The concept of antenna gain is pretty important for calculation of link budget and is something I've had contact with. I know that it tells me how focused the waves emitted by the antenna are and I know how to read antenna gain charts.
What I don't know is how to calculate antenna gain for a given antenna.
So what are the general steps required in order to calculate antenna gain?
If it matters, I'd like to calculate gain of a J-pole antenna, but I'd like to keep this question general, if possible.
UPDATE: Just to be clear, what I really want to know is how to actually do the calculations, not just how to use already calculated results. I do expect the "perfect" answer to be extremely long and complicated. Because of that, I'm willing to accept answers which list the necessary steps and provide information in which direction I should do research next for the required steps.