[[Image:FDTD48.png|thumb|450px|EM.Tempo's Port Definition dialog.]]
[[Image:FDTD49.png|thumb|300px|Reassigning sources to ports and defining coupled ports in EM.Tempo.]]
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.Tempo, 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.