It depends on how many floquet modes you want to excite. As higher is the waveguide height the more attenuation of the high modes you have and the less influence has the higher order modes. In HFSS you can calculate the atenuation of these modes.

If your structure will be impinged by plane waves I recommend you to use only the first two modes so you should have an enough height to attenuate the other modes (this height is usually lambda/2). 

I recommend you to read:

http://lsun.people.ysu.edu/ECEN%203742/ANSYS%20HFSS%20Module/HFSSFloquet_14.5.pdf

All the best!

Dear Humberto,

Thank you for the prompt reply and for the useful referral. I read the entire tutorial and I found it very useful at some points, however, I still have these questions if you would help:

1- I am using the only two specular modes (TE00 & TM00) which has ZERO attenuation, however every time I change the height I get different high impedance frequency for the AMC I am simulating, so I am still wondering which height is correct?

2- What is the difference between using Waveport and Floquet-Port while simulating HIS, AMC, EBG, or any periodic structure?

3- What should be the boundary assignment for the bottom face of the air-box surrounding the unit-cell (sides: periodic boundaries, top: Floquet-Port, bottom: ??) ??

Thank you in advance,,,

You should change the height of the box untill you realize that the resonant frequency does not change.The different resonances are not real. These are due to the mesh which is applied to solve the EM problem is not suitable.

As far as I know waveport and floquet-port are two ways to resolve the Maxwell equations in the waveguide. You must use Floquet-port if you work with Master-Slave boundary conditions. Similar results are obtained if you use PEC/PMC and Waveport.

The bottom face boundary depends on whether you have a grounded metallic plate at the bottom of your HIS if so you do not need to add any port if not and you want to simulate transmission properties you should add another floquet port.

Hope this information helps you

Thank you very much ...

 Dear Humberto

Do you have any similar tutorial about modelling of FSS or EBG materials for CST Microwave Suite? 

Dear Alexey,

You could take the following webinar where they talk about FSS:

https://www.cst.com/Events/Details/3452

I think that EBG simulation could be a little bit complicated.

@Humberto

I have tried changing the height through a parametric sweep, and still getting different high high-impedance frequency for each height. There must be a correct value !! Any suggestions??

@Alexey

The following tutorial is for EBG simulation using CST. This tutorial teaches how to extract the dispersion diagram for a periodic EBG structure. As far as I know "Dispersion diagram" the primary result you want to look at when dealing with EBG.

Dear colleagues, thank you for your answers

working on

Hi Ahmed Abdelraheem

The ports should be placed at a safe distance from the FSS to let the evanescent modes weaken sufficiently. To determine this distance, the attenuation values of the undesired higher order modes should be known. One can get these values from the HFSS, by using modes calculator.

Usually, lambda/4 is a good distance of Floquet port from the FSS.

@ Lamine N. A Bamogho

How to decide the Lambda value....what I mean to say is For example if we are working X Band(8-12GHz) which frequency should we take?...and why...please specify the reason?

@ Mohammad Abdulshukoor

First of all, the lambda/4, should the minimum distance of Floquet port from the FSS to ensure that higher order modes are sufficiently attenuated ( more than 50dB) before hitting the FSS surface. So we could eliminate those modes.

Therefore, the Lambda value should be computed with the lowest frequency of the frequency band. This remains true for dual band systems.

You can also use mode calculator in HFSS to check the effects.

However, it is important to choose an operating frequency inside the frequency band, which would be the solution frequency necessary to set the mode calculator in HFSS. So Lambda should be computed based on the solution frequency.

@ Lamine N A Bamogho

Hello sir,

1.can you please tell me how to check the field patterns at resonant frequency (In HFSS its only showing @Solution Frequency but not @ Resonant frequency). Most of the time iam not getting resonant frequency as solution frequency.

2..Can we take same procedure ...mean mid of frequency of sweep as solution frequency...and lambda/4 of this solution frequency as radiation box for Multiband and Broadband too??? Is it applicable

Hi Mohammad Abdulshukoor,

1- For this question, assuming that you know the position of the resonant frequency from a first simulation.

I would suggest to reduce the step size in the frequency sweep editor to have HFSS compute the field at that frequency. You can check it in the preview table. So you will be able to plot the field pattern.

2- No, I do not think so. Actually, Multiband and Broadband cases, are configured differently in HFSS. Maybe this link will help you to understand the principle: https://docplayer.net/64182125-Ansys-hfss-broadband-adaptive-meshing.html

- Using lambda/4 as a minimum size of the radiation box avoid the higher order modes propagation, in the unit-cell as explain above.

However, using lambda/4 also also allow to to keep the smallest size of the box to optimize the necessary computation time and resources.

I agree with Lamine N. A Bamogho