how to determine the Bandwidth in CST

Hello guys
i have a problem with measuring the Bandwidth of a dual band PIFA antenna.
lets look at the following S Parameter figure ,, we see that the lowest magnitude is -7.9dB

so How do i measure it ,, do i subtract -3dB from the loweset value and take -4.3dB as upper and lower cut off frequencies like showing in the following figure

i hope u can answer my question and sorry for being long
Regardz
Rio

Hi Rio,
I guess the return loss of the antenna is quite high..i think it should be at least -10dB for about 70% transmission. Then the bandwidth is taken from the level of -10dB.