How to determine the central frequency from a vectorial spectrum analyzer?
The S11 represents the relative amount of signal that is reflected from the antenna, so the lower the better, because lower means more radiated by the antenna.

Usually, we consider the antenna to be "working" at a given frequency if its S11 is lower than -10 dB (of course, this requirement can be changed according to the application. spatial antennas are more around -20 dB for example).

Therefore, your antenna seems to be working at three distinct frequencies (with the s11 &lt; -10 dB criterion). Is this a problem ? It depends on the hardware you put behind it during operation.