A record of every field component at every point in space throughout an entire simulation would constitute an excessive amount of data. To obtain useful information it is necessary look at cross-sections through this mass of data. The 3D-TLM Simulator offers the following subsets of the total available information:
• Time Domain Output: Data for the entire duration of the simulation, but at a limited number of points in space, or reduced to a limited number of signals. This output may subsequently be transformed to the frequency domain.
• Space Domain (Time): Data for the entire mesh (or a part of it), but at a limited number of instants in time.
• Space Domain (Frequency): Data for the entire mesh (or a large part of it), but at a limited number of frequencies.
• Equivalent surface (Frequency): for obtaining near and far field results.
• ISD output: Data for interfacing to CST CABLE STUDIO
Quantities available for time domain output include:
• Electromagnetic Field components in selected cells in the mesh.
• Electromagnetic Field components at points outside the mesh.
• The current at selected points on wires.
• Currents and voltages in lumped-circuits.
• The state of ports.
• Integral of voltage along a path
• Integral of current through a closed loop
When output of field components (Ex, Ey, Ez, Hx, Hy, Hz), is requested for a point within the mesh, then the values obtained are for the center of the cell containing the given point.
To obtain field components at points outside the mesh, it is necessary to specify an equivalent surface (or Huygen’s surface), within the mesh. This surface encloses all material bodies and sources contributing to the field (see below). Field values at points outside the surface (and outside the mesh), are obtained by an integral over the surface. Unlike time-domain output points within the mesh, those outside of it are not snapped to cell-centers. Output at points outside the mesh is computationally expensive; the expense being approximately proportional to the number of output points. Note that points more distant than (duration ´ c) will not be reached by any signal before the simulation terminates.
For wire current output, the current is taken as positive when flowing in the direction in which the wire-path was constructed.
The state of each port is defined in CST MICROSTRIPES by between two and six different variables. Although these quantities are available to the user in the time-domain output, they are primarily intended for subsequent processing (in the calculation of scattering parameters).
A ‘snapshot’ of the electromagnetic field, surface current and wire current distributions may be obtained at a number of instants during the simulation. This is probably the least useful form of output available.
Electromagnetic field, surface current and wire current distributions may be obtained at a number of frequencies. A further space domain frequency option is the equivalent surface (see below). This is used by the near to far field transformation module to obtain the radiation pattern.
Space domain (Frequency) electromagnetic field requests are computationally intensive and will increase the run time and memory requirement of a simulation. Space domain surface current and equivalent surface requests will not significantly affect the run time and memory requirements of a simulation.
An additional space domain frequency output option is energy stored and power lost into a metal or medium body. It can be used to determine the loaded and unloaded Q of a cavity, or the true gain (i.e. including material losses) of an antenna. The loss (or energy) may be determined in total, or for each separate material body.
To determine the energy stored and power loss in mediums, the 3D-TLM Simulator calculates electric and magnetic fields in the space-domain region at each frequency of interest - whether or not output of these components is requested. From these components and from the material properties, the energy and loss are calculated and written to a file with the extension .sdi ("space-domain-integrals").
Similarly, to determine the loss on metal surfaces, the Simulator calculates surface current in the space-domain region - whether or not output of these components is requested. From these components and from the metal definition, the loss is calculated and included in the .sdi output. No loss is ever calculated for the external boundaries of the problem space (unless they are coated with resistive metal panels).
Loss and energy calculation for mediums may be obtained by selecting "Integrate Energy and Loss over volume" in the Build module’s Space Domain (Frequency) Output Window. Loss and calculation for metals may be obtained by selecting "Integrate Loss over surfaces" in the same window. Note that the integrals are only performed over the specified regions.
The 3D-TLM Simulator cannot calculate energy integrals over regions containing thin panels or wires. It cannot calculate loss integrals over regions containing thin panels, and it neglects losses in wires.
In CST MICROSTRIPES an equivalent surface is used to obtain time domain output outside of the mesh. Furthermore, space domain (single frequency and broadband) output of the equivalent surface current distribution may be requested, and used to calculate the radiation pattern at the same frequencies.
The electric and magnetic equivalent surface currents are used to represent the entire structure enclosed by the equivalent surface. As a consequence, the space domain region of the equivalent surface must totally enclose all of the structure being modelled that would affect the radiated fields. All metal and medium data types and wires must lie inside the space domain region.
The only exception to this rule is the case of a dielectric structure which extends the whole length of the mesh used for microstrip simulations. For this type of simulation, some of the dielectric may extend beyond the equivalent surface block. The simulator will check to ensure that the scattering structure is enclosed within the equivalent surface block.