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[[Image:Tutorial_icon.png|30px]] '''[[EM.Cube#EM.Libera_Tutorial_Lessons Libera_Documentation | EM.Libera Tutorial Gateway]]'''
[[Image:Back_icon.png|30px]] '''[[EM.Cube | Back to EM.Cube Main Page]]'''
You can use [[EM.Libera]] either for simulating arbitrary 3D metallic, dielectric and composite surfaces and volumetric structures or for modeling wire objects and metallic wireframe structures. [[EM.Libera]] also serves as the frequency-domain, full-wave '''MoM3D Module''' of '''[[EM.Cube]]''', a comprehensive, integrated, modular electromagnetic modeling environment. [[EM.Libera]] shares the visual interface, 3D parametric CAD modeler, data visualization tools, and many more utilities and features collectively known as [[Building Geometrical Constructions in CubeCAD | CubeCAD]] with all of [[EM.Cube]]'s other computational modules.
[[Image:Info_icon.png|30px]] Click here to learn more about '''[[Getting_Started_with_EM.CUBE Cube | EM.Cube Modeling Environment]]'''.
=== Advantages & Limitations of EM.Libera's Surface MoM & Wire MoM Solvers ===
== Building the Physical Structure in EM.Libera ==
[[Image:wire_pic1.png|thumb|350px|EM.Libera's Navigation Tree.]]
All the objects in your project workspace are organized into object groups based on their material composition and geometry type in the "Physical Structure" section of the navigation tree. In [[EM.Libera]], you can create three different types of objects:
| style="width:250px;" | Solid objects
| Surface MoM solver only
|-
| style="width:30px;" | [[File:Virt_group_icon.png]]
| style="width:150px;" | [[Glossary of EM.Cube's Materials, Sources, Devices & Other Physical Object Types#Virtual_Object_Group | Virtual Object]]
| style="width:300px;" | Used for representing non-physical items
| style="width:250px;" | All types of objects
| None
|}
Both of [[EM.Libera]]'s two simulation engines, Wire MoM and Surface MoM, can handle metallic structures. You define wires under '''Thin Wire''' groups and surface and volumetric metal objects under '''PEC Objects'''. In other words, you can draw lines, polylines and other curve objects as thin wires, which have a radius parameters expressed in project units. All types of solid and surface CAD objects can be drawn in a PEC group. Only solid CAD objects can be drawn under '''Dielectric Objects'''.
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[[Image:wire_pic1.png|thumb|350px|EM.Libera's Navigation Tree.]]
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Once a new object group node has been created on the navigation tree, it becomes the "Active" group of the project workspace, which is always listed in bold letters. When you draw a new CAD object such as a Box or a Sphere, it is inserted under the currently active group. There is only one object group that is active at any time. Any object type can be made active by right clicking on its name in the navigation tree and selecting the '''Activate''' item of the contextual menu. It is recommended that you first create object groups, and then draw new CAD objects under the active object group. However, if you start a new [[EM.Libera]] project from scratch, and start drawing a new object without having previously defined any object groups, a new default PEC object group is created and added to the navigation tree to hold your new CAD object.
[[Image:Info_icon.png|40px30px]] Click here to learn more about '''[[Defining_Materials_in_EM.CubeBuilding Geometrical Constructions in CubeCAD#Moving_Objects_among_Material_Groups Transferring Objects Among Different Groups or Modules | Moving Objects among Material Different Groups]]'''.
{{Note|In [[EM.Cube]], you can import external CAD models (such as STEP, IGES, STL models, etc.) only to [[Building_Geometrical_Constructions_in_CubeCAD | CubeCAD]]. From [[Building_Geometrical_Constructions_in_CubeCAD | CubeCAD]], you can then move the imported objects to [[EM.Libera]].}}
== EM.Libera's Excitation Sources ==
|-
| style="width:30px;" | [[File:gap_src_icon.png]]
| [[Glossary of EM.Cube's Excitation Materials, Sources, Devices & Other Physical Object Types#Strip Gap Circuit Source |Strip Gap Circuit Source]]
| style="width:300px;" | General-purpose point voltage source
| style="width:300px;" | Associated with a PEC rectangle strip, works only with SMOM solver
|-
| style="width:30px;" | [[File:gap_src_icon.png]]
| [[Glossary of EM.Cube's Excitation Materials, Sources, Devices & Other Physical Object Types#Wire Gap Circuit Source |Wire Gap Circuit Source]]
| style="width:300px;" | General-purpose point voltage source
| style="width:300px;" | Associated with an PEC or thin wire line or polyline, works only with WMOM solver
|-
| style="width:30px;" | [[File:hertz_src_icon.png]]
| [[Glossary of EM.Cube's Excitation Materials, Sources, Devices & Other Physical Object Types#Hertzian Short Dipole Source |Hertzian Short Dipole Source]]
| style="width:300px;" | Almost omni-directional physical radiator
| style="width:300px;" | None, stand-alone source
|-
| style="width:30px;" | [[File:plane_wave_icon.png]]
| [[Glossary of EM.Cube's Excitation Materials, Sources, Devices & Other Physical Object Types#Plane Wave |Plane Wave Source]]
| style="width:300px;" | Used for modeling scattering
| style="width:300px;" | None, stand-alone source
|-
| style="width:30px;" | [[File:huyg_src_icon.png]]
| [[Glossary of EM.Cube's Excitation Materials, Sources, Devices & Other Physical Object Types#Huygens Source |Huygens Source]]| style="width:300px;" | Used for modeling equivalent sourced sources imported from other [[EM.Cube]] modules
| style="width:300px;" | Imported from a Huygens surface data file
|}
Click on each category to learn more details about it in the [[Glossary of EM.Cube's Excitation Materials, Sources, Devices & Other Physical Object Types]].
