Help needed on the currents exported from the .os file FEKO

Altair Forum User

I have come across a project, which requires some calculations of the currents output from FEKO MoM. I was able to export the .OS file from FEKO, but I do not have very good understanding of all the different components of the real and imaginary currents such as Jx, Jy, Jz, Jcorn1, 2 and 3, Jx, y z _c1, c2 and c3.

 

Basically, I only need the surface currents from each meshed triangle, induced by plane waves from all directions - this can be represented by an N by M current matrix, where N is the number of incident plane waves and M is the number of small triangles. These matrices (one such matrix for each triangle) of currents will also directly contribute to the calculation of far-field radiation.  Please, advise me how to utilize the data from the .OS file (or other files from FEKO) to obtain the aforementioned currents. Also, what is the different between surface currents and segment currents in FEKO?

 

Thank you very much!

Shane

Mel

Hi Shane

 

The currents are given in component form (real and imaginary) at the triangle position centre and at the 3 triangle corners (Jcorn1 for instance).

Segment currents are wire segment currents (e.g. currents on the wire of a monopole)

 

See the UserManual.pdf, part of your FEKO installation, for 'Description of the output file -> currents and charges'. Also see the section on the OS card.

Maybe the currents output to the out file (request this OUT file output in CADFEKO) is more convenient.

 

 

 

OS card :

<?xml version="1.0" encoding="UTF-8"?>

 

Another alternative is to use the surface currents dataset in the Lua scripting environment. It's up to you.

Currents dataset:

<?xml version="1.0" encoding="UTF-8"?>

 

 

Altair Forum User

Hi Mel,

Thank you for the clarification on the file format. I have already checked the manual regarding the .os file.

 

However, my question would be how to obtain a complex scalar of the surface current (from each meshed triangle in MoM) from the complex current density vectors: Jx, Jy and Jz. Or how would FEKO do this numerically.

 

Hi @JIF

I have seen your previous answers that are somewhat similar to my question regarding the currents exported from FEKO . Would you mind helping me out on this one? Thank you in advance.

JIF

Hello Shane,

 

I'm not sure what you are trying to do. Are you trying to get hold of the the current at the middle of each triangle (magnitude only, or complex or also the vector direction) for each triangle for each incident plane wave? You can get the info from the OS file or the DataSet as already indicated. The both the DataSet and the OS file provides the complex vector current at each vertex. If you want the current at the middle of the triangle, simple add the three values and divide by 3.

Jc = (1/3)*(J1 + J2 + J3)

 

Now, we can calculate the magnitude of the current at the centre of the triangle as
|Jc| = sqrt( |Jcx^2| + |Jcy^2| + |Jcz^2|) = sqrt( (Re(Jcx)^2 + Im(Jcx)^2) + (Re(Jcy)^2 + Im(Jcy)^2) + (Re(Jcz)^2 + Im(Jcz)^2) )
where Jcx is the x-component of the vector Jc (complex vector current density at the centre of the triangle). Note that this is not necsesarily the maximum that the current will be, but rather the upper limit of the maximum since it assumes that the three vector components are in phase and will be a maximum at the same time. If we wanted to really calculate the maximum, then we would have to calculate the phase (wt) where the maximum will occur and then use the magnitudes of the vector components when the phase is equal to wt_max. This means that the instantaneous value calculated below might never actually be equal to the maximum calculated here.

 

We can also calculate the instantaneous value of the current at the centre of the triangle.
Jci(wt) = Re(Jc*exp(jwt)) = Re(Jc)*cos(wt) - Im(Jc)*sin(wt)
Note that Jci(wt) is a scalar vector (not complex). Now, the magnitude of Jci(wt) indicating the colour in POSTFEKO is calculated as before using:
|Jci(wt)| = sqrt(Jcix^2 + Jciy^2 + Jciz^2) .

 

This explains what POSTFEKO displays in the 3D view.

 

I hope that helps. If you do use the DataSet and not the OS file, you will see that each triangle index is repeated 3 times and this represents the current at vertex 1, 2 and 3 for that triangle. To know the exact location in space of the vertex, you need to access the mesh info (also via the API).

 

I hope that helps.

Altair Forum User

Hi @JIF

 

Thank you very much for the detailed explanation!

 

Basically, I want to solve the EFIE for Method of Moment using the currents obtained from Z*I = V. I am hoping to obtain the complex current vector I for each triangle for each incident plane wave (basic radar cross section process).

 

Are the currents at the middle of the triangles or the so-called 'triangle currents' the complex vector I that I am looking for to solve EFIE equation? In addition, are the complex currents Jx, Jy and Jz (as shown in the attached picture) the currents at vertices or the currents at the middle of the triangles?

<?xml version="1.0" encoding="UTF-8"?>

 

Thanks to your thorough explanation, I now know how to calculate the magnitude of these currents. However, how do I somehow combine the imaginary part (or phase) of the x,y and z components of the currents so that I only have one complex current value per triangle per incident angle? As previously mentioned, I need to obtain a vector of complex currents for each triangle for each plane wave for the EFIE calculation.

 

Thank you again for taking your time to answer my questions. I really appreciate your help.

 

Cheers,

Shane