6.2.12 Fields Monitoring
The time domain field monitoring is very useful and flexible but is most informative with a sinusoidal excitation at a particular frequency. Thus one frequency can be thoroughly investigated at a time.
QuickWave enables another useful feature, which allows watching the field distribution at selected frequencies from data extracted by Fourier transforming of the results of simulation, obtained with pulse excitation. Thus we obtain the frequency domain fields from the time domain ones via Fourier transform. Such a post-processing is called FD-Monitors and may be described as a frequency domain monitors (FDM). It performs Fourier-transformation of the field components chosen from the selection of: Ex, Ey, Ez, Hx, Hy, Hz, at frequencies specified by the user, in the selected sections of the circuit called Field Monitor (called also FD Monitor or Monitor box). This way of watching the fields is complementary to their unconstrained display. With FD-Monitors post-processing we need to pre-define sections (Monitor box) and frequencies of interest, but then we obtain fields at multiple frequencies from one simulation with a pulse source (otherwise consecutive simulations, with harmonic excitation at consecutive frequencies would be needed).
The Fields Monitor is one of specific QuickWave simulation objects and must be introduced to the scenario to enable FD-Monitors post-processing. It takes a form of a box or a plane (in V2D projects) and must surround the space, in which the Fourier-transformation of the time domain field components should be performed.
In QW-Simulator, in Fields Monitor window, the FD-Monitors post-processing delivers the following results:
- magnitude, real and imaginary parts of the selected field components
- real and imaginary parts of total electric and magnetic field
- real and imaginary parts of the Poynting vector (if available from the selected field components)
- magnitude of the dissipated power (total, electric and magnetic) and dissipated power density (if losses are present in the scenario).
FD-Monitors post-processing is available only if at least one Monitor box has been set in the simulation project. Theoretically, any number of Fields Monitors and frequencies describing them can be considered. However, collecting the data for many frequencies and FD Monitors may become quite consuming in terms of computer resources. It should be also mentioned that when the area defined in FD Monitor contains a large number of FDTD cells it may introduce a significant slow-down in computing.For that reason, to accelerate data collection, in FD‑Monitors post-processing, the sparsity factor in space has been introduced. Sparsity factor can be set separately for each direction. Setting the sparsity factor N>1 causes that only one of N FDTD cells is considered in the calculation of the field component in FD-Monitors post-processing. Please note that for the FD-Monitors sparsity N greater than 1 the following components: E total, H total, ExH, Sx, Sy, Sz, PdE, PdH, Pd and pd values will be calculated not from the nearest cell but will be estimated from the nearest N-th cell.
The FD-Monitors post-processing may be convenient in many applications although it has two disadvantages:
- the frequencies and the structure sections of interest need to be defined prior to starting the simulations,
- when many frequencies and many sections are chosen the FD-Monitoring may become quite time- and memory-consuming.
It is worth noting at this point that in the Fields Monitor window of QW-Simulator, the user is provided with the distribution of the field amplitudes (real, and imaginary parts or absolute values) for the considered frequency, as the FD-Monitors post-processing results, but he can also monitor the field components (Ex, Ey, Ez, Hx, Hy, Hz, E total, H total, ExH, Sx, Sy, Sz) in time via Animation feature. The animation of fields takes into account the amplitudes and phase relations calculated from FD-Monitors post-processing. Let us note here that the time variation here is based on a different principle than when we watch the fields concurrently with FDTD simulation. Here we use the field amplitude and phase obtained for a particular frequency through a Fourier transformation of the time-domain signals during FDTD simulation and reproduce them in artificial time not synchronized with the time of FDTD simulations.
The user is encouraged to follow the description of application of frequency domain monitors (FDM) in User Guide 3D: Application of Frequency Domain Monitors (FDM) for microwave heating.