I was doing an experiment for finding the radiation pattern of Pyramidal Horn Antennas at RF frequencies(as part of my course curriculum). There I came across the Friis Transmission Equation
What I don't understand here is the two terms Gt and Gr. What does these gain terms refer to? Gain is something associated with active devices like amplifiers. But how come antennas have gain? They don't have any source of energy so that they can amplify the power, they are transmitting or receiving.