I am trying to simulate a UHF RFID tag in HFSS. The tag has a center module and a meander dipole antenna made of conductive wire. The center module is used to match the imaginary part of the RFID chip’s impedance. I used an online calculator -the link I added- to design the center module. I used a circular spiral coil. I calculated the chip impedance as 10 – 189j ohms. The spiral coil passes between the two points where the chip is connected. Because of this, I raised these points and added a lumped port between them. I set the lumped port impedance to match the chip’s impedance. I defined the meander dipole as a PEC material.

The center module and the meander dipole are supposed to be connected with inductive coupling. But at this point, something went wrong. When I simulate with this structure, the S11 result looks the same as the S11 of only the center module. The center module and the meander dipole have different impedances, I think I should not see just one resonance point in the S11 graph. It seems like the coupling is not happening.

I placed a radiation boundary based on the wavelength of the working frequency. I am using the modal solver. Maybe I made a mistake in the solver type, boundary condition, or port definition. How can I understand if the coupling is happening?

https://www.circuits.dk/calculator_planar_coil_inductor.htm