Mode expansion coefficients in FDTD ports?

- April 15, 2022 at 5:28 pm
  
  zhxzhang
  Subscriber
  
  The results of mode expansion coefficients in ports seem to have a drastic change over the years, and now I'm having trouble interpretating the results of the a and b coefficients.
  
  For example, the forward propagating source port usually has |a|~1 before, but now it's around the same magnitude as N coefficient (~1e-15), and I can't seem to find the relation between these two.
  
  Is this a bug, or there is some change in definition of these coefficients? Thanks!
- April 29, 2022 at 10:46 pm
  
  Guilin Sun
  Ansys Employee
  
  Thank you for your post. Sorry to reply you late.
  I tested and indeed the magnitudes of coefficients a nd b are quite small. Although no documentation, I guess that, it borrows the method used in EME where to increase the accuracy it assumes the power of each mode is 1 Watt. However this is done in calculating S, not passed to a and b properly, and not in N and P. In addition, since port now has the necessary data of S and T, so the values of a and b are not important, so let them not scaled accordingly. I believe that right now the S parameters are not calculated as b/a from the outputs. I would suggest not to use a and b, but S only.
- April 29, 2022 at 11:11 pm
  
  zhxzhang
  Subscriber
  
  Thanks for the response! So I figured out a couple days ago that I can get the correct a and b by right click on the result and select "calculate" (see attached screenshot).
  However I don't think this operation can be done by any script. Please fix this bug because I do need to use a and b in my study. Thanks!
- April 29, 2022 at 11:27 pm
  
  Guilin Sun
  Ansys Employee
  
  yes, when a and b are correct, all other are not. It is less likely to be fixed soon.

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