Radiation patterns illustrating the definition of effective isotropic radiated power (EIRP) of a radio transmitter. The axes represent radiated power density in watts per meter at some given distance from the antennas.  It shows a typical radiation pattern of a directive (high gain) antenna Ra{\displaystyle R_{\text{a}}} and an ideal isotropic antenna| Riso{\displaystyle R_{\text{iso}}}  which has the same power flux density  S{\displaystyle S} as the directive antenna in the direction of its maximum signal strength (main lobe).  The transmitted power that would have to be applied to the isotropic antenna to radiate the same signal strength is the effective isotropic radiated power (EIRP).
Own work
Chetvorno