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Validating SAR Calculations

In this project, you will learn how to:

  • Model a tissue-simulating liquid with a dipole.
  • Define the properties of the environment.
  • Add a feed to the dipole and simulate its effects.
  • Add a point sensor and measure E-field at the center of the liquid.
  • Add SAR sensors and retrieve raw and averaged SAR data.

Getting Started

This section briefly describes how to set the display units for the SAR Validation project. To set up a project for the first time, refer to Application Preferences Appendix for instructions about how to configure project preferences and navigate through the display units tab.
In the Project Properties Editor window, navigate to the Display Units tab:

  1. Select SI Metric in the Unit Set drop down list.
  2. Change Length to millimeters(mm). This changes the value of Unit Set to Custom.
  3. Click Done.

Creating the Geometry

The geometry for this example consists of a Flat Phantom, Phantom Shell, and a dipole made of two cylinders.

Modeling the Flat Phantom

First, you will create the rectangular extrusion named Flat Phantom which represents the tissue simulating liquid used for SAR measurements. You will perform the simulation at 835MHz, so the phantom dimensions will be 220x150mm with an extrusion in the +Z direction of 150mm.

  1. Right-click the Parts branch of the Project Tree. Choose Create New > Extrude from the context menu.
  2. Name the object by typing Flat Phantom in the Name text box.
  3. Choose View > Standard Views > Top(-Z) orientation.
  4. Choose the Rectangle tool from the Shapes toolbar.
  5. Click the mouse on the origin of the coordinate system.
  6. Press Tab to display the creation dialog for the second point. Enter 225mm,150mm and click OK to complete the rectangle.
  7. Navigate to the Extrude tab to extrude the rectangular region. Enter the Extrude Distance of 150mm.
  8. Click Done to finish the Flat Phantom geometry.

Modeling the Phantom Shell

Next, you will create the rectangular extrusion named Phantom Shell. This shell is a plastic vessel that will hold the simulating liquid. For this simulation, you need to add only the bottom of the vessel that separates the liquid from the dipole source. This shell size will match the phantom size in X and Y, and have a thickness of 2mm.

  1. Right-click the Parts branch and choose Create New > Extrude from the context menu.
  2. Under the Specify Orientation tab, define the origin at (0, 0, 2mm).
  3. Under the Edit Cross Section tab, type Phantom Shell in the Name text box.
  4. Choose View > Standard Views > Bottom(+Z) orientation.
  5. Choose the Rectangle tool from the Shapes toolbar.
  6. Trace the new cross-section over the existing cross-section (of the flat phantom) since they are of equal width and length.
  7. Navigate to the Extrude tab to extrude the rectangular region a distance of -2mm.
  8. Click Done to finish the Phantom Shell geometry.

Modeling the Dipole

Now you will create the dipole geometry, which comprises two cylindrical extrusions. Typically the dipole will have a balun structure as well, but we will omit that for simplicity in this example. The dipole will have a radius of 1.8mm and a length of 161mm.

  1. Right-click the Parts branch and choose Create New > Extrude from the context menu.
  2. Under the Specify Orientation tab, define the origin at (113mm, 75mm, -15mm).
    • Redefine the orientation of the sketching plane by selecting the YZ Plane under the Presets drop-down list.
  3. Under the Edit Cross Section tab, type Cylinder1 in the Name box.
  4. Choose View > Standard Views > Left(+X) orientation.
  5. Choose the Circle center radius tool from the Shapes toolbar.
    • Click the mouse on the origin of the coordinate system.
    • Press Tab to display the creation dialog for the radius. Enter 1.8mm and click OK.
  6. Navigate to the Extrude tab to extrude the cylinder. Enter a distance of 80mm.
  7. Click Done to finish the Cylinder1 geometry.

Create the second extrusion

Now you will create the second part of the dipole, Cylinder2.

