How to view higher order modes propagation in waveguides using Feko?
Hello Altair Community,
I have a questions regarding a waveguide simulation. I have designed a WR-1800 waveguide in Cadfeko and would like to view which modes can propagate in my given frequency range and if, how the field lines look like. I have tried setting up a CMA but I see some issues with the results. When I plot the fields in Postfeko (for example Mode 1), the field lines do not look as expected (assuming Mode 1 means the fundamental mode TE10 for rectangular waveguides).
My waveguide has the dimensions of a WR-1800 waveguide and is simulated at a frequency of 450 MHz. The model looks as following (simple cuboid):
It is closed on both sides because Feko gives the error "Modal analysis not possible with ports". After meshing, I simulate the model with the following requests:
CMA:
Near Field Request:
After the simulation is finished, I would like to view the fields inside the waveguide. To do that, I have simply plotted the near fields in Postfeko also activating the arrows that show the field direction. Assuming that "Mode 1" is the fundamental mode (TE10), I would expect the field lines to look like this:
However, Feko shows the field lines for the Mode 1 as following:
I am not quite sure if I have a correct simulation setup for my purpose of showing the field lines of different modes or if I am interpreting Mode 1 correctly as TE10. If this is not correct, could you please help me to set up a simulation for my purpose?
I have attached my Cadfeko model here.
Thank you in advance!
Best regards
Fatih
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Hi Fatih,
I think you have confused the CMA with the waveguide modal analysis.What you want can be obtained by setting a Standard Configuration where you define the waveguide port and source and excite that source either using the fundamental mode only, or by defining as many modes you wish:
You can then request a near field that cuts your waveguide at a given cross-section. For instance here I have set it at the mid-length of the waveguide:
After you run your simulation, you will be able to view your near field in PostFeko, and activate the Arrow Display of the field direction in the 3D view:
I have attached the model (created in v.2024.1).
Best Regards
Hi Zeina,
thank you very much for your fast reply - it helped me a lot!
I have now understood how to excite the different modes and view them. I just have one last questions resulting from this. I have excited the modes TE10, TE11, and TE20 in my model. After simulating it, I would like to view the near fields of the different modes separately in POSTFEKO. However, I do not find any function that enables to plot the near fields depending on the mode excitation. I see different waveguide sources in the left tab, but no near field options when I want to plot the fields when I select a waveguide source. I can see the different sources on the left corresponding to the three modes I have excited, but no plot option that lets me choose which field type I would like to plot. Is there a way to change the field plot below to the other modes I have simulated?
Another small remark: I specify the modes I would like to excite in the window below. However, for the fundamental TE10 mode I have to write the indices the other way round. For example, for the TE10 mode I would expect to input 1 for m and 0 for n. But simulating this and viewing the fields does not give the correct fields in POSTFEKO. I have to input a 0 for m and a 1 for n to view the TE10 correctly. Same goes for the TE20 mode which I only get correct results for if I input a 0 for m and a 2 for n. Should this not be the other way round?
Maybe I am having a wrong thought about this. I would appreciate your help with this issue.
Thank you!
Best regards
Fatih
Altair EmployeeHi Zeina,
thank you very much for your fast reply - it helped me a lot!
I have now understood how to excite the different modes and view them. I just have one last questions resulting from this. I have excited the modes TE10, TE11, and TE20 in my model. After simulating it, I would like to view the near fields of the different modes separately in POSTFEKO. However, I do not find any function that enables to plot the near fields depending on the mode excitation. I see different waveguide sources in the left tab, but no near field options when I want to plot the fields when I select a waveguide source. I can see the different sources on the left corresponding to the three modes I have excited, but no plot option that lets me choose which field type I would like to plot. Is there a way to change the field plot below to the other modes I have simulated?
Another small remark: I specify the modes I would like to excite in the window below. However, for the fundamental TE10 mode I have to write the indices the other way round. For example, for the TE10 mode I would expect to input 1 for m and 0 for n. But simulating this and viewing the fields does not give the correct fields in POSTFEKO. I have to input a 0 for m and a 1 for n to view the TE10 correctly. Same goes for the TE20 mode which I only get correct results for if I input a 0 for m and a 2 for n. Should this not be the other way round?
Maybe I am having a wrong thought about this. I would appreciate your help with this issue.
Thank you!
Best regards
Fatih
Hi Fatih,
I'm glad that this helped.
As for your question regarding the numbering of the modes, this will depend on the reference direction you have set while creating your waveguide port:
I invite you to check this section of the user guide where this is detailed: Reference Vector
Altair EmployeeHi Fatih,
I'm glad that this helped.
As for your question regarding the numbering of the modes, this will depend on the reference direction you have set while creating your waveguide port:
I invite you to check this section of the user guide where this is detailed: Reference Vector
Additionally, regarding the field display given different excitation modes of the source, you can only see that if you create multiple configurations where you define one source excitation mode per configuration. If you excite a source with multiple modes, the field results you will be able to observe are those resulting from all the excited modes in your source.
Altair EmployeeHi Zeina,
thank you very much for your fast and clear answers.
I have understood now how the modal analysis should be treated.
Thank you!
You're Welcome!
Don't hesitate to mark the replies you found helpful as correct answers so that other forum users in the futur who have the same questions can directly see them 
. Thanks!
Hi Fatih,
I think you have confused the CMA with the waveguide modal analysis.
What you want can be obtained by setting a Standard Configuration where you define the waveguide port and source and excite that source either using the fundamental mode only, or by defining as many modes you wish:
You can then request a near field that cuts your waveguide at a given cross-section. For instance here I have set it at the mid-length of the waveguide:
After you run your simulation, you will be able to view your near field in PostFeko, and activate the Arrow Display of the field direction in the 3D view:
I have attached the model (created in v.2024.1).
Best Regards