XFDTD Tutorial for analyzing antennas parameters
(See the tutorial “Beginning XFDTD” before you use this one)
XFDTD uses two file types for any simulation. The Geometry file contains information about grid spacing and size, material types, and the location of objects in the grid. Its extension is ".id". The other file type is the fdtd file. This file contains all simulation parameters, including the location of sources, number of time steps, and data to be saved during the simulation.
To run a simulation, a Geometry file has to be created. After the Geometry file has been saved, an fdtd file needs to be created. The fdtd file automatically references the geometry file that was open when it is created. After the fdtd file is created and saved, calcfdtd can be run. The data will be stored in the directory that xfdtd was opened in. After the simulation run is complete, the fdtd file needs to be reloaded into xfdtd for data to be viewed and plotted.
Two types of simulations can be run, steady state or transient.
Steady State
Steady state simulations use the sinusoid source found in the FDTD – Run Parameters - Voltage Source menu. This is very useful to determine exact parameters of an object at the specified input frequency. This includes SAR values, efficiency, radiation patterns, and input impedance. Many of these values can be found by simply looking at the Steady State Option in the Plot menu.
Transient
Transient simulations are excellent for determining the general parameters of an antenna over a range of frequencies. Transient simulations use a gaussian pulse or one of its variations as a source. By running a transient simulation. One can easily determine the input impedance over a large frequency range. This allows you to find the resonant frequency of an antenna, and gives you the information needed to make changes to your design.
Note on number of time steps to use.
For steady state simulations, enough time steps must be included to reach steady state conditions. Otherwise, the results will be invalid. To determine if convergence has been reached. Store near zone values. Look at the near zone values to determine if Steady State has been reached.
For transient simulation, more time steps will allow larger FFT transforms. This will give plots more data points, giving smoother graphs.
To determine resonant frequency or compute input impedance over a frequency range:
1. Set up for a transient simulation as described above.
2. Under the FDTD - Compute Input Impedance menu, set the FFT size. It must be greater that the number of time steps specified in the run parameters. Close box with OK button.
3. SAVE FDTD parameter file.
4. Run Calcfdtd
5. After simulation is finished, reload FDTD file.
6. Go to Plot Menu, Plot Input Impedance for both real and imaginary.
7. Resonant frequencies are found where real impedance goes high, and the imaginary impedance crosses zero.
8. Note: the results may not be valid for the entire frequency range plotted.
If real results are negative, the results are not valid. Scaling x-axis down can provide a better visual.
To computer SAR Max.
1. Set up for steady state simulation as described above.
2. In the FDTD-Save All Steady-State Quantities-SAR Planes, add at least one of the planes to the SAR list.
3. Save FTDT file.
4. Run simulation
5. Reload FDTD file.
6. In the Plot menu, Compute SAR Statistics for 1 gram SAR values.
7. After calculations are complete, go to Display SAR information in Plot menu.
To compute Efficiency
1. Run steady state simulation.
2. Reload FDTD file.
3. Under plot menu, view Steady State Data.
To compute Radiation Patterns
1. Select Steady State Far Zone Transformation Type for Sinusoidal Source in the FDTD-Run Parameters-Voltage Source menu.
2. Run steady state simulation.
3. Reload FDTD file.
4. In the Plot - Compute Far-Zone Plot Data menu, enter desired angles.
5. View results in the Plot-Display Plot menu.
(Note: Results will be invalid if any material is within 6 cells of the boundaries.
To compute half power beam width
1. Compute radiation patters.
2. On the plot, note the peak. Then find how wide the beam is before dropping 3 dB.
To compute S parameters
For Transient Simulation
1. Under the FDTD-Run Parameters-Voltage Source menu, set the S-parameter computation to YES.
2. Run Simulation
3. Reload FDTD file.
4. In the Plot - Compute s-parameters menu, compute the S parameters.
5. View results in the Plot-Display Plot menu.
For Steady State Simulation
1. Under the FDTD-Run Parameters-Voltage Source menu, set the S-parameter computation to YES.
2. Run Simulation
3. Reload FDTD file.
4. Under plot menu, view Steady State Data.
(Note: This is the procedure for one source only. See the manual if using multiple sources.)
To compute antenna bandwidth.
1. Compute S parameters for a Transient Simulation
2. Find the lowest point on the S11 plot.
3. Up 3 dB to the left is the lower cutoff frequency.
4. Up 3 dB to the right is the upper cutoff frequency.