What is the ratio of the radiation intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna was radiated isotropically (in all directions)?

• A. Directivity
• B. Radiation pattern
• C. Antenna efficiency
• D. ABSOLUTE GAIN (GABS)

Answer: (D)

The main idea is comparing how strongly the antenna radiates in a specific direction to how strongly an ideal isotropic radiator would radiate if it were fed with the same input power. The radiation intensity in a direction is the power per unit solid angle there. An isotropic source fed with the same input power would emit P_in/4π in every direction. So the ratio U(θ,φ) / (P_in/4π) defines the directional gain for that direction. This quantity is called the absolute gain (Gabs). It measures how much more (or less) intensity you get in that direction compared to the isotropic case with the same input power, and it naturally includes the effect of radiation efficiency when the antenna isn’t lossless.

Directivity, by contrast, uses the average radiation intensity of the actual antenna, U_avg = P_rad/4π, as the baseline, so it would be U(θ,φ) / (P_rad/4π); this equals gain only if losses are negligible. Antenna efficiency is just the ratio P_rad/P_in, not a directional ratio. The radiation pattern is the full directional distribution itself, not the ratio.