what's to know? x band is 8 to 12 ghz, you need x band to target track.
Yes AESA radar is capable of frequency agility, but the range of frequencies and power is dictated by number of T/R modules, power and size of the antenna. Remember I said size is important, the small antenna on the PL 15 plus the power and compute resource in a small frame is the limiting factor. This is why the S 400 is far more lethal than the R-77, larger antenna means more real estate for a larger electronic brian.
In most cases jamming mid course updates is usually more than enough to defeat the missile. In all this you have to remember that missile launch (plume) and missile exhaust can be tracked by both radar and IR sensor, the greater this distance the easier it is to defeat the missile.