Which term describes the ratio of radiation intensity in a given direction to the intensity from an isotropic radiator?

• A. Directivity
• B. Antenna efficiency
• C. Power gain
• D. ABSOLUTE GAIN (GABS)

Answer: (D)

The main idea is directivity. Directivity measures how concentrated an antenna’s radiation is in a specific direction compared to an isotropic radiator that would spread the same total power uniformly in all directions. It is defined as the ratio of the radiation intensity in a given direction, U(θ, φ), to the radiation intensity from an isotropic source with the same total radiated power, U_iso. Since U_iso equals the total radiated power divided by 4π, directivity tells you how much stronger (or weaker) the beam is in that direction relative to the uniform baseline.

Antena efficiency is about how effectively input power is converted to radiated power, not about directional concentration. Power gain combines directivity with efficiency (G = ηD), so it changes if efficiency isn’t perfect. Absolute gain isn’t a standard term for this specific ratio. So the ratio described in the question is best captured by directivity.