What is the difference between gain and directivity?

• A. Directivity is how focused the antenna radiates, while gain is the relative signal strength including efficiency.
• B. Directivity includes efficiency; Gain is solely radiation pattern.
• C. Gain and directivity are identical.
• D. Directivity measures impedance match; Gain measures beam width.

Answer: (C)

The difference hinges on what each metric accounts for. Directivity describes how concentrated the antenna’s radiated power is in a particular direction compared with an isotropic source, and it depends only on the antenna’s geometry and excitation—essentially the best-case focusing without considering losses. Gain, on the other hand, includes losses in the system (ohmic, dielectric, mismatches). It tells you how much of the input power is actually radiated in a direction, not just how the pattern concentrates energy. The relationship is G = ηD, where η is the radiation efficiency (0 to 1). So directivity can be high even if the antenna isn’t very efficient, and gain will be less than directivity when efficiency is below 100%. They are identical only if the antenna is lossless (η = 1). Impedance matching and beam width are separate concepts and aren’t what directivity measures.