[[Image:MOM6B.png|thumb|320px|EM.Libera's Strip Gap Source dialog.]]
Your 3D physical structure must be excited by some sort of signal source that induces electric linear currents on thin wires, electric surface currents on metal surface and both electric magnetic surface currents on the surface of dielectric objects. The excitation source you choose depends on the observables you seek in your project. EM.Libera provides the following source types for exciting your physical structure (click on each type to learn more about it):
* {| class="wikitable"|-! scope="col"| Source Type! scope="col"| Applications! scope="col"| Restrictions|-| '''[[Common_Excitation_Source_Types_in_EMGlossary of EM.Cube's Excitation Sources#Lumped_.26_Gap_Sources Strip Gap Circuit Source | Strip Gap SourcesCircuit Source]]'''* | style="width:300px;" | General-purpose point voltage source | style="width:300px;" | Associated with a PEC rectangle strip, works only with SMOM solver|-| '''[[Common_Excitation_Source_Types_in_EMGlossary of EM.Cube's Excitation Sources#Lumped_.26_Gap_Sources Wire Gap Circuit Source | Wire Gap SourcesCircuit Source]]'''* | style="width:300px;" | General-purpose point voltage source| style="width:300px;" | Associated with an PEC or thin wire line or polyline|-| '''[[Common_Excitation_Source_Types_in_EMGlossary of EM.Cube's Excitation Sources#Hertzian_Dipole_Sources Hertzian Short Dipole Source |Hertzian Short Dipole SourcesSource]]'''* | style="width:300px;" | Almost omni-directional physical radiator| style="width:300px;" | None, stand-alone source|-| '''[[Common_Excitation_Source_Types_in_EMGlossary of EM.Cube's Excitation Sources#Plane_Wave_Sources Plane Wave | Plane Wave SourcesSource]]'''* | style="width:300px;" | Used for modeling scattering & computation of reflection/transmission characteristics of periodic surfaces| style="width:300px;" | None, stand-alone source|-| '''[[Hybrid_Modeling_using_Multiple_Simulation_EnginesGlossary of EM.Cube's Excitation Sources#Working_with_Huygens_Sources Huygens Source | Huygens SourcesSource]]'''| style="width:300px;" | Used for modeling equivalent sourced imported from other [[EM.Cube]] modules | style="width:300px;" | Imported from a Huygens surface data file|}Â Click on each category to learn more details about it in the [[Glossary of EM.Cube's Excitation Sources]].
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.