Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
Gain in dB scale of the transmitting antenna is:
$$G=10\cdot\log \frac{P_o}{P_i}$$
The input power will always be larger than the output power. However, I see a lot of radiation pattern graphs with positive gains in dB. How is that possible? Shouldn't the gain value ALWAYS be negative as the power ratio P_o\P_i will always be less than zero?
I suggest that you read the Wikipedia article about Antenna gain
Antenna gain is defined as the power radiated in a certain direction (so for a directional antenna, you would obviously choose the direction to which most power is radiated) relative to the power if the antenna was omnidirectional. Meaning: it radiates an equal amount of power in all directions.
So a high gain antenna will "focus" all power in one (or more) directions and that is defined as "gain".
