Antenna efficiency, η, is a term associated with aperture antennas, e.g., parabolic dishes. Antenna efficiency is different from and contrasted with radiation efficiency, which applies to any antenna type.
Antenna efficiency Wikipedia
Antenna efficiency is defined as the ratio of the aperture effective area, Ae to its actual physical area, A. It describes the percentage of the physical aperture area which actually captures radio frequency (RF) energy. Thus, the effective area of an aperture antenna is the surface area of a theoretically perfect aperture that would collect the same energy as the actual aperture with associated antenna efficiency.
Effective area is an important concept in the study of antennas, because the ratio of the gain of an antenna to its effective area can be shown to be a universal constant.
IEEE defines several other antenna parameters which include the word efficiency, such asaperture illumination efficiency for aperture-antennas.
polarization efficiency; polarization mismatch factor.
These are closely related and easily misunderstood definitions. Care must be taken in engineering applications to ensure that the efficiency specifications are specific, clear, and unambiguous.