For antennas and planar circuits, where you typically define one or more ports, you usually use lumped sources. [[EM.Libera]] provides two types of lumped sources: strip gap and wire gap. A Gap is an infinitesimally narrow discontinuity that is placed on the path of the current and is used to define an ideal voltage source. Wire gap sources must be placed on '''Thin Wire Line''' and '''Thin Polyline''' objects to provide excitation for the Wire MoM solver. The gap splits the wire into two lines with a an infinitesimally small spacing between them, across which the ideal voltage source is connected.
A short dipole provides another simple way of exciting a 3D structure in [[EM.Libera]]. A short dipole source acts like an infinitesimally small ideal current source. You can also use an incident plane wave to excite your physical structure in [[EM.Libera]]. In particular, you need a plane wave source to compute the radar cross section of a target. The direction of incidence is defined by the θ and φ angles of the unit propagation vector in the spherical coordinate system. The default values of the incidence angles are θ = 180° and φ = 0° corresponding to a normally incident plane wave propagating along the -Z direction with a +X-polarized E-vector. Huygens sources are virtual equivalent sources that capture the radiated electric and magnetic fields from another structure that was previously analyzed in another [[EM.Cube]] computational module.
[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Common_Excitation_Source_Types_in_EM.CubePreparing_Physical_Structures_for_Electromagnetic_Simulation#Defining_FiniteModeling_Finite-Sized_Source_Arrays | Using Source Arrays in Antenna Arrays]]'''.
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In [[EM.Libera]], you can define simple lumped elements in a similar manner as gap sources. In fact, a lumped element is equivalent to an infinitesimally narrow gap that is placed in the path of the current, across which Ohm's law is enforced as a boundary condition. You can define passive RLC lumped elements or active lumped elements containing a voltage gap source. The latter case can be used to excite a wire structure or metallic strip and model a non-ideal voltage source with an internal resistance. [[EM.Libera]]'s lumped circuit represent a series-parallel combination of resistor, inductor and capacitor elements. This is shown in the figure below:
[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Modeling_Lumped_Elements,_Circuits_%26_Devices_in_EM.CubePreparing_Physical_Structures_for_Electromagnetic_Simulation#Defining_Lumped_Elements_in_EM.Picasso_.26_EM.Libera Modeling_Lumped_Elements_in_the_MoM_Solvers | Defining Lumped Elements]]'''.
[[Image:Info_icon.png|40px]] Click here for a general discussion of '''[[Modeling_Lumped_Elements,_Circuits_%26_Devices_in_EM.CubePreparing_Physical_Structures_for_Electromagnetic_Simulation#Linear_Passive_Devices A_Review_of_Linear_.26_Nonlinear_Passive_.26_Active_Devices | Linear Passive Devices]]'''.
=== Defining Ports ===
Ports are used to order and index gap sources for S parameter calculation. They are defined in the '''Observables''' section of the navigation tree. By default, as many ports as the total number of sources are created. You can define any number of ports equal to or less than the total number of sources. All port impedances are 50Ω by default.
[[Image:Info_icon.png|40px]] Click here to learn more about the '''[[Common_Excitation_Source_Types_in_EMGlossary_of_EM.Cube%27s_Simulation_Observables_%26_Graph_Types#The_Port_Definition_Observable Port_Definition_Observable | Port Definition Observable]]'''.
