[[EM.Cube]]'s FDTD Modules currently offers the following types of observable:
* '''Field ProbesProbe''' for monitoring E- and H-field components at a fixed location in both time and frequency domains. * '''Field Sensors* Domain EnergySensor''' for monitoring E- and H-field components on a cross section of the computational domain in both time and frequency domains.* '''Far Field - Radiation PatternsPattern''' for monitoring the radiation behavior of your structure.* '''Far Field - RCS''' for monitoring the scattering behavior of your structure.* '''Huygens Surface Data''' for collecting tangential field data on a box. * '''Port Characteristics (Definition''' for calculating the S/Y/Z [[Parametersparameters]] and voltage standing wave ratio (VSWR). * Reflection '''Domain Energy''' for calculating the total electric and magnetic energy in the computational domain.* '''Periodic Characteristics''' for calculating the reflection and Transmission Coefficientstransmission coefficients when your periodic structure is excited by a plane wave source.
Field probes monitor the field components at a certain point in the computational domain. They record the time-domain field data during the entire time loop and compute their frequency spectrum using a discrete Fourier transform. Field sensors are primarily intended for observation of near field maps on a certain cross section of the computational domain. The field sensor planes are parallel to one of the three principal XY, YZ or ZX planes. When you run a frequency sweep or parametric sweep, multiple maps are generated for each sample of your sweep variable, and you can animate these maps. You can also animate the evolution of the near fields in the time domain over the course of the simulated time loop. [[EM.Cube]] can also keep track of the electric, magnetic and total energy of the computational domain as functions of the time step.
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Using asymptotic near-to-far-field transformations, [[EM.Cube]] calculates the far fields of your physical structure in the standard spherical coordinate system. The radiation patterns are indeed the spherical electric field components E<sub>θ</sub> and E<sub>φ</sub> expressed as functions of the observation angles θ and φ over a unit sphere. The far field data are calculated in the frequency domain at a specified frequency, which is equal to your project's center frequency by default. When your excitation source is a plane wave or a Gaussian beam, the far field data actually represent the scattering behavior of your "target". In the case of a plane wave source, the FDTD simulation engine can also compute the radar cross section of you target. If your structure is periodic, then the reflection and transmission coefficients of the periodic surface are also calculated over the entire bandwidth of your project.
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You can define ports for lumped sources, waveguide sources and distributed sources. In that case, the FDTD simulation engine calculates the scattering (S) [[parameters]] of your multiport network over the entire bandwidth specified in your project. From the scattering matrix, [[EM.Cube]] determines the impedance and admittance matrices of your network over the operational bandwidth. You can plot the S/Y/Z [[parameters]] in EM.Grid. If your project has more than one port, the FDTD time loop will be run as many times as the number of ports, N. In each time loop run j (j = 1, 2, ..., N), the source(s) associated with the jth port is (are) excited with a unit amplitude and all the other sources are turned off. In this run, all the S<sub>ij</sub> parameters (i = 1, 2, ..., N) are calculated. At the end of the Nth run, the entire S matrix is completed.
===Probing Fields in Time and Frequency Domains===
[[Image:FDTD75.png|thumb|300px|FDTD Field Probe Dialog]]
Field probes monitor the field components at a certain point in the computational domain. They record the time-domain field data during the entire time loop and compute their frequency spectrum using a discrete Fourier transform. By computing the time domain fields at a certain location, you can examine the transient response of a system at that location. This is also very useful for monitoring the convergence of FDTD time marching loop. [[EM.Cube]]'s field probes allow you to save the temporal values of a field component at a specified point in the computational domain during the entire time marching loop. You can plot the time domain field components as a function of the time step index. You can also plot the spectral contents of those field components, i.e. their Fourier transform, over the project's specified frequency bandwidth. To define a new field probe, follow these steps:
* Right click on the '''Field Probe''' item in the '''Observables''' section of the Navigation Tree and select '''Insert New Observable...'''