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Finite Z component of E field for TE modes

lostarmour

Hi everyone,

I have been trying to simulate a waveguide junction problem with waveguide ports to calculate the field distribution on the aperture. I have a 5 circular waveguide problem where I use only TE modes at the small waveguide port. On the other end I have used a port with 0 excitation from all the modes to ensure an absorbing boundary condition.

The problem is that I still get a finite z component for the aperture E field. However, from theory we know that the aperture fields for a TE mode should have a 0 z component. This I have observed in only 1 waveguide problem too. This also happens when the discretization is lambda/25. Is that a numerical artifact? I am adding the cfx file also to this thread.

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Mel

The waveguide is way overmoded. Even though you apply the TE mode, because of the dimensions many modes are propagating.

From the OUT file:

TE 1 1 4.55156E+09 3.98372E+02 0.00000E+00 0.00000E+00 2.77479E+02 Propagating
TM 0 1 5.94492E+09 3.41078E+02 0.00000E+00 0.00000E+00 2.65650E+02 Propagating
TE 2 1 7.55032E+09 4.47366E+02 0.00000E+00 0.00000E+00 2.47090E+02 Propagating
TE 0 1 9.47229E+09 5.11613E+02 0.00000E+00 0.00000E+00 2.16061E+02 Propagating
TM 1 1 9.47229E+09 2.77408E+02 0.00000E+00 0.00000E+00 2.16061E+02 Propagating
TE 3 1 1.03857E+10 5.61795E+02 0.00000E+00 0.00000E+00 1.96761E+02 Propagating
TM 2 1 1.26957E+10 1.58787E+02 0.00000E+00 0.00000E+00 1.23672E+02 Propagating
TE 4 1 1.31454E+10 1.09505E+03 0.00000E+00 0.00000E+00 1.00945E+02 Propagating
TE 1 2 1.31798E+10 1.11703E+03 0.00000E+00 0.00000E+00 9.89584E+01 Propagating
TM 0 2 1.36461E+10 8.41727E+01 0.00000E+00 0.00000E+00 6.55583E+01 Propagating

If the waveguide is made longer, at least the evanescent modes (again see OUT file) should have sufficiently died away over the length of the waveguide.

lostarmour

The waveguide is way overmoded. Even though you apply the TE mode, because of the dimensions many modes are propagating.

From the OUT file:

TE 1 1 4.55156E+09 3.98372E+02 0.00000E+00 0.00000E+00 2.77479E+02 Propagating
TM 0 1 5.94492E+09 3.41078E+02 0.00000E+00 0.00000E+00 2.65650E+02 Propagating
TE 2 1 7.55032E+09 4.47366E+02 0.00000E+00 0.00000E+00 2.47090E+02 Propagating
TE 0 1 9.47229E+09 5.11613E+02 0.00000E+00 0.00000E+00 2.16061E+02 Propagating
TM 1 1 9.47229E+09 2.77408E+02 0.00000E+00 0.00000E+00 2.16061E+02 Propagating
TE 3 1 1.03857E+10 5.61795E+02 0.00000E+00 0.00000E+00 1.96761E+02 Propagating
TM 2 1 1.26957E+10 1.58787E+02 0.00000E+00 0.00000E+00 1.23672E+02 Propagating
TE 4 1 1.31454E+10 1.09505E+03 0.00000E+00 0.00000E+00 1.00945E+02 Propagating
TE 1 2 1.31798E+10 1.11703E+03 0.00000E+00 0.00000E+00 9.89584E+01 Propagating
TM 0 2 1.36461E+10 8.41727E+01 0.00000E+00 0.00000E+00 6.55583E+01 Propagating

If the waveguide is made longer, at least the evanescent modes (again see OUT file) should have sufficiently died away over the length of the waveguide.

Yes, I have all the other modes propagating as you mentioned. Therefore, I also tested when all these modes are active (excited) at the port and the result is different. And I even excited only the TM modes out of these modes and the result is again different. I have a theoretical model on MATLAB and I use z component for TE as 0 and for TM non-zero.

My only TM mode simulation in FEKO matches the results of MATLAB (all modes excited). Therefore, I expected that the only TE mode excitation in FEKO should give me almost 0 z component of electric field.

Mel

Your meshing should also be fine enough to avoid the warning:

WARNING 4682: Coarse triangular mesh of a waveguide port for representing the field distribution of the modes included in the mode expansion

lostarmour

Yes, I have all the other modes propagating as you mentioned. Therefore, I also tested when all these modes are active (excited) at the port and the result is different. And I even excited only the TM modes out of these modes and the result is again different. I have a theoretical model on MATLAB and I use z component for TE as 0 and for TM non-zero.

My only TM mode simulation in FEKO matches the results of MATLAB (all modes excited). Therefore, I expected that the only TE mode excitation in FEKO should give me almost 0 z component of electric field.

For example, take this example file I am attaching. In this I have excited only one TE mode at a frequency where the only TE mode is propagating. In this case also, in spite of having a mesh size of lambda/40, I still get some z component of E field. I guess because of this, when I add more modes, the z components due to only TE modes also get added.

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