The above would typically be measured using a Vector Network Analyzer (VNA), which can plot S11. The above figure implies that the antenna radiates best at 2.5 GHz, where S11=-10 dB. Further, at 1.5 GHz the antenna will radiate virtually nothing, as S11 is close to 0 dB (so all the power is reflected). The antenna bandwidth can also be determined from the above figure. If the bandwidth is defined as the frequency range where S11 is to be less than -6 dB, then the bandwidth would be roughly 1 GHz, with 3 GHz the high end and 2 GHz the low end of the frequency band.