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| style="width:30px;" | [[File:currdistr_icon.png]]
| style="width:150px;" | Current Distribution Maps
| style="width:150px;" | [[Glossary of EM.Cube's Simulation Observables& Graph Types#Current Distribution |Current Distribution]]
| style="width:300px;" | Computing electric surface current distribution on metal and dielectric objects, magnetic surface current distribution on dielectric objects and linear current distribution on wires
| style="width:250px;" | None
| style="width:30px;" | [[File:fieldsensor_icon.png]]
| style="width:150px;" | Near-Field Distribution Maps
| style="width:150px;" | [[Glossary of EM.Cube's Simulation Observables& Graph Types#Near-Field Sensor |Near-Field Sensor]]
| style="width:300px;" | Computing electric and magnetic field components on a specified plane in the frequency domain
| style="width:250px;" | None
| style="width:30px;" | [[File:farfield_icon.png]]
| style="width:150px;" | Far-Field Radiation Characteristics
| style="width:150px;" | [[Glossary of EM.Cube's Simulation Observables& Graph Types#Far-Field Radiation Pattern |Far-Field Radiation Pattern]]
| style="width:300px;" | Computing the radiation pattern and additional radiation characteristics such as directivity, axial ratio, side lobe levels, etc.
| style="width:250px;" | None
| style="width:30px;" | [[File:rcs_icon.png]]
| style="width:150px;" | Far-Field Scattering Characteristics
| style="width:150px;" | [[Glossary of EM.Cube's Simulation Observables& Graph Types#Radar Cross Section (RCS) |Radar Cross Section (RCS)]]
| style="width:300px;" | Computing the bistatic and monostatic RCS of a target
| style="width:250px;" | Requires a plane wave source
| style="width:30px;" | [[File:port_icon.png]]
| style="width:150px;" | Port Characteristics
| style="width:150px;" | [[Glossary of EM.Cube's Simulation Observables& Graph Types#Port Definition |Port Definition]]
| style="width:300px;" | Computing the S/Y/Z parameters and voltage standing wave ratio (VSWR)
| style="width:250px;" | Requires one of these source types: lumped, distributed, microstrip, CPW, coaxial or waveguide port
| style="width:30px;" | [[File:huyg_surf_icon.png]]
| style="width:150px;" | Equivalent electric and magnetic surface current data
| style="width:150px;" | [[Glossary of EM.Cube's Simulation Observables& Graph Types#Huygens Surface |Huygens Surface]]
| style="width:300px;" | Collecting tangential field data on a box to be used later as a Huygens source in other [[EM.Cube]] modules
| style="width:250px;" | None
|}
Click on each category to learn more details about it in the [[Glossary of EM.Cube's Simulation Observables& Graph Types]].
Depending on the types of objects present in your project workspace, [[EM.Libera]] performs either a Surface MoM simulation or a Wire MoM simulation. In the former case, the electric and magnetic surface current distributions on the surface of PEC and dielectric objects can be visualized. In the latter case, the linear electric currents on all the wires and wireframe objects can be plotted.
You need to define a far field observable if you want to plot radiation patterns of your physical structure in [[EM.Libera]]. After a 3D MoM simulation is finished, three radiation patterns plots are added to the far field entry in the Navigation Tree. These are the far field component in Theta direction, the far field component in Phi direction and the total far field.
[[Image:Info_icon.png|40px30px]] Click here to learn more about the theory of '''[[Data_Visualization_and_ProcessingDefining_Project_Observables_%26_Visualizing_Output_Data#Using_Array_Factors_to_Model_Antenna_Arrays Using_Array_Factor_to_Model_Antenna_Arrays | Using Array Factors to Model Antenna Arrays ]]'''.
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[[EM.Libera]] features two simulation engines, Wire MoM and Surface MoM, which require different mesh types. The Wire MoM simulator handles only wire objects and wireframe structures. These objects are discretized as elementary linear elements (filaments). A wire is simply subdivided into smaller segments according to a mesh density criterion. Curved wires are first converted to multi-segment polylines and then subdivided further if necessary. At the connection points between two or more wires, junction basis functions are generated to ensure current continuity.
On the other hands, [[EM.Libera]]'s Surface MoM solver requires a triangular surface mesh of surface and [[Solid Objects|solid objects]].The mesh generating algorithm tries to generate regularized triangular cells with almost equal surface areas across the entire structure. You can control the cell size using the "Mesh Density" parameter. By default, the mesh density is expressed in terms of the free-space wavelength. The default mesh density is 10 cells per wavelength. For meshing surfaces, a mesh density of 7 cells per wavelength roughly translates to 100 triangular cells per squared wavelength. Alternatively, you can base the definition of the mesh density on "Cell Edge Length" expressed in project units.
[[Image:Info_icon.png|40px30px]] Click here to learn more about [[EM.Libera]]'s '''[[Mesh_Generation_Schemes_in_EMPreparing_Physical_Structures_for_Electromagnetic_Simulation#Working_with_EM.Cube#The_Triangular_Surface_Mesh_Generator .27s_Mesh_Generators | Triangular Surface Working with Mesh Generator ]]'''.
