https://www.antenna-theory.com/ is a good starter for apprentice-level RF voodoo.

30 dBm = 30 decibels wrt to 1 milliwatt. 10 decibels = 1 bel = 10-fold increase of the base quantity, so 30 decibels = 10 x 10 x 10 = 1000x increase, so 30 dBm = 1000x 1 mW = 1 W.