RIS Array: Radiation Pattern Unchanged Despite Lumped Impedances in HFSS

[agupta]

Hello,

I am currently simulating a Reconfigurable Intelligent Surface (RIS) in HFSS and have encountered an issue that I hope someone can clarify.

Setup:

• Each unit cell contains lumped R/C elements.

• When I simulate the unit cell with an impedance sweep, the S11 clearly varies with the impedance values, which confirms the elements are being modeled correctly at the unit cell level.

• To build the array, I duplicated the unit cell geometry (instead of using “link/array” commands), since I need to apply different impedance values to each element of the RIS.

• For the open region, I am using an FE-BI boundary condition.

• Excitation has been tried in two ways:

  1. Incident plane wave.

  2. Illumination with a horn antenna.

Problem:
When I simulate the entire array, the far-field radiation pattern looks the same whether or not the lumped impedances are included. I expected the reflection phase/amplitude variations from the lumped elements to change the overall pattern compared to a uniform reflecting surface, but no such variation is observed.

Question:
Could anyone suggest what mistake I might be making? Is there something specific about:

• How lumped elements are treated in arrays,

• The boundary condition setup (FE-BI), or

• The excitation method (plane wave vs horn),

that could prevent the impedance variations from influencing the radiation pattern?

Any guidance or suggestions would be greatly appreciated.

Thank you!