Changes

The FDTD method solves computational electromagnetic problems within a finite region of space. Thus, a finite-extent solution domain must be defined before the problem can be solved. This is rather straightforward in the case of a shielded structure, where a typical PEC enclosure box defines the computational domain. The difficulty arises when you try to model an open-boundary structure like an antenna. In that case, the computational domain must be truncated using appropriate termination boundary conditions. For open-boundary circuit-type structures like filters, couplers, etc., without a shielding, it is still possible to place a PEC enclosure box at a distance from the actual structure. However, the distance of the box walls from the structure must be large enough to eliminate any coupling effects. Furthermore, you must also watch for spurious box resonance effects, as you effectively place your structure inside a resonant cavity. The objective of termination boundary conditions is to eliminate the reflections from the walls of the domain box back to the computational domain. The previous releases of [[EM.Cube]] featured various termination boundary conditions like absorbing boundary conditions (ABC) and uniaxial perfectly matched layer (UPML). In this release of [[EM.Cube]]'s [[FDTD Module]], we have phased out all the previous diverse boundary conditions in favor of Convolutional Perfectly Matched Layer (CPML), which has demonstrated a superior performance for a wide range of applications.

In [[FDTD Module]], you can define two types of domain box. A "'''Default'''" type domain box is placed at a specified offset distance from the largest extents of your physical structure (global bounding box). In addition, you have two options to specify the offset distance, either in free-space wavelengths or in terms of the number of free-space grid cells. A "'''Custom'''" type domain box, on the other hand, is completely defined by you in the World Coordinate System (WCS). In this case, you have to specify the lower left front corner (Corner 1) and upper right back corner (Corner 2) of the domain box. These coordinates can be set arbitrarily, but you need to make sure that your custom domain box completely encloses your structure. When you start a new project in [[FDTD Module]], a default-type domain is automatically created with a default offset value set equal to a quarter free-space wavelength (0.25λ₀). As soon as you draw your first object, a blue domain box is placed around your object. As you add more objects and increase the overall size of your structure, the domain box grows accordingly to encompass your entire physical structure. When you delete objects from the project workspace, the domain box also shrinks accordingly.

===Computational Domain Settings===

[[EM.Cube]] provides special waveguide sources that can excite either the TE_mn or TM_mn modes of a rectangular waveguide which is oriented along one of the three principal axes. In other words, the plane of the waveguide source must be parallel to one of the principal (XY, YZ or ZX) coordinate planes.

{{Note|In order to create a waveguide source, you must have at least one "Hollow" Box object with no caps or only one end cap in your project.}}

* The default waveguide more to be excited is TE₁₀. You can select '''TE''' or '''TM''' mode types with arbitrary "m" and "n" modal indices.

* In the '''Source Properties''' section, you can specify the source '''Amplitude''' in Volts, and the '''Phase''' in Degrees.

=== Distributed Sources ===

[[Image:FDTD45.png|thumb|300px|[[FDTD Module]]'s Distributed Source dialog]]

Waveguide sources are a special case of distributed sources in [[EM.Cube]]'s [[FDTD Module]]. A Distributed Source is defined in a rectangular plane of finite extents, parallel to one of the three principal coordinate planes. An impressed electric field component is assumed across the specified rectangular area, which pumps energy into the computational domain. The current version of [[EM.Cube]] provides three spatial field profiles for a distributed source:

The sinusoidal type has the functional form cos(py/w), and the edge-singular type has the functional form 1/v(1-(2y/w)^2), where y is the coordinate along the direction of field variation measured from the center of the rectangular area and w is its total width.

To create a new distributed source, follow these steps:

* In the '''Source Properties''' section, you can select the '''Profile''' from three options: '''Uniform''', '''Sinusoidal''' and '''Edge-Singular'''. You can also specify the source '''Amplitude''' in Volts, and '''Phase''' in Degrees and the source's internal '''Resistance '''in Ohms .

Ports are used to order and index sources for circuit parameter calculations like S/Y/Z [[parameters]]. That is why they are defined in the '''Observables''' section of Navigation Tree. In [[EM.Cube]]'s [[FDTD Module]], you can define ports at the location of '''Lumped Sources''', '''Waveguide Sources''' and '''Distributed Sources'''. In other words, ideal sources or other types of sources cannot be used to define ports or calculate port characteristics.

Ports are defined in the '''Observables''' section of the Navigation Tree. Right click on the '''Port Definition''' item of the Navigation Tree and select '''Insert New Port Definition...''' from the contextual menu. The Port Definition Dialog opens up, showing the default port assignments. If you have N sources in your physical structure, then N default ports are defined, with one port assigned to each source according to their order on the Navigation Tree.

You can define any number of ports equal to or less than the total number of sources in your project. The Port List of the dialog shows a list of all the ports in ascending order, with their associated sources and the port's characteristic impedance, which is 50O by default. You can delete any port by selecting it from the Port List and clicking the '''Delete '''button of the dialog. Keep in mind that after deleting a port, you will have a source in your project without any port assignment. Make sure that is what you intend. When you delete one or more ports in your project, their associated sources become free and "available" for either defining new ports or reassignment to the other ports. To define a new port, click the '''Add '''button of the Port Definition dialog to open the "Add Port" dialog. On the left side of this dialog, you will see a table containing all the available sources. Select one or more ports and use the right arrow ('''--->''') button to move them to the table on the right side, labeled "Associated". These ports are now associated with the new port being defined. You can move sources from the "Associated" table back to the "Available" table on the left using the left arrow ('''<---''') button of the dialog. You can associate more than one source with the same port. In that case, you will have coupled sources, collectively representing a coupled port.

{{Note|In order to obtain correct results, the port impedance must equal the characteristic impedance of the transmission line on which the port is established. This is not done automatically in [[EM.Cube]].}}