Computer lab: Multimode analysis - Canvas

DD2370 Computational Methods for Electromagnetics

Computer lab: Multimode analysis

1


1. Main goal:

The main goal of this lab is to develop and validate your own multimode analysis code.

You will validate this code through two examples:

- Periodic repetition of corrugations inserted in a parallel plate waveguide.

- Periodic repetition of glide-symmetric corrugations inserted in a parallel plate waveguide.

2. Intended learning outcomes:

With the exercises included in this lab, you will:

- Learn the fundamentals of the electromagnetic response of periodic structures.

- Discover new functionalities of commercial software CST Microwave Studio (CST MWS).

- Understand the differences between eigenmode and frequency/time domain analyses.

- Understand the limitations of eigenmode analyses to calculate the phase constant in structures

with unbounded boundary conditions and the attenuation constant of any periodic structure.

- Learn how commercial software creates and processes waveguide ports in frequency/time

domain analyses.

- Understand the importance of knowing the physics of a specific problem in order to accurately

simulate it.

- Learn how commercial software can be combined with your own electromagnetic knowledge to

solve advance problems beyond the capabilities offered by the own software.

3. Structures under analysis:

There are two structures that you will analyze. These structures are periodic with respect to z direction,

and invariant in the transversal direction. They are bounded up and down with a metallic wall, i.e. the

corrugations are embedded in a parallel plate configurations.

These are depicted as follows, where gray represents metal:

Conventional periodic corrugation Glide-symmetric periodic corrugation

The dimensions for this lab will be:

• 𝑝 = 0.75 mm

• 𝑎 = 0.5 mm

• ℎ = 1 mm

• ℎ1 = 0.45 mm



2

4. Tasks and deliverables

4.1) Eigenmode analysis: Simulate the structures with the eigenmode analysis of CST (see

Appendix A for more information on how to use this tool). Export the achieved modes.

Hint:

- You don’t need to define the top and bottom ground planes.

- Simulate at least 2 modes for the case of one corrugation, and 4 modes for glide-symmetric case.

Questions:

- What boundary conditions have you used? Explain the selection.

- Do you need a background material? Explain the decision.

- What is the thickness of the corrugations that you chose? Why?

4.2) Frequency domain analysis: Simulate the structures with the frequency domain analysis of

CST (see Appendix B for more information on how to use this tool). Export the S-parameters for all (or

selected) the ports.

Hints:

- You don’t need to define the top and bottom ground planes.

- Each waveguide port requires at least 3 modes.

Questions:

- What boundary conditions have you used? Explain the selection.

- Do you need a background material? Explain the decision.

- What is the thickness of the corrugations that you chose? Why?

4.3) Multi-mode analysis: Write your multi-mode analysis method in Matlab (or any other software)

and compare the results of the eigenmode analysis (given by CST) and your multi-mode code

(S-parameters obtained in the frequency domain of CST and Maltab).

Hint:

- The expected results should look similar to:

Single corrugations



3

Glide-symmetric corrugations

4.4) Deliverables and reflections on the results:

You must upload in Canvas:

- You own code in Matlab (or any alternative, such as Python) with adequate comments to explain

the operation.

- A report with the results and explanations. Maximum 5 pages. The report must contain:

1. The results, with correct axis and clear format as the figures in the figures above.

2. Explanation on the selection of boundary conditions and background materials with

either eigenmode and multimode analysis.

3. Explanation on the “conceptual” differences between the modes calculated with the

eigenmode and multi-mode analysis, and the modes of the waveguide ports in the

frequency domain.

4. Explanation on the convergence with the number of “waveguide” modes. Which type of

modes are required to produce an accurate solution?



4

APPENDIX: Brief introduction to CST MWS:

This is a tool for the simulation of 3D structures at high frequency. CST MWS enables analysis of high

frequency (HF) devices such as antennas, filters, couplers, planar and multi-layer structures and SI and

EMC effects.

Here, we provide a short explanation of how to use the two main solvers that you will need for this LAB:

eigenmode and frequency domain analysis.

A. Eigenmode solver:

You will have to follow the next steps:

1) Open CST Studio Suite.

Select CST Microwave Studio (new project template):

2) Select the most convenient configuration.

