Artificial Magentic Conductor (AMC) S parameter issues

i have read few papers of yours about AMC ground plane.
I wish to ask you few questions that i hope can clear up my mind.

i) what is the reference value for S11 for AMC unit cells? In some papers i read, the S11 is smaller than -10 dB and some are bigger like -2dB (for instance). But as far as i understand, S11 is the signal from input port 1 to output port 1 too. AMC react as reflector in antenna (for my interest) therefor S11 should have bigger value eg. -2dB and not below -10dB?

ii) In measurement/simulation of AMC periodic sturucture to observe the bandgap characteristics which is denoted by S21 (magnitude). As far as i understand the Structure is added up with 50 ohm transmission line on top, with port 1 and port 2 at each end of the transmission line (as in attachment). This would make a multilayer substrate With AMC in the bottom substrate and transmission line on the top susbstrate. I am courius, when doing this kinda of measurement, does it require S11 and S22 to be below than -10 dB?

(p/s: FYI, the AMC unit cell, exhibit 0 degree reflection phase at my frequency interest 5.8GHz)

I attach you some of the simulated resulttht ive obtain.

Your Help is much appreciated. thank u.