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MoM matrix and eigencurrent

Altair Forum User

Hello,

I would need some clarification to enable me understand what I am doing on FEKO. I am relatively new to using FEKO but I am having a hard time understanding some terminology.

1. What is the difference between MoM matrix and MoM impedance matrix. If there is a difference, how to I obtain the MoM impedance matrix on FEKO else I guess they are both the same.

2. What is the major difference between the surface current, current density vector and eigen-current. How do I obtain the eigen current on FEKO

3. The MoM matrix does not tend to match in terms of dimension with the current density vector on FEKO, how can I reconcile this

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Hello ozuem,

Do you need the clarification because the manual is using these terms interchangeably or what is the reason for needing the clarification? I'm just asking so that we can improve the documentation if that is required.

1. What is the difference between MoM matrix and MoM impedance matrix. If there is a difference, how to I obtain the MoM impedance matrix on FEKO else I guess they are both the same.

There is no difference, they are the same thing. The matrix can be exported to a *.mat file. See comments in the following post for converting the file to ASCII format.

2. What is the major difference between the surface current, current density vector and eigen-current. How do I obtain the eigen current on FEKO

It is difficult for me to answer this without some context. The surface current and the current density is just the surface current density on surfaces. When you refer to the eigen vector, it is a reference to the basis function weights (solution of the MoM matrix). If you know the basis functions and the associated weights, you can calculate the current (surface current density) anywhere on the surface. I hope I didn't add to the confusion and it is now more clear?

3. The MoM matrix does not tend to match in terms of dimension with the current density vector on FEKO, how can I reconcile this

As indicated above, the MoM matrix (or MoM impedance matrix) will have the dimensions of the number of basis functions (exported to the *.mat file) - basis functions are associated with each non-boundary edge. This should match with the solution matrix (exported to the *.str file). The currents (exported to the *.os file) will have a different dimension since it calculates the currents on the triangles and thus it will have dimensions related to the number of triangles.

I hope it is a bit more clear now.

Altair Forum User

Okay. Thank you very much, you really cleared my doubts. I am so much grateful for this. I have had this confusion for a while. One more question which I think I did not state is the eigen-current part. Is the eigen-current now the same as surface current or the current vector. I am referring in context of calculating the Q factor of an antenna where I need to multiply the eigen-current by the imaginary part of the impedance. Hope to hear from you soon. Thank you.

Altair Forum User

It was not from the manual I got my confusion, its from the display message when running the FEKO solver but it is clearer now

Altair Forum User

Hello ozuem,

The eigen-currents will be treated in the exact same way as surface currents in terms of the values and coefficients written to external files. Thus, the values exported to an *.os file will have the same dimension as normal currents exported to an *.os file.

Altair Forum User

Thank you so much at RenierM but at this point, the MoM matrix does not match the eigen-current vector hence the X of the MoM cannot multiply the eigen-current if you understand my worry. May be there is a way I can match this. Hope to hear from you soon.

Altair Forum User

Unfortunately you will not be able to get the coefficients for the CMA solution as they are not exported in any form (no STR file) - the currents that you get in the OS files (and in POSTFEKO) are the currents at triangle centres and corner points.

The dimensions of the MoM matrix is associated with the amount of edge basis functions and this is not equal to the number of triangles - it is thus expected that these dimensions do not match.

Altair Forum User

Unfortunately you will not be able to get the coefficients for the CMA solution as they are not exported in any form (no STR file) - the currents that you get in the OS files (and in POSTFEKO) are the currents at triangle centres and corner points.

The dimensions of the MoM matrix is associated with the amount of edge basis functions and this is not equal to the number of triangles - it is thus expected that these dimensions do not match.

Thank you so much for your reply and your assistance.

Altair Forum User

@RenierM, any idea on how to get the current for the edges from the data provided by postfeko?

Altair Forum User

@ozuem, if you want to manipulate the eigen currents such as create a combination of these, you can access and process them in POSTFEKO Lua.

Unfortunately we do not support accessing the eigenvector - if you want to do an analysis that require this, you will have to solve the eigen problem yourself using the matrix that is exported to the .mat file.