We are going to study a periodic structure:

Therefore, we will assume that our structure is a periodic unit cell:



5

And we would like to obtain a dispersion diagram:

Therefore, the simulations will be done with the Eigenmode analysis:

Then, you will select the units. They will be typically “mm” for the lengths, “GHz” for the frequency, and

“ns” for the time analysis. Temperature will not be used in this laboratory.



6

Then, you will arrive to the following set-up, and you will click “finish”.

3) Define your structure:

3.1) Define the structure.

Here, you will use the modeling functions to create the metallic corrugations. Only one brick is needed

for the case corrugated periodic structure. Only two bricks are needed for the glide-symmetric

structure.

3.2) Define frequency, background materials and boundary conditions.

Reflect about the background material that you need, and if you need to define some space between

corrugations and the ground planes.

The boundary conditions should be metallic top and bottom. Your structure should be periodic in the

along z axis. Reflect about the boundary conditions in the third dimension.

The frequency will be set from 10 to 250 GHz. The highest frequency will define your mesh.



7

3.3) Define a variable “phase”

Introduce this variable in the phase shift/scan in the z-direction. This is the direction of your periodicity.

3.4) Launch the simulation

Select at least 2 modes in the analysis for the corrugated structure, and 4 modes for the glide-symmetric

configuration.

Select ‘Parametric sweep’ function and introduce a variation of your variable ‘phase’ from all the possible

phases: 0.1-180 degrees.

Add the analysis (“Result template”): Select “2D and 3D Field results”, and later on “3D Eigenmode

results”. The results that you would like to obtain is the operational frequency of the mode for each

phase. You must select all modes separately, i.e. there are 2 or 4 modes.



8

Close the window and start the parametric analysis.

4) Visualize the results:

The dispersion diagram for the first 2 or 4 modes will be in “1D Results” in the navigation bar at the left.

This graph shows you the operational frequency of each mode for different values of phase between the

initial point of the waveguide and the final one.

If you launch the simulation without parametric study, for a single phase (for example, ‘phase = 1’), you

will be able to check the modes in the Electric field plot in the “2D/3D Results”. Plot the modes at 90

degrees and reflect about the electric field distributions of the first 2 or 4 modes.



9

B. Frequency domain solver:

The frequency solver consists of a MoM simulation. This type of technique has been studied during the

course. CST has his own implementation, which is general and can be applied for a large number of

structures and configurations.

To carry out one simulation, you will have to follow the next steps:

1) Open CST Studio Suite.

Select CST Microwave Studio (new project template):

2) Select the most convenient configuration.

This configuration can be later on modified with the boundary conditions and background materials.

For example, you can select the ‘Antennas’ template, even though you will not simulate an antenna.

3) Select the most convenient option for the analysis.

For this lab, most of the options will fit. Select, for example, “waveguides”.



10

4) Follow the indications of the quick start:

4.1) Define the solver: Frequency domain:

4.2) Define the units:

Units will be typically “mm” for the lengths, “GHz” for the frequency, and “ns” for the time analysis.

Temperature will not be used in this laboratory.

4.3) Define the frequency of study and the monitors:

The frequency will be set from 10-250 GHz.



11

4.4) Check background material:

Important note: The following background material is only for illustration. It is not necessarily the

one that you must use in this LAB.

4.5) Boundary conditions:

You must define the boundary conditions for your specific problem.

Important note: The following boundary conditions are only for illustration. It is not necessarily the

one that you must use in this LAB.

4.6) Define the structure:

Here, you will use the modeling functions to create the metallic corrugations. Only one brick is needed

for the case corrugated periodic structure. Only two bricks are needed for the glide-symmetric

structure.



12

3.7) Define the ports:

In this LAB, you will need to create two waveguide ports (one at each side of the periodic structure)

which are facing each other. You need to define at least 3 modes at each of the waveguide ports.

Important: Reflect about the meaning of these ports.

3.8) Launch the simulation:

Due to some detected bugs in CST, it is recommended to use “hexahedral” mesh.

For the first simulation is recommended to select “calculate port modes only” and to check the validity

of the modes. They will be represented in 2D/3D results in the “navigation bar”.

Computer lab: Multimode analysis - Canvas

Documents

Transcript of Computer lab: Multimode analysis - Canvas