I am confused as some researchers took s11 below -10 db. and s11 around 0 db. or in some journals s11 approaches to 0 db. and s21 is below -10 db. what is the exact consideration of s11 and s21 parameters while designing a unit cell for lens antenna.
S11 is a coefficient that reflects the amount of energy reflected by a material or surface. For example, if you want to design a unit cell for a frequency selective surface (FSS), the goal should be to have maximum reflection (0dB) and minimum transmission (for example, -20 dB) at resonance frequencies. Similarly, if you want to design a metamaterial that passes maximum signal and has less reflection, then S11 should be minimized (i.e., -20 dB), and transmission should be maximized (S21 ~ 0 dB). The specific application of the unit cell determines the desired values for S11 and transmission.
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