How much number of TE and TM modes should be?

Hi all;

Can anyone tell me that of How much number of TE and TM modes should be enough? for simulating a periodic surface (Frequency selective surface) in hfss. Mean to say if i keep 10 modes both TE and TM the resulting S21 value is -60dB at 4 GHz frequency so if i increase the number modes to 14 then the convergence issue is faced enev i increased the number of passes. Kindly tell me the difference what would happen if use 10 modes or 14 modes in mode calculator in floquet port in HFSS. :?:

As the period of your device increases relative to your operating wavelength, the device will diffract into more modes. It is critical that you use enough modes to include all the propagating ones. The others are cutoff and decay quickly with distance. There is a button called something like "Analyze Modes" that will calculate the attenuation of these cutoff modes. If you do not include these, you need to make sure that there is enough distance between your device and your Floquet port so that they attenuate to essentially zero at the port. Don't let the evanescent modes touch your Floquet port. There is a deembed option that will numerically move the port away from the device, I think. I haven't played with that much and I am not completely sure what it actually does.

Hope this helps!

rrumpf said:

As the period of your device increases relative to your operating wavelength, the device will diffract into more modes. It is critical that you use enough modes to include all the propagating ones. The others are cutoff and decay quickly with distance. There is a button called something like "Analyze Modes" that will calculate the attenuation of these cutoff modes. If you do not include these, you need to make sure that there is enough distance between your device and your Floquet port so that they attenuate to essentially zero at the port. Don't let the evanescent modes touch your Floquet port. There is a deembed option that will numerically move the port away from the device, I think. I haven't played with that much and I am not completely sure what it actually does.

Hope this helps!

Click to expand...

I really did not get any thing you are saying. As the period of your device increases relative to your operating wavelength, the device will diffract into more modes. what do want to say by this line??There is a button called something like "Analyze Modes" that will calculate the attenuation of these cutoff modes. . Do you want to say that for checking the attunation of modes as the angle in the modes calculator is varied am i right?

I am using 10 modes both TE and TM in my design and kindly tell me what happen if i change the angle in the floquet port -> excitations -> Modes setup -> modes calculator then which angle should be changed phi or theta and what would that effect in the graph of S21 . which one should be kept as constant and which one varied.

Thanks for reply

"As the period of your device increases relative to your operating wavelength, the device will diffract into more modes."

This line has nothing to do with HFSS. It has to do with the physics of the device. A wave incident on a periodic structure will be diffracted by it and "split" into discrete waves propagating at different angles. The longer the period, the more modes diffraction will produce. For some theoretical background on this, refer to Lecture 9 here:

**broken link removed**

So in HFSS, you must include all of the propagating modes that will be produced by your device or the port condition will not work. That is, the number of modes you choose has to be consistent with the physics of the device. In HFSS you enter the frequency and calculate the modes. Modes that are cutoff will be given an attenuation coefficient. If you do not include the cutoff modes, then you must ensure there is enough space between your device and the port so that these decay sufficiently.

Angle of incidence can affect all of this. This can cause some modes previously cutoff to be propagating and some that were propagating to be cutoff. Use the Mode Calculator to determine this. You must include all propagating modes, but can leave off the others if you have sufficient space between your device and the port.

Thank you rrumpf!
Can you help me to find the number of modes in CST MWS FSS simulation.
Also CST FSS simulation supports TE and TM polarization only. Is it possible to simulate for other angle of polarization (Ex: 60 Degree). Thanks in advance.

I do not know how to use CST so I can't be of much help to you there. Sorry!

Fortunately, in LHI materials there are only TE and TM modes so these will account for everything. Any angle of incidence is possible so you shouldn't be limited in any way.