{{Note|Keep in mind that you are always responsible for the choice of excitation source and the project observables. In other words, [[EM.Cube]] does not automatically provide a default excitation source or does not suggest default observables.}}
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===FDTD Observable Types===
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In [[EM.Cube]], project observables are the simulation data that are generated by the simulation engine at the end of each simulation run. [[EM.Cube]]'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 space, the engine has to destroy the temporal field data from each time step to the next and reuse the memory. 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.
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[[EM.Cube]]'s FDTD Modules currently offers the following types of observable:
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* Field Probes
* Field Sensors
* Domain Energy
* Far Field - Radiation Patterns
* Far Field - RCS
* Huygens Surface Data
* Port Characteristics (S/Y/Z [[Parameters]] and VSWR)
* Reflection and Transmission Coefficients
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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.
=== The FDTD Simulation Engine Settings ===
Of [[FDTD Module]]'s observables, the near fields, far fields and all of their associated [[parameters]] like directivity, RCS, etc., are calculated at a certain frequency that is specified as part of the definition of the observable. 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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== Working with FDTD Simulation Data ==
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===The FDTD Observable Types===
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In [[EM.Cube]], project observables are the simulation data that are generated by the simulation engine at the end of each simulation run. [[EM.Cube]]'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 space, the engine has to destroy the temporal field data from each time step to the next and reuse the memory. 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.
Â
[[EM.Cube]]'s FDTD Modules currently offers the following types of observable:
Â
* Field Probes
* Field Sensors
* Domain Energy
* Far Field - Radiation Patterns
* Far Field - RCS
* Huygens Surface Data
* Port Characteristics (S/Y/Z [[Parameters]] and VSWR)
* Reflection and Transmission Coefficients
Â
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.
Â
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===
In [[EM.Cube]] you can visualize the near fields at a specific frequency in a specific plane of the computational domain. At the end of an FDTD simulation, all the time domain electric and magnetic field values are available at all mesh nodes. These temporal quantities are transformed into the frequency domain using discrete Fourier transforms to calculate the electric and magnetic fields on a specified sensor plane. To define a new Field Sensor, follow these steps:
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* Right click on the '''Field Sensors''' item in the '''Observables''' section of the Navigation Tree and select '''Insert New Observable...'''