how to express the bandwidth percentage? help please

I want to calculate the percentage bandwidth of an antenna from the parameter S11. How I do the calculation? In light of the example is in the picture how to express the bandwidth percentage? I just know that we look for an antenna, area where the S11 is less than -10dB.

Resonant frequency is approximately 36GHz, the -10dB Return Loss bandwidth (2:1 VSWR) is approximately 6GHz, that means the bandwidth is approximately 16%.

% BW is calculated by the formula
100*BW (@ required RL) / Center frequency (Fc).
in this case, as per approximation from Vfone, Fc is 36GHz and BW 6GHz.
This results 100*6/36=16.67%

thus, % BW can change with the same bandwidth but with other center frequence?! example, if my BW is always 6Ghz but center frequence=20Ghz then %BW=30%?!

You are right.

((f2-f1)/(center frequency))*100=banwidth percentage