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Plane Wave

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Hi, I am struggling to simulate a plane wave propogating in air. It should be very straightforward, but I am unable to get satisfactory results in COMSOL 5.3a.

Basically, I use a rectangular 2-D model with the RF module (electromagnetic waves, frequency domain). The wave propogates in the X direction, and I specify two user-defined ports at the vertical edges of the rectangle (x=0; wave excitation) and (x=L; no wave excitation) of the rectangle. I specify the input quantity as electric field, with the electric mode being Ex,Ey = 0 and Ez = 1. The propogation constant (beta) is set to emw.k. Along the 2 horizontal edges, I set the boundary conditions as perfect electric conductors. I don't know if using a perfect electric conductor is an appropriate boundary condition.

When I compute, the electric field in the z direction (Ez) doesn't oscillate around 0 and in fact goes to 0 almost immediately.


3 Replies Last Post Jul 11, 2018, 1:25 PM EDT

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Posted: 11 months ago Jun 17, 2018, 4:10 AM EDT

I think you should try first given examples in RF module to understand basics and then try yours own model.

I think you should try first given examples in RF module to understand basics and then try yours own model.

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Posted: 11 months ago Jul 5, 2018, 11:58 PM EDT

Hi José,

I would not try a user-defined port first. It mostly depends on your structure, for example a rectangle in 2D is a simplified model for a rectangular waveguide. Hence, if the field goes to 0 immediatly, I would think that this is because you are sending a mode that cannot propagate into your rectangle.

You can try a numeric port, but if you want to be more specific, I would go first for a rectangular port, using a TE10 mode, or a TM11 modes (they have the lowest cutoff frequencies in a rectangular waveguide). Hence, you are not obliged to specify the components of E.

Another solution might be that you are propagating at the wrong frequency. Because your waveguide is finite, I would do an eigenmode study first, then pick a good frequency (lowest imaginary part and which looks like a TE or TM or whatever mode you are sending), and then do the Frequency domain study with this frequency as an input. That will ensure you that this frequency is allowed to propagate.

I don't know if you did but don't forget so specify a Boundary Mode Analysis in your study, not for the eigenfrequency study but before the Frequency domain step in the other study.

About the boundary conditions, a PEC is appropriate as long as your are propagating in a metal : it will reflect everything. Otherwise, if it is not a metal, I would put the structure in a box of air, with scattering boundary conditions on the sides to absorb the field (and add PML around if you want).

Attached is a TE10 mode propagating in a semiconducture waveguide, surrounding by air and with PML to give you an example. I did it in 3D.

Best,

François Swiadek

Hi José, I would not try a user-defined port first. It mostly depends on your structure, for example a rectangle in 2D is a simplified model for a rectangular waveguide. Hence, if the field goes to 0 immediatly, I would think that this is because you are sending a mode that cannot propagate into your rectangle. You can try a numeric port, but if you want to be more specific, I would go first for a rectangular port, using a TE10 mode, or a TM11 modes (they have the lowest cutoff frequencies in a rectangular waveguide). Hence, you are not obliged to specify the components of E. Another solution might be that you are propagating at the wrong frequency. Because your waveguide is finite, I would do an eigenmode study first, then pick a good frequency (lowest imaginary part and which looks like a TE or TM or whatever mode you are sending), and then do the Frequency domain study with this frequency as an input. That will ensure you that this frequency is allowed to propagate. I don't know if you did but don't forget so specify a Boundary Mode Analysis in your study, not for the eigenfrequency study but before the Frequency domain step in the other study. About the boundary conditions, a PEC is appropriate as long as your are propagating in a metal : it will reflect everything. Otherwise, if it is not a metal, I would put the structure in a box of air, with scattering boundary conditions on the sides to absorb the field (and add PML around if you want). Attached is a TE10 mode propagating in a semiconducture waveguide, surrounding by air and with PML to give you an example. I did it in 3D. Best, François Swiadek


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Posted: 11 months ago Jul 11, 2018, 1:25 PM EDT
Updated: 11 months ago Jul 11, 2018, 1:27 PM EDT

If you want to generate/apply an open-space plane wave, consider using the scattered wave formulation (a background wave) instead of attempting to launch a plane wave from a port. And, in that case, don't use PECs for the boundary conditions anywhere that they wouldn't already be a perfect fit to the plane wave in question.

If you want to generate/apply an open-space plane wave, consider using the scattered wave formulation (a background wave) instead of attempting to launch a plane wave from a port. And, in that case, don't use PECs for the boundary conditions anywhere that they wouldn't already be a perfect fit to the plane wave in question.

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