Antenna gain
= Antenna gain
{title2=$G(\widehat{\mathbf r})=4\pi r^2S_r/P_{\rm in}$}
{wiki}
<Antenna gain> compares the far-field <energy flux> in a specified direction with the flux that an isotropic radiator would produce using the same input <power>. Thus $S_r=GP_{\rm in}/(4\pi r^2)$. Gain includes radiation efficiency; it must be distinguished from a voltage or current <amplifier> gain and from directivity when losses matter.