Table 3. Efficiency and Loss (dB)
As shown in the table above, antenna efficiency in percentage can be converted to a dB scale by a 10 log (%) function.
The efficiency of a typical embedded antenna can range from about 40 to 75%. Greater than 75% efficiency is challenging to obtain from a fully embedded antenna and lower than 40% efficiency will typically cause certification failures. In most cases, the efficiency goal should be 60%, with 40% as an absolute minimum. It is important to understand that good return-loss performance can be inadvertently achieved at the expense of efficiency. An extreme example of this concept is a 50 ohm resistor: the resistor has an excellent return loss but has virtually 0% efficiency, and is obviously not an antenna. Therefore, an understanding of the return loss and efficiency concepts is critical to good antenna design.
Antenna Fundamentals – Technical Brief