dBi and dBd are used to measure the focusing power (gain) of antennas. Digi specifies all antennas in dBi only.
dBi is a measurement that compares the gain of an antenna with respect to an isotropic radiator (a theoretical antenna that disperses incoming energy evenly over the surface of an imaginary sphere.)
dBd compares the gain of an antenna to the gain of a reference dipole antenna (defined as 2.15 dBi gain).
To convert dBi to dBd:
Gain in dBd = gain in dBi - 2.15 dB
Gain in dBi = gain in dBd + 2.15 dB
Also note that antennas with higher gain focus the energy over a smaller area. Digi recommends avoiding the highest gain antennas in most applications because they are more difficult to work with.
Recommended antenna gains:
Omnidirectional: 3 dBi to 6 dBi
Directional: 8 dBi to 11 dBi
Note: High gain antennas are of limited use in extending the range in a non-Line-of-Sight (non-LOS) environment. In a non-LOS environment the obstructions contribute more losses to the system than the antennas are able to overcome. Also, the obstructions cause the signals to bounce and arrive at the antenna from different angles so it is desirable to have an antenna with a wide beam width and a lower gain.