groups. Some of these objects have flat faces such as boxes, pyramids, rectangle or triangle strips, etc. Some others contain curved surfaces or curved boundaries such as cylinders, cones, etc. All the non-flat surfaces have to be discretized in the form of a collection of smaller flat facets. EM.Cube uses a triangular surface mesh generator to discretize the penetrable and impenetrable [[Surface Objects|surface objects ]] of your propagation scene. This mesh generator is very similar to the ones used in EM.Cube's two other modules: MoM3D and Physical Optics (PO).
You can build a variety of surface and [[Solid Objects|solid objects ]] using EM.Cube's native "Curve" CAD objects like lines, polylines, circles, etc. You can use tools like Extrude, Loft, Strip-Sweep, Pipe-Sweep, etc. to transform curves into surface or [[Solid Objects|solid objects]]. '''However, keep in mind that all the "Curve" CAD objects are ignored by the SBR mesh generator and are therefore not sent to the simulation engine.'''
=== Viewing SBR Mesh ===