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
- Recall, the characteristic impedance of a coaxial cable is
Now we are ready to build our model in HFSS.
where a is the radius of the inner conductor, b
is the radius of the outer conductor, and μ and ε are the
permeability and permittivity, respectively, of the dielectric
between the two conductors. So, a coaxial cable with an inner
conductor radius a of 1mm, an outer conductor radius b
of 7.71mm, and a dielectric with ε = 6εo and μ = μo,
will have a characteristic impedance of approximately 50 Ω.
Creating the Geometry
The geometry of this model will consist of three objects.
Let's begin by drawing the inner conductor. To do this, click the
"Draw cylinder" option at the top of the page.
- A cylinder for the inner conductor.
- A cylindrical tube for the dielectric (having a hole down the
axis for the inner conductor).
- A cylindrical tube for the outer conductor (having a hole down
the axis for the dielectric and inner conductor).
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.
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
This should result in something like what is shown below:
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:
**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).
With both cylinders selected, click the `Subtract' button at the top
of the screen.
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'.
If you view your model from the end at this point, with Cylinder3
selected, you will see that it is now an empty tube.
**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.
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."
Type `PEC' into the search-by-name field, then select it in the list
and click `OK'.
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.
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
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
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
7. In the 3D modeler window, click the center of the selected circle
(the cursor will look different when it hovers over the exact
8. Click the top of the selected circle (the cursor will look
different when it hovers over the exact top).
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
14. In the 3D modeler window, click the center of the selected
circle (the cursor will look different when it hovers over the exact
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.
Perform the following steps to set up the analysis options:
1. Right click on Analysis in the Project Tree, and select "Add
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
Perform the following steps to set up the frequency sweep:
1. Under the Analysis item in the Project Tree, right-click on
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
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
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
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:
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.
Copyright 2021, Kathryn Leigh Smith. All rights reserved.