The Electromagnetic Waves, Asymptotic Scattering Interface
The Electromagnetic Waves, Asymptotic Scattering (ewas) interface (), found under the Radio Frequency branch () when adding a physics interface, is used for quick studies of the far-field response of a 3D or 2D object to a given background field. The physics interface sets up a surface electric background field for the far-field transformation, using the Stratton–Chu formula, performed in the postprocessing.
Use this physics interface in 2D and 3D when approximating the scattered far-field of an object configured only by a perfect electric conductor boundary condition. The physics interface supports the Frequency Domain study type. The Frequency Domain study can also be used for sweeping the background field by a sequence of frequencies.
When this physics interface is added, these default nodes are also added to the Model Builder: Asymptotic Scattering, Far-Field Calculation, and Initial Values. No additional boundary feature is needed in general. However, from the Physics toolbar, a new Far-Field Calculation can be added to process the far-field calculation on user-defined boundary selections. You can also right-click Electromagnetic Waves, Asymptotic Scattering to select physics features from the context menu.
Settings
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is ewas.
Physics-Controlled Mesh
The physics-controlled mesh is controlled from the Mesh node’s Settings window (if the Sequence type is Physics-controlled mesh). In the table in the Physics-Controlled Mesh section, find the physics interface in the Contributor column and select or clear the checkbox in the Use column on the same row for enabling (the default) or disabling contributions from the physics interface to the physics-controlled mesh.
When the Use checkbox for the physics interface is selected, this invokes a parameter for the maximum mesh element size in free space. The physics-controlled mesh automatically scales the maximum mesh element size as the wavelength changes.
When the Use checkbox is selected for the physics interface in the section for the physics interface below the table, choose one of the four options for the Maximum mesh element size control parameter — From study (the default), User defined, Frequency, or Wavelength. When From study is selected, 1/5 of the vacuum wavelength from the highest frequency defined in study step is used for the maximum mesh element size. For the option User defined, enter a suitable Maximum element size in free space. For example, 1/5 of the vacuum wavelength or smaller. When Frequency is selected, enter the highest frequency intended to be used during the simulation. The maximum mesh element size in free space is 1/5 of the vacuum wavelength for the entered frequency. For the Wavelength option, enter the smallest vacuum wavelength intended to be used during the simulation. The maximum mesh element size in free space is 1/5 of the entered wavelength.
Formulation
For Scattered field select a Background wave type according to the following table:
Table 4-2: Background wave type based on component dimension.
Component
Background wave type
2D
User defined (default)
3D
User defined (default), Linearly polarized plane wave
User Defined
Enter the component expressions for the Background electric field Eb (SI unit: V/m). The entered expressions must be differentiable.
Notice that expressions including coupling operators are not differentiable and cannot be used as background fields.
Linearly Polarized Plane Wave
The initial background wave is predefined as E0 = exp(−jkxx)z. This field is transformed by three successive rotations along the roll, pitch, and yaw angles, in that order. For a graphic representation of the initial background field and the definition of the three rotations; compare with Figure 4-1 below.
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Enter an Electric field amplitude E0 (SI unit: V/m). The default is 1 V/m.
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Enter a Roll angle (SI unit: rad), which is a right-handed rotation with respect to the +x direction. The default is 0 rad, corresponding to polarization along the +z direction.
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Enter a Pitch angle (SI unit: rad), which is a right-handed rotation with respect to the +y direction. The default is 0 rad, corresponding to the initial direction of propagation pointing in the +x direction.
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Enter a Yaw angle (SI unit: rad), which is a right-handed rotation with respect to the +z direction.
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Enter a Wave number k (SI unit: rad/m). The default is ewas.k0 rad/m. The wave number must evaluate to a value that is the same for the domains the scattered field is applied to.
Figure 4-7: Schematic of the directions for the wave vector k, the electric field E0, and the roll, pitch, and yaw rotations. The top image represents an initial wave propagating in the x direction with a polarization along the z direction.
Dependent Variables
The dependent variables (field variables) are for the Electric field E and its components (in the Electric field components fields). The name can be changed but the names of fields and dependent variables must be unique within a model.
Discretization
Select the shape order for the Electric field dependent variable — Linear, Quadratic (the default), or Cubic. For more information about the Discretization section, see Settings for the Discretization Sections in the COMSOL Multiphysics Reference Manual.