HFSS Phase of the reflection coefficient for an AMC material

Hello,
I have a problem calculating the phase of the reflection coefficient for an AMC material. My structure is formed by a silicon substrate and 2 oxides on top of it.
The AMC properties are generated by a spiral strip of metal placed in the bottom oxide at the interface with the top oxide.
I use HFSS v11 to simulate it.
Since the single spiral should be part of an infinite plane of those kind of structures,
I've placed master and slave boundaries on the side walls. At the bottom I've placed a PMC boundaries to avoid unwanted reflections.
Since my field is gonna come from an inductor placed at the top of the top oxide, I've placed a floquet port at that position. Since I wanna know the reflection coefficient at the interface of the 2 oxides, I've embedded the port till there.
At this point, the phase of the reflection coefficient should be the phase of S11, right?
What happened is that I plot the S11 phase but, even changing the dimensions of the AMC spiral, the plot remain the same, it doesn't change at all.
So I guess there is something wrong in my setup.

I'm attaching my HFSS v11 project.
Can someone please take a look at it and tell me what I do wrong to calculate the phase of the reflection coefficient?

Thanks a lot