# '''Huygens Source''': A distributed source defined based on know tangential electric and magnetic field components on the surface of a virtual box object.
[[Image:FDTD48.png|thumb|300px|The Port Definition dialog]]
[[Image:FDTD49.png|thumb|300px|Reassigning sources to ports and defining coupled ports.]]
===Defining a New Source===
===Defining Ports===
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[[Image:FDTD48.png|thumb|250px|The Port Definition dialog]]
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.
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[[Image:FDTD49.png|thumb|350px|Reassigning sources to ports and defining coupled ports.]]
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.