Altair Forum User

Please I need a quick assistance because I am stuck on one problem of using FEKO for some months. I am able to extract the MoM impedance matrix, I have access to the .out file as well but then I need to get the current at the edges since my MoM matrix is given for the edges. Any idea on how to go about this? I will appreciate a quick and fast response. Thank you.

Altair Forum User

The currents (or then the solution to the matrix problem) is exported in the STR-file. This will however not be the Eigen currents as this is what you will calculate using the CMA Eigen equation.

Altair Forum User

Ok @ RenierM, thank you so much so confirm that eigen current can be referred to as the eigen vector?

Altair Forum User

Also @ ReinerM, i have no access to the str file. Anyway i could go around this?

Altair Forum User

The values exported to the STR-file can be accessed using the str2ascii tool, located at http://www.feko.info/support/helpcenter/how-to/how-to-read-the-.mat-.lud-.rhs-files-and-.str-files

The values exported to the STR-file are the matrix solution coefficients - if RWG basis functions are used the values correspond to the currents on the triangles.

If you solve the CMA eigen equation using the matrix (which you exported above), you will calculate the eigen vector which in the case of RWG-basis functions then correspond to the eigen current.

Please note that these values are not exported by FEKO for CMA solutions. You will have to calculate them yourself.

Altair Forum User

I am so grateful to you RenierM, I guess my knowledge in the use of FEKO has been broadened and my knowledge has been increased even though I am still getting to learn a lot by the day. However, is there any formula I can use to get the current on the edge

Hi Ozuem.

Since you are asking for a way to access the eigen current on the mesh edges I assume you are looking for basis function coefficients (eigen vector)?
If you would like to access and manipulate these vectors your best approach would be to set up the generalised eigen equation yourself, using the mom matrix you have exported from FEKO.

Altair Forum User

Thanks @ Wilco. Any function to perform this on MATLAB?

We don't have access to MATLAB internally so I cannot confirm. You will have to look through the linear algebra functions in the documentation.

Altair Forum User

We don't have access to MATLAB internally so I cannot confirm. You will have to look through the linear algebra functions in the documentation.

Please what documentation?

You should search through the Matlab documentation for the functions you require.

Altair Forum User

Okay. Thank you

Altair Forum User

You should search through the Matlab documentation for the functions you require.

But I am also concerned about the issue of modal tracking. I really do not intend to solve this equation from scratch.

Unfortunately if you intend to do your own modal manipulation and further post processing, solving the eigen equation from scratch will be the only way to achieve this.

Altair Forum User

Okay. Thanks for you support. Now a question about the MoM matrix. How can the matrix of the other frequency be exported because it seems only the MoM of the first frequency of analysis is made available when you check the .mat file

 Syntax:  mat2ascii FILENAME [b]   Filename:   name of the *.mat file (with extension)   Options:     b         block number (e.g. for multiple frequencies)               (default is first block, b=1)

Altair Forum User

Please can you send me an example of the syntax

Altair Forum User

okay i got it. It id (b) not where b is for block number

Altair Forum User

I am so grateful for your continuous support. I still have questions as I continue to use FEKO and I hope you don't mind.

Please, I want to know if it is possible to get the information of the external edges. I also want to confirm that the values given for the current density vector on the triangle is for the triangle centre or for the whole triangle?

Assuming you are referring to edges that form the boundary of a surface, what information about these edges are you looking for?

The surface current (which is a vector quantity represented as a surface density in Ampere/meter (A/m)) can be exported to the *.out or a *.os file where the vector quantity will be reported on the triangle centre and the current magnitude will be reported for triangle corners. The *.str file reports the weighting coefficients of the matrix solution so that can be used to construct the surface current at any point on the model.

Altair Forum User

The currents (or then the solution to the matrix problem) is exported in the STR-file. This will however not be the Eigen currents as this is what you will calculate using the CMA Eigen equation.

This statement is confusing me. What is the difference between the eigen-current and the solution of the matrix problem. Please I would love clarification

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