Desperate help needed in Birdcage Simulation with HFSS.

Dear all,
I am trying to simulate MRI birdcage coild with ANSYS HFSS. The primary aim is to achieve homogeneous magnetic field within the coil at 64 MHz. It is a highpass birdcage coil with 16 rungs, 30 capacitors and two ports. I can achieve very good homogeneous magnetic field with circular polarization with capacitance of 75 pF. However, my S11 parameter is always around 67 MHz. How can i bring S11 at 64 MHz without disturbing the H-field?

I can bring the S11 parameter at 64 MHz by changing capacitance of the capacitors but then I no longer have the homogeneous magnetic field.
I have also noticed that the dimensions of my lumped capacitors also affect the magnetic field as well as S11 parameters. Currently I have 2D sheet of 10 mm x 10 mm to model the lumped capacitors. I have observed that by decreasing height of the lumped capacitors, S11 parameter decreases and vice versa.
This also means that by changing height of the lumped capacitors, I can have my S11 parameter pointing at 64 MHz but as I said I would no longer have homogeneous B1 field.

Please anyone can help me in this very basic and annoying problem.
I would be extremely extremely grateful.

regards