== Defining an Excitation Source ==
An electromagnetic boundary value problem requires an excitation source that typically appears as the right-hand-side (RHS) term of a differential or integral equation. A source can be an impressed electric or magnetic field or an impressed electric or magnetic current that produces an impressed electric or magnetic field. Sources can be lumped, confined to a single point or an infinitesimal region, or they can be distributed over a sizable area or volume of the computational domain. In each of [[EM.Cube]]'s computational modules, you need to choose the right source type depending on the purpose of your simulation and based on the observables you define for your project. For example, for computing the radar cross section (RCS) of a target, you need a plane wave source. If you are interested in computing the S/Z/Y parameters of your structure, then you have to choose a source type like a gap or lumped source that supports a "Port Definition" observable.
[[EM.Cube]] provides a large variety of source types for the excitation of your physical structure. Some source type have identical definitions in different computational modules. For example, plane waves behave exactly the same way in [[EM.Tempo]], [[EM.Illumina]], [[EM.Libera]] or [[EM.Picasso]]. Or Hertzian short dipole sources are identical in [[EM.Terrano]], [[EM.Illumina]], [[EM.Libera]] or [[EM.Picasso]]. On the other hand, some source types apply only to certain computational modules. For example, Gaussian beam sources appear only in [[EM.Tempo]]. In all of [[EM.Cube]]'s computational modules except for [[EM.Ferma]], the source types are listed under the '''Sources''' section of the navigation tree. In these modules, the sources either exist on their own as virtual objects or they are associated with the existing geometric objects in the project workspace. In [[EM.Ferma]], electrostatic and magnetostatic sources like volume charges or wire currents are defined as geometric objects side by side of material objects in the project workspace.