  1. Right-click the Parts branch and choose Create New>Extrude.
  2. Under the Specify Orientation tab, define the origin at (32mm, 75mm, -15mm).
    • Redefine the orientation of the sketching plane by selecting the YZ Plane under the Presets drop-down.
  3. Under the Edit Cross Section tab, type Cylinder2 in the Name text box.
  4. In the View Tools toolbar, select the Right (-X) orientation.
  5. Choose the Circle center, Radius tool from the Shapes toolbar.
    • Click the mouse on the origin of the coordinate system.
    • Click Tab to display the creation dialog for the radius. Enter 1.8mm and click OK.
  6. Navigate to the Extrude tab to extrude the cylinder. Enter a distance of 80mm.
  7. Click Done to finish the Cylinder2 geometry.
    The following figure displays a view of the finished geometry before materials are added.

Creating Materials

After creating four new objects, you will assign materials to them. Cylinder1 and Cylinder2 will be perfect electric conductors, PEC. The Flat Phantom and Phantom Shell objects will be isotropic materials named Phantom Liquid and Phantom Shell, respectively.

Define material, PEC

  1. Right-click the Definitions:Materials branch of the Project Tree. Choose New Material Definition from the context menu.
  2. Set the perfect electric conductor material properties as follows:
    • Name: PEC
    • Electric: Perfect Conductor
    • Magnetic: Freespace
  3. If desired, navigate to the Appearance tab to set the display color of the PEC material.

Define Material, Phantom Liquid

  1. Right-click the Definitions:Materials branch of the Project Tree and select New Material Definition.
  2. Set the material properties as follows:
    • Name: Phantom Liquid
    • Electric: Isotropic
    • Magnetic: Freespace
    • Under the Electric tab:
      • Type: Nondispersive
      • Entry Method: Normal
      • Conductivity: 0.9 S/m
      • Relative Permittivity: 41.5
        Editing the color of the Phantom Liquid material
  3. Under the Physical Parameters tab, enter 1000 kg/m^3 as the Density.
  4. Navigate to the Appearance tab and assign the Phantom Liquid material a new color to distinguish it from PEC.
  5. Click Done to add the new material, Phantom Liquid.

Define material, Phantom Shell

  1. Right-click the Definitions:Materials branch of the Project Tree and select New Material Definition.
  2. Set the material properties as follows:
    • Name: Phantom Shell
    • Electric: Isotropic
    • Magnetic: Freespace
    • Under the Electric tab:
    • Type: Nondispersive
    • Entry Method: Normal
    • Conductivity: 0 S/m
    • Relative Permittivity: 3.7
  3. Navigate to the Appearance tab and assign the Phantom Shell material a new color to distinguish it from PEC.
  4. Click Done to add the new material, Phantom Shell.

Assigning Materials

  1. Click and drag the PEC material object located in the Project Tree and drop it on top of Cylinder1 and Cylinder2.
  2. Assign the Phantom Liquid material to the Flat Phantom object.
  3. Assign the Phantom Shell material to the Phantom Shell object.
    The finished geometry with applied materials is seen in the following figure.
The Project Tree

Creating the Grid

Now, you will define characteristics of the cells in preparation to perform an accurate calculation.

Define cell size and padding

  1. Open the Geometry browser window, select Grid Tools and click Edit Grid.
  2. Navigate to the Size tab.
    • Define Base Cell Sizes as Target 1mm and Merge 0.8 in all directions, with the Ratio boxes selected.
    • Free Space Padding: 10 in all directions except Lower Z, which will be 20.
  3. Click Done to apply the grid settings.

Creating a Mesh

In the FDTD branch of the Project Tree, double-click the Mesh icon. This displays the mesh view and automatically create the mesh. If you switch to the 3D Mesh view of All Edges, note that the grid does not align with the CAD view of the geometry objects This is because the cell size does not overlap the geometry dimensions exactly.

To align the mesh, you can turn on the fixed points for several of the geometry objects. This will adjust the mesh so that the grid lines overlap the edges of the CAD geometry objects.

  1. From the Parts branch, right-click the Phantom Shell object.
  2. Select Gridding Properties from the menu.
  3. In the Gridding Properties Editor dialog box, select Use Automatic Fixed Points.
  4. Click Apply to apply the fixed points extraction to this geometry object.
  5. Click Copy to clipboard to save these settings.
  6. Click Done.

