over the band of interest. Efficiency is simply a measure of what portion of the power supplied to the antenna, including any reflection loss, is actually radiated by the antenna.
The efficiency of small antennas that are tightly integrated into a small product can be affected substantially. Nearby grounded conductors and dielectric materials (like a typical plastic housing) will constrain and absorb the near-fields of the antenna and cause significant losses. 2 below illustrates the efficiency concept.
Figure 2. Efficiency Concept
Common plastic materials, like polycarbonate or ABS (Acrylonitrile Butadiene Styrene), can become significant absorbers of RF energy when very close to an antenna. Energy loss will occur during both receive and transmit operation. Efficiency is typically expressed as a percentage, but it is helpful to translate efficiency into the same logarithmic scale used in other antenna performance parameters, as shown in 3.
Antenna Fundamentals – Technical Brief