# '''[[Common_Excitation_Source_Types_in_EM.Cube#Gaussian_Beam_Sources |Gaussian Beam Source]]''': A distributed source with a complex-valued focused Gaussian beam profile defined using a virtual box object enclosing the entire physical structure.
A lumped source is the most commonly used way of exciting a structure in EM.Tempo. A lumped source is an ideal source that must be placed on a line object that is parallel to one of the three principal axes and shows up as a small red arrow on the host line. Lumped sources are typically used to define ports and compute the port characteristics like S/Y/Z [[parameters]]. Using simple lumped sources, you can simulate a variety of transmission line structures in [[EM.Tempo]] including filters, couplers or antenna feeds. This approach may become less accurate at very high frequencies when the details of the feed structures become important and can no longer be modeled with highly localized lumped ports. In such cases, it is recommended to use âDistributed Sourcesâ, which utilize accurate modal field distributions at the ports for calculation of the incident and reflected waves. Waveguide source is used to excite the dominant TE<sub>10</sub> mode of a hollow rectangular waveguide. Waveguide sources typically provide more accurate results for scattering [[parameters]] of waveguide structures compared to lumped sources as they represent the actual dominant propagating modes at the transmission line ports.
[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Common_Excitation_Source_Types_in_EM.Cube#Lumped_.26_Gap_Sources | Lumped Sources]]'''.
{{Note|In order to define a waveguide source, you must have at least one hollow box object with no caps or only one end cap or a hollow box array in your project.}}
A plane wave source is another a popular excitation method that is used for calculation of the radar cross section of targets or reflection and transmission characteristics of periodic surfaces. Â [[A Gaussian beam source is another source type that is highly localized as opposed to the uniform plane wave. For both plane wave and Gaussian beam sources, EM.Tempo]] requires a finite plane wave incidence surface to calculate the excitation. When you create either of these sources, a plane wave source, box or a plane wave Gaussian box is created as part of its their definition. A trident symbol on the box shows the propagation vector as well as the E-field and H-field polarization vectors. The time domain plane wave or Gaussian beam excitation is calculated on the surface of this box and injected into the computational domain. The plane wave box is displayed in the project workspace as a purple wireframe box enclosing the structure, while the Gaussian beam box appears as a green wireframe box. InitiallyBoth boxes have an initial default size with an offset of 0.2λ<sub>0</sub> from the largest bounding box enclosing your entire physical structure. In both source dialogs, the radio button '''Size: Default''' is selected. With this option, the boundaries of the excitation box always have a distance of three cells from the bounding box of the geometry and cannot be changedby default. The radio button '''Size: Custom''' allows you to set the excitation box manually. The values for the coordinates of '''Corner 1''' and '''Corner 2''' can now be changed. Corner 1 is the front lower left corner and Corner 2 is the rear upper right corner of the box. The box has to be corner coordinates are defined in the world coordinate system (WCS).
[[EM.Tempo]] gives you an option to illuminate objects with a focused beam instead of a uniform plane wave. The focused beam is a Gaussian beam, which is a solution of the paraxial approximation to the Helmholtz equation. The fundamental Gaussian beam is rotationally-symmetric about its propagation axis, and its transverse field distribution follows a Gaussian function profile. The critical parameter is the beam radius w<sub>0</sub>; it is the point where the field drops by 1/e from its value at the center. The beam opens up into a cone along the propagation direction, with a cone angle of tan θ = λ<sub>0</sub>/(π.ω<sub>0</sub>) (λ<sub>0</sub> is the free-space wavelength).
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Similar to the plane wave source, a Gaussian beam is define by spherical angles of incidence Theta and Phi in degrees. You can also set the '''Polarization''' of the Gaussian Beam and choose from the three options: '''TM<sub>z</sub>''', '''TE<sub>z</sub>''' and '''User Defined'''. A default Excitation Box three cells away from the bounding box of the geometry is initially suggested, i.e. the radio button '''Size: Default''' is selected by default. The radio button '''Size: Custom''' allows you to set the excitation box manually by modifying the coordinates of '''Corner 1''' (front lower left) and '''Corner 2''' (back upper right) of the box in the world coordinate system (WCS). The Gaussian beam box is displayed in the project workspace as a green wireframe box enclosing the structure. A translucent green circle normal to the direction propagation shows the footprint of Gaussian beam at its focal (waist) point.
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Unlike plane waves, a Gaussian beam is a localized field. Therefore, you need to specify the '''Beam Properties'''. This includes the coordinates of the beam's '''Focus''', which is the beam's waist center in the world coordinate system as well as the beam's '''Radius''' in project units.
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[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Common_Excitation_Source_Types_in_EM.Cube#Plane_Wave_Sources | Plane Wave Sources]]'''.