Antenna efficiency is the share of the power fed to an antenna that it radiates as radio waves, rather than losing as heat or reflecting back to the radio. Antenna efficiency is quoted as a percentage or in dB, where 100% or 0 dB would be a perfect antenna.

Antenna efficiency converts to dB on a logarithmic scale: 50% efficiency is -3 dB and 25% is -6 dB. The losses come from resistance in the antenna's metal, energy absorbed by the materials around it, and mismatch with the cable and radio. Radiation efficiency counts only the losses inside the antenna. Total efficiency also includes the mismatch measured by VSWR or return loss, and is the more useful figure.

Antenna efficiency and antenna gain are linked: gain is efficiency multiplied by directivity, which is how strongly the antenna's shape focuses energy. Two antennas with the same shape but different efficiency have different gain, and a low-efficiency antenna cannot make up the shortfall by focusing its beam.

Antenna efficiency matters most on small antennas built into IoT devices. An embedded antenna much smaller than the wavelength it works at loses efficiency, and the effect is strongest on the low cellular bands below 1 GHz, where wavelengths are longest. The device's casing, battery, circuit board and mounting surface also absorb energy, so an embedded antenna's efficiency must be measured in the finished product, not only on the antenna maker's test board.

For a cellular router with external antennas, the datasheet gain figure already reflects efficiency, so gain is the figure to compare. For a device with a built-in antenna, ask for total efficiency across each band the device will use.