===Understanding the FDTD Observable Types===
In EM.Tempo, project observables are the simulation data that are generated by the simulation engine at the end of each simulation run. [[EM.Tempo]]'s FDTD simulation engine calculates all the six electric and magnetic field components (E<sub>x</sub>, E<sub>y</sub>, E<sub>z</sub>, H<sub>x</sub>, H<sub>y</sub> and H<sub>z</sub>) at every mesh grid node at all time steps from t = 0 until the end of the time loop. However, in order to save memory usage, the engine discards the temporal field data from each time step to the next. Storage, manipulation and visualization of 3D data can become overwhelming for complex structures and larger computational domains. Furthermore, calculation of some field characteristics such as radiation patterns or radar cross section (RCS) can be sizable, time-consuming, post-processing tasks. That is why [[EM.Cube]] asks you to define project observables to instruct why types of simulation data you seek in each simulation effort.
EM.Tempo offers the following types of observable:
* '''[[Hybrid_Modeling_using_Multiple_Simulation_Engines#Generating_Huygens_Surface_Data | Huygens Surface]]''' for collecting tangential field data on a box.
Of [[EM.Tempo]]'s frequency domain observables, the near fields, far fields and all of their associated [[parameters]] like directivity, RCS, etc., are calculated at a certain single frequency that is specified as part of the definition of the observable. To compute those frequency domain data at several frequencies, you need to define multiple observables, one for each frequency. On the other hand, port characteristics like S/Y/Z [[parameters]], VSWR and periodic characteristics like reflection and transmission coefficients, are calculated over the entire specified bandwidth of your project.
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