[[Image:Info_icon.png|40px30px]] Click here to learn more about '''[[Preparing_Physical_Structures_for_Electromagnetic_Simulation#The_Triangular_Surface_Mesh_Generator | EM.Libera's Triangular Surface Mesh Generator ]]'s '''. <table><tr><td> [[Mesh_Generation_Schemes_in_EMImage:Mesh5.Cube#The_Linear_Wireframe_Mesh_Generator png| thumb|400px|EM.Libera's Mesh Settings dialog showing the parameters of the linear wireframe mesh generator.]] </td></tr></table> === The Linear Wireframe Mesh Generator === You can analyze metallic wire structures very accurately with utmost computational efficiency using [[EM.Libera]]'s Wire MoM simulator. When you structure contains at least one PEC line, polyline or any curve CAD object, [[EM.Libera]] will automatically invoke its linear wireframe mesh generator. This mesh generator subdivides straight lines and linear segments of polyline objects into or linear elements according to the specified mesh density. It also polygonizes rounded [[Curve Objects|curve objects]] into polylines with side lengths that are determined by the specified mesh density. Note that polygonizing operation is temporary and solely for he purpose of mesh generation. As for surface and solid CAD objects, a wireframe mesh of these objects is created which consists of a large number of interconnected linear (wire) elements. {{Note| The linear wireframe mesh generator discretizes rounded curves temporarily using CubeCAD's Polygonize tool. It also discretizes surface and solid CAD objects temporarily using CubeCAD's Polymesh tool.}} <table><tr><td> [[Image:Mesh6.png|thumb|200px|The geometry of an expanding helix with a circular ground.]] </td><td> [[Image:Mesh7.png|thumb|200px|Wireframe mesh of the helix with the default mesh density of 10 cells/λ<sub>0</sub>.]] </td><td> [[Image:Mesh8.png|thumb|200px|Wireframe mesh of the helix with a mesh density of 25 cells/λ<sub>0</sub>.]] </td><td> [[Image:Mesh9.png|thumb|200px|Wireframe mesh of the helix with a mesh density of 50 cells/λ<sub>0</sub>.]] </td></tr></table>
=== Mesh of Connected Objects ===
[[Image:MOM3.png|thumb|300px|EM.Libera's Mesh Hierarchy dialog.]]
All the objects belonging to the same PEC or dielectric group are merged together using the Boolean union operation before meshing. If your structure contains attached, interconnected or overlapping solid objects, their internal common faces are removed and only the surface of the external faces is meshed. Similarly, all the surface objects belonging to the same PEC group are merged together and their internal edges are removed before meshing. Note that a solid and a surface object belonging to the same PEC group might not always be merged properly.
When two objects belonging to two different material groups overlap or intersect each other, [[EM.Libera]] has to determine how to designate the overlap or common volume or surface. As an example, the figure below shows a dielectric cylinder sitting on top of a PEC plate. The two object share a circular area at the base of the cylinder. Are the cells on this circle metallic or do they belong to the dielectric material group? Note that the cells of the junction are displayed in a different color then those of either groups. To address problems of this kind, [[EM.Libera]] does provide a "Material Hierarchy" table, which you can modify. To access this table, select '''Menu > Simulate < > discretization < > Mesh Hierarchy...'''. The PEC groups by default have the highest priority and reside at the top of the table. You can select an group from the table and change its hierarch hierarchy using the {{key|Move Up}} or {{key|Move Down}} buttons of the dialog. You can also change the color of junction cells that belong to each group. <table><tr><td> [[Image:MOM3.png|thumb|300px|EM.Libera's Mesh Hierarchy dialog.]] </td></tr></table>
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=== Using Polymesh Objects to Connect Wires to Wireframe Surfaces ===
If the project workspace contains a line object, the wireframe mesh generator is used to discretize your physical structure. From the point of view of this mesh generator, all PEC [[Surface Objects|surface objects]] and PEC [[Solid Objects|solid objects]] are treated as wireframe objects. If you want to model a wire radiator connected to a metal surface, you have to make sure that the resulting wireframe mesh of the surface has a node exactly at the location where you want to connect your wire. This is not guaranteed automatically. However, you can use [[EM.Cube]]'s polymesh objects to accomplish this objective.
{{Note|In [[EM.Cube]], polymesh objects are regards regarded as already-meshed objects and are not re-meshed again during a simulation.}}
You can convert any surface object or solid object to a polymesh using [[CubeCAD]]'s '''Polymesh Tool'''.
[[Image:Info_icon.png|40px30px]] Click here to learn more about '''[[Discretizing_ObjectsGlossary_of_EM.Cube%27s_CAD_Tools#Converting_Objects_to_Polymesh Polymesh_Tool | Converting Object to Polymesh]]''' in [[EM.Cube]].
Once an object is converted to a polymesh, you can place your wire at any of its nodes. In that case, [[EM.Libera]]'s Wire MoM engine will sense the coincident nodes between line segments and will create a junction basis function to ensure current continuity.
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