How to interpret Floquet Ports in HFSS

[alienmindsinc]

Good afternoon,

I'm trying to run a simulation of an infinite array of antennas, but I'm running into a problem. The requirement is to use Floquet ports, and as far as I can tell, I'm doing it correctly, but the results are all wrong.

I started off with a basic 25 mm square patch on 30 mm square dielectric with a height of 0.2 mm and no ground plane. According to the results, this was a perfect reflector. So I removed the dielectric, and got very similar results. Finally, I removed the patch, and again, same results.

Until this point, the air box was defined as follows: Floquet port on top with a axis on x, b axis on y. Master 1 with matching slave one on left and right, master 2 and slave 2 on front and back, no boundary on the bottom (it was the bottom of the dielectric).

The reason for this is that we are supposed to model the main array in Rx mode such that we can excite a plane wave away from the array and see how much of the energy reflects back. For this, I was told that S11 of the floquet port will be the reflection coefficient (meaning the amount of energy that returns to the port), and that S12 for the floquet port will be the transmission coefficient (meaning the amount that does not return/is absorbed by the array). Are these assumptions correct?

Anyway, after digging for a while, I found out that HFSS treats all surfaces as closed unless you specify otherwise, so for my last simulation, I used an empty air box bounded on the bottom with a radiation boundary, the top with a floquet port, and I left the sides in their original master/slave configuration. Once again, I'm getting confusing results from the S parameters.

I guess, I'm asking the following questions:
1. What am I doing wrong?
2. Is the model of an incoming plane wave bouncing off of a structure to supply the reflection coefficient a good way of thinking of the Floquet S parameters as everyone in the lab keeps telling me?
3. If I'm wrong, can you please explain what the Floquet port is doing?

I'm attaching my final empty cell so that you can see what I've done, and comment about it if it is wrong.

Thanks in advance.

 

[TheArcane]

I started off with a basic 25 mm square patch on 30 mm square dielectric with a height of 0.2 mm and no ground plane. According to the results, this was a perfect reflector. So I removed the dielectric, and got very similar results. Finally, I removed the patch, and again, same results.

Sounds like you left your antenna open circuited. In that case, the power from the incident wave has nowhere to go but back to the port, so you see full reflection.
Terminate the antenna with a conjugate matched load or some sort of port with the proper impedance and you will see proper absorption.

 

[alienmindsinc]

I'm sorry about the delay in response, I didn't get a notice that there had been a reply (it got sent to spam).

I'm fairly new to HFSS, and the documentation I received was a rather slim grouping of random tutorials. Which port would be best for this? I would have thought a perfectly matched load boundary would be the thing to attach.

Thanks.

 

[gultepee]

Hi,

I'm also new in hfss and i have a question rather than an answer.

How do you apply an incident wave to a port? So far, I thought incident waves and ports are two different types of excitations, but clearly you can mix them which makes it a lot easier to calculate s-parameters. Appreciate any help...

Thanks,

 

[alienmindsinc]

This really should be its own topic.

You don't apply an incident wave to a port, you apply it to an entire structure by assigning excitation -> incident wave, and then filling in parameters. You measure its effects on a structure after simulation.

The floquet port seems to use this basic model of a plane wave coming in from the place you assign it, and then expanding over an infinite array of structures in your unit cell.

-M

 

[gultepee]

So, you are applying two different type of excitations:
1- incident wave,
2- two ports.
And the s-parameters you calculate are essentially the parameters between the two ports, included the effect of the incident wave.
Do I understand you correctly?

 

[alienmindsinc]

I don't think you quite understand what you're asking.

S parameters are ratios between two ports. They are essentially the voltage at one port with respect to the voltage at another port. So if you choose two arbitrary points in space and label them 1 and two, as below:

1. .2

You get the following:

S11 => The ratio of the voltage output at port 1 to the voltage input at port 1 in the forward direction.
S12 => The ratio of voltage output at port 1 to voltage input at port 2 in the direction of port 1 to 2.
S21 => The ratio of voltage output at port 2 to voltage input at port 1 in the direction of port 2 to 1.
S22 => The ratio of the voltage output at port 2 to the voltage input at port 2 in the reverse direction.

Now if you apply an incident wave to this structure, you get something much different if you include the effects of that wave. I don't think the resultant S parameters will be valid.

If you are measuring the effects of the wave at a single port, I think your results will be okay. However, measuring S parameters between ports will give deceptive data that is unlikely to be useful or accurate.

From what I can tell, a Floquet Port accomplishes what you are trying to figure out. However, I can give you no information on the inner workings of the floquet port as I, myself, still cannot get the expected results on a well documented model (with years of data) measuring what I assume to be the same quantities.

 

[tallface65]

You may want to be careful in your setup. TheArcane has a good point. Unless you defined the feed of the patch antenna with a 50 Ohm RLC boundary condition or a lumped port definition with a 50 Ohm reference, your antenna is not matched and is rather left OPEN!! This will of course lead to very strong reflections. As a solution, ensure your antenna is seeing a matched load at the feed (in HFSS this does not need to be complex conjugate matched, just real matched )

Also, what information are you specifically looking for from your simulation. If you want active S parameters, just feed your antenna with a lumped/wave port and keep the Master/Slave boundaries and the Floquet port as it is a perfect termination (assuming enough modes are selected, see HFSS help) If you want active element pattern, do the same setup, but now sweep the angle of incidence to get the single element active Gain using equation given in their antenna training ppt

 

[notius]

As TheArcane and tallface65 said, you should defined the feed of the patch antenna.

I think, what you are modeling now is sth like FSS - frequency selective surface (act as a filter) ; apply two Floquet port, one on top, one at bottom, and apply Master/Slave Boundary at walls of the cell, that's exactly a simulation of FSS.

 

[gultepee]

tallface65:

If you are using two floquet ports in the design, you do not need to use a lumped port, right?

 

[tallface65]

Gultepee, that is sort of correct. You need either a lumped port at 50 Ohms or an RLC boundary at 50 Ohms to match the feed of the antenna. If you do not do this, the antenna will always "see" itself as open.

A floquet port simply allows for an easy excitation/termination definition for the use of periodic structures. The nice thing, though confusing to some, is that the Floquet ports return complex S Parameters. Floquet ports simply excite plane waves, but each plane wave mode is orthogonal and you can get any given plane wave (incidence angle) from a superposition of the propagating Floquet modes. The key here is periodicity and one can look up Floquet's theorem for specifics on the math (Or in HFSS help)

The maximum number of Floquet ports in a simulation is two, and Notius is correct in that if you care about both reflection and transmittance, then you will need two Floquet ports to do so. In the case of a reflectarray foe example, it is backed by a ground plane so you would only need one Floquet port since you will only see reflections.

Hope this helps