Now, you will turn on Fixed Points for the cylinders. Select both Cylinder1 and Cylinder2 from the Parts branch. Right-click and select Edit> Paste to copy the clipboard contents to these two objects. This will turn on fixed points for the dipole as well. The resulting geometry view should now show that the grid overlaps well with the CAD objects.

Adding a Feed

Now, you will add a Feed to the geometry. We want to place the feed in the gap between the two cylinders made of PEC materials. The following figure displays a 3D Mesh View of the gap.

  1. Right-click the Circuit Components branch of the Project Tree. Choose New Circuit Component with> New Feed Definition from the context menu.
  2. Define the endpoints of the feed.
    • Endpoint 1: X: 113 mm, Y: 75 mm, Z: -15 mm
    • Endpoint 2: X: 112 mm, Y: 75 mm, Z: -15 mm
  3. Navigate to the Properties tab, and enter the following:
    • Name: Feed
    • Component Definition: 50 ohm Voltage Source
    • Direction: Auto
    • Polarity: Positive
    • Select the This component is a port checkbox.
  4. Click Done to add the Feed.

Editing the Waveform

An associated waveform was automatically created for the feed definition.

  1. Navigate to the Definitions:Waveforms branch of the Project Tree. Double-click the Broadband Pulse waveform to edit its properties.
  2. Set the properties of the waveform as follows:
    • Name: Sinusoid
    • Type: Ramped Sinusoid
    • Frequency: 0.835 GHz
  3. Click Done to apply the changes.

Defining the Outer Boundary

  1. Double-click the FDTD:Outer Boundary branch of the Project Tree to open the Outer Boundary Editor.
  2. Set the outer boundary properties as follows:
    • Boundary: Absorbing for all boundaries
    • Absorption Type: PML
    • Layers: 7
  3. Click Done to apply the outer boundary settings.

Requesting Output Data

Recall that the project already contains one port sensor named Feed that will request results. You can collect SAR results by adding an SAR Sensor.

Adding an SAR Sensor

  1. Right-click the Sensors:SAR Sensors branch of the Project Tree. Select Properties from the context menu.
    • Select the Collect Raw SAR Data checkbox.
    • Select the Full Grid box. It requires that the data be saved over the full grid if Averaged SAR values will be computed.
  2. Click Done to finish editing the SAR Sensor.

To collect averaged SAR data, you must define a sensor.

  1. Right-click the Sensors:SAR Averaging Sensor branch of the Project Tree. Select Properties from the context menu.
    • Check the Collect 1-gram Avg. SAR data and Collect 10-gram Avg. SAR data boxes.
    • Select the Box Region box, and enter the following coordinates:
      • Corner 1: (0 mm, 0 mm, 0 mm)
      • Corner 2: (225 mm, 150 mm, 150 mm)
  2. Click Done to finish editing the SAR Averaging Sensor.

Adding a Point Sensor Definition

A Point Sensor may be saved inside the Flat Phantom object to monitor the convergence of the fields during the calculation. First, ypu will create its definition.

  1. Right-click the Definitions:Sensor Data Definitions branch of the Project Tree. Choose New Point Sensor Definition from the context menu.
  2. Set the properties of the surface sensor definition as follows:
    • Name: E-field vs. Time
    • Field vs. Time: E
    • Sampling Interval: timestep
  3. Click Done to finish editing the definition.

Adding a Point Sensor

  1. Right-click the Sensors:Near Field Sensors branch of the Project Tree. Select New Point Sensor from the context menu.
    • Enter its Location as (112.5 mm, 75 mm, 15 mm).
    • Under the Properties tab, enter the following:
      • Name: E-field
      • Sensor Definition: E-field vs. Time
      • Sampling Method: Snapped to E-Grid
  2. Click Done to finish editing the E-field Sensor.

Running the Calculation

If you have not already saved your project, do so by selecting File>Save Project. After the project is saved, a new simulation can be created to send to the calculation engine.

