Ansys HFSS Tutorial: Coaxial Cable


This tutorial will go through the process for building and simulating a section of coaxial cable in Ansys HFSS.


If you are a UNCC student, and this is your first time using HFSS, please see Initializing HFSS.

Design of a Coaxial Cable

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Zo_coax.png
Now we are ready to build our model in HFSS.

Creating the Geometry

The geometry of this model will consist of three objects.
  1. A cylinder for the inner conductor.
  2. A cylindrical tube for the dielectric (having a hole down the axis for the inner conductor).
  3. A cylindrical tube for the outer conductor (having a hole down the axis for the dielectric and inner conductor).
Let's begin by drawing the inner conductor. To do this, click the "Draw cylinder" option at the top of the page.
drawcylinder
If you do not get a pop-up dialog box, hit F4 on your keyboard. In the dialog box that comes up, change the "Axis" to `X', then enter a variable-defined radius of `innercondrad'.  Hit enter.
definevariable.png
In the "Add variable" dialog box that comes up, enter a value of 1 mm.  This is the radius of the inner conductor. Click `OK'.  Back in the `Create Cylinder' Dialog box, enter a variable-defined height of `coaxlength', and set its value to be 150 mm. In the `Center Position' field, enter the following comma-separated sequence:
This should result in something like what is shown below:
createcylinder.png
Repeat this process to create a second cylinder of radius `outercondrad' = 7.71~mm. Note that the outer radius of the dielectric is the inner radius of the outer conductor.  Repeat the process again to create a third cylinder of radius `shieldrad' = outercondrad+0.3mm. Note that since all the fields are inside the coax, only the inner radius of the outer conductor tube is significant.  We have arbitrarily chosen to give it a thickness of 0.3mm here.
Now we have three cylinders that overlap one another, which should look something like:
3cylinders.png
**not necessarily drawn to scale**

Now we need to create a dielectric-sized hole in the outer conductor. To do this, click the largest cylinder in the project tree (by default, it should be Cylinder3), then, while holding the `Ctrl' key, click the middle cylinder (by default, Cylinder2).
selectcylinders.png
With both cylinders selected, click the `Subtract' button at the top of the screen. 
subtract.png
In the dialog box that comes up, make sure `Cylinder3' appears in the "Blank Parts" field, and `Cylinder2' appears in the "Tool Parts" field. Check the box that says `Clone tool objects before operation', and click `OK'.
subtract2.png
If you view your model from the end at this point, with Cylinder3 selected, you will see that it is now an empty tube.
emptytube.png
**not necessarily drawn to scale**

Repeat this process with Cylinders 1 and 2, subtracting Cylinder1 (the inner conductor) from Cylinder2 (the dielectric).
Now we have all the pieces of our geometry.

Assigning Materials

At this point, HFSS assumes that all our model objects are vacuum. We need to make the material assignments, so that they are PEC and dielectric. To do this, let's first select our conductors - Cylinder1 and Cylinder3. Remember you can click on one in the project tree, then hold the `Ctrl' key and click on the other, to select both. Then right-click on one of the selected objects and click "Assign Material."
assignmaterial.png
Type `PEC' into the search-by-name field, then select it in the list and click `OK'. 
PEC.png
Back in the project tree, select `Cylinder2', right click, and click `Assign Material'. Since we arbitrarily chose to use a dielectric with r = 6 for this example, this does not necessarily correspond to any actual material in the default materials list. Therefore, we will create our own non-standard material. To do this, click the 'Add Material' button at the bottom of the dialog box. In the new dialog box that comes up, enter `6' into the `Relative Permittivity' field.  Otherwise, leave the default values. Click `OK', then click 'OK' in the material selection dialog.
creatematerial.png

Excitations

Next, we will create a "Waveport" excitation at each end of the circuit. A waveport in HFSS defines a location where energy is allowed to enter and exit the system.  To make this assignment, we perform the following steps:

1. Create a circle (near the 'Create Cylinder' button at the top of the page).
2. Set the center position of the circle to (coaxlength/2, 0,0), the axis of the circle to `X', and the radius of the circle to `shieldrad'.
createcircle.png
3. Create a second circle, exactly the same as the first, except located at the other end of the coax (-coaxlength/2, 0, 0).
4. Select the first circle.
5. Right click in the 3D modeler window and select Assign Excitation => Wave Port.
6. Under "Integration Line," click the word None, and select New Line...
7. In the 3D modeler window, click the center of the selected circle (the cursor will look different when it hovers over the exact center).
8. Click the top of the selected circle (the cursor will look different when it hovers over the exact top).
WP2.png
9. Click Next
10. Click Finish
11. Select the second circle.
12. Right click in the 3D modeler window and select Assign Excitation => Wave Port.
13. Under "Integration Line," click the word None, and select New Line...
14. In the 3D modeler window, click the center of the selected circle (the cursor will look different when it hovers over the exact center)
15. Click the top of the selected circle (the cursor will look different when it hovers over the exact top).
16. Click Next
17. Click Finish

Now your model is complete.

Analysis

Perform the following steps to set up the analysis options:
1. Right click on Analysis in the Project Tree, and select "Add Solution Setup"
AddSolution.png
2. Under the General tab:
    (a) Set the solution frequency to 5 GHz,  This is the frequency at which HFSS will refine the field solution. For a non-resonant simulation, it should always be set to the highest frequency of interest.
    (b) Set the maximum number of passes to 30
    (c) Set maximum Delta S to 0.01 (this sets an upper limit on the uncertainty of our solution).
3. Under the Options tab:   
    (a) Set the Maximum Refinement per pass to 20 %
    (b) Set the Order of Basis Functions to Second Order
Click `OK'.

Perform the following steps to set up the frequency sweep:
1. Under the Analysis item in the Project Tree, right-click on Setup1.
AddSweep.png

2. Select Add Frequency Sweep...
3. Set "Sweep Type" to Discrete.
4. Set "Distribution" to Linear Step.
5. Set start frequency to 1 GHz.
6. Set stop frequency to 5 GHz (note: for a non-resonant structure, the upper limit of the sweep should never be higher than the solution frequency).
7. Set step size to 0.25 GHz.
8. Click OK.

Final Checks and Running the Simulation

Save the project by clicking on the save icon at the top of the screen.
Select HFSS => Validation Check... to ensure the project is prepared for simulation (click close).
Right-click Setup1 under Analysis in the project tree and select Analyze to begin the simulation. At this point the progress window should show the progress of the simulation, beginning with the mesh generation.

Simulation Results

To view the results of the simulation, perform the following steps:
1. Right click on the results item in the Project Tree
2. Click Create Modal Solution Data Report => Rectangular Plot
3. Under the trace tab, select the S-parameter -> S(1,1) -> dB and click New Report
4. You may also add additional S-Parameters by highlighting them and clicking Add Trace.
5. Click Close.

As an example, a plot of both S(1,1) and S(2,1) in dB, from 4 GHz - 5 GHz, is shown here:
4to5.png
Notice that there are no abrupt changes in the S-parameters.  This indicates that the structure is non-resonant (at least at these frequencies) and supports the decision we made in the beginning to set the solution frequency to the highest frequency of the sweep.

You may also check that the characteristic impedance of your coax matches your design, by performing the following steps:
1. Right-click on `Setup' in the project manager
2. Click `Matrix data'
3. Check the box marked `Z'
4. Under the column `Port Z', check that the values of Z is approximately what you calculated it to be. Note that the two Z values correspond to the two ports of the network.
portimpedance.png


Copyright 2021, Kathryn Leigh Smith.  All rights reserved.
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