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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. In EM.Libera, a gap and is used to define an excitation source in the form of an ideal voltage source. Gap Wire gap sources can 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. Strip gap sources must be placed on long, narrow, '''PEC Rectangle Strip''' objects to provide excitation for the Surface MoM solver. The gap splits the strip into two strips with a an infinitesimally small spacing between them, across which the ideal voltage source is connected. Only narrow rectangle strip object that have a single mesh cell across their width can be used to host a gap source.

{{Note|If you want to excite a curved wire antenna such as a circular loop or helix with a wire gap source, first you have to convert the curve object into a polyline using [[CubeCAD]]'s Polygonize Tool.}}  Gap sources can also be placed on long, narrow, PEC '''Rectangle Strip''' objects to provide excitation for the Surface MoM solver. The gap splits the strip into two strips with a an infinitesimally small spacing between them, across which the ideal voltage source is connected. Only narrow rectangle strip object that have a single mesh cell across their width can be used to host a gap source. [[Image:Info_icon.png|40px]] Click here to learn more about '''[[Common_Excitation_Source_Types_in_EM.Cube#Lumped_.26_Gap_Sources | Defining Gap Sources]]'''.

A short dipole provides a another simple way of exciting a 3D structure in EM.Libera. A short dipole source acts like an infinitesimally small ideal current source. To define a short dipole source, follow these steps: [[Image:Info_icon.png|40px]] Click here You can also use an incident plane wave to learn more about '''[[Common_Excitation_Source_Types_in_EM.Cube#Hertzian_Dipole_Sources | Hertzian Dipole Sources]]'''. Your excite your physical structure in EM.Libera can be excited by an incident plane wave. 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. [[EM.Libera]] provides the following polarization options: TMz, TEz, Custom Linear, LCPz and RCPz. [[Image:Info_icon.png|40px]] Click here to learn more about '''[[Common_Excitation_Source_Types_in_EM.Cube#Plane_Wave_Sources | Plane Wave Sources]]'''. {{Note|In the spherical coordinate system, normal plane wave incidence from the top of the domain downward corresponds to θ of 180°. }}