bandwidth measurement

Hi,
how to measure bandwidth of antenna from return loss(s11) graph or how to analysis bandwidth whether it is increasing o decreasing from return loss (s11)graph of an antenna

although S21 is the graph that will define bandwidth.....
but if you want to extract Bandwidth info from S11 than for general understanding u can say ... "the band on which the magnitude or S11 is below -10db or more (-20db, -30db etc) is your bandwidth... as because where S21 has its positive peak on the same frequency the S11 will have its negative peak...!
hope that answers your question