  1. Open the Simulations workspace window. Click the New Simulation button in the upper-left to set up a new simulation.
  2. Type a descriptive name for the simulation, such as Flat Phantom at 835MHz.
  3. Most of the default settings are sufficient. Navigate to the Specify Termination Criteria tab. Set up the termination criteria as follows:
    • Maximum Simulation Time: 10000 * timestep
    • Dectect Convergence: Checked
    • Threshold: -30 dB
  4. Select Create and Queue Simulation to close the dialog and run the new simulation.

Viewing the Results

The Output tab of the Simulations workspace window displays the progress of the simulation. After the Status column shows that the simulation has completed, you can view the results in the Results workspace window.

E-field Results

Now, you can view the E-field results retrieved from the center of the Tissue.

  1. To filter the list accordingly, select the following options in the columns in the top pane of the Results window. (You may need to change your column headings first.)
    • Output Object: E-field
    • Result Type: E-field (E)
  2. Right-click the result and select Create Line Graph.
    • Select X as the Component, and click View. The plot of the E-field at the center of the Flat Phantom object will appear.

      Note

      It is possible to view the data before the simulation is complete. The plot will update automatically as more data is computed.


      The resulting plot indicates that the fields inside the phantom are at steady-state as a smooth sine wave is visible. This confirms our convergence condition of -30 dB that was set during the simulation setup.
  3. You may close the window when you are finished viewing the results.

System Efficiency Results

Now you can view data from the point.

  1. To view the system efficiency results, select the following:
    • Output Object: System
    • Result Type: Net Input Power
  2. Double-click on the result. The powers in the simulation are displayed. As you can see, the power delivered to the antenna is relatively small, just under 2.5mW. For many SAR analyzes, the power is adjusted to a value such as 1W to normalize all results. You can do this by clicking on the System Sensor Output window.
  3. Click the power value to the right of Net Input Power (0.002498 W).
  4. Type a value of 1W and click Enter. The powers should now scale to the 1W input. This will also scale the SAR value.
  5. You can close the window when you are finished viewing the results.

To view the Feed results:

  1. In the Results workspace window, select:
    • Output Object: Feed
    • Result Type: S-Parameters
  2. Double-click the result under the Discrete domain. The following results will appear showing the impedance at the feed, the input power delivered, and the return loss.

    You can see from the table that our return loss is less than -30 dB, so you have a good match at the selected frequency.
  3. You can close the window when you are finished viewing the results.

SAR Sensor Data

Now you will load the SAR data into the field viewer.

  1. To view the SAR sensor data, select the following:
    • Output Object: SAR Sensor (Raw)
    • Result Type: SAR (Specific Absorption Rate)
  2. Double-click the result in the filtered list. The plot will appear in the Geometry workspace window.
  3. Under the Setup tab, adjust the following settings:
    • Sequence Axis: X
    • Display Mode: Flat
    • Decimation: Normal
    • Under Axis Ranges:
      • Y: Full
      • Z: Full
  4. Toggle the Parts Visibility to turn off the display of the geometry, and select the Left (+X) orientation. The resulting image should appear.
  5. Under the Sequence tab, define Showing: 112. The following SAR image appears.
  6. Under the Statistics tab, choose View all SAR Stats. A summary table of the SAR values appears, as shown in the following figure:

For some situations, the SAR results should be normalized to the feed point current rather than the forward power.

  1. To make this adjustment, return to the Results workspace window and select:
    • Output Object: Feed
    • Result Type: Current
  2. Double-click the result under the Discrete domain.
  3. On this screen, the current value may be edited by clicking on the numerical value. Type in a value of 200mA and all results, including the SAR values, will be adjusted to a feed point current of 200mA.
  4. You may close the window when you are finished viewing the results.
    It may be of interest to plot the SAR as a function of distance along a line extending above the feed point.
  5. In the Results workspace window, select the SAR data.
  6. Right-click on the result and select Create Line Graph.
    • Independent Axis: Z
    • X: 112
    • Y: 75 mm
  7. Click View to see a line plot of the SAR as a function of distance from the feed point in the center of the phantom, as shown in the following figure:
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