What is the difference between the units dBm, dBi and dBd?
dBm is absolute power: 0 dBm = 1 mW, 10 dBm = 10 mW, 14 dBm = 25 mW, 20 dBm = 100 mW, 27 dBm = 500 mW. dBi and dBd are antenna gain: dBi is referenced to an isotropic antenna, dBd to a half-wave dipole. Since the dipole itself has 2.15 dBi of gain, the conversion is dBi = dBd + 2.15.
The source of the confusion is that all three start with "dB" but measure different things.
- dBm is a power level. Its reference is 1 mW. Every 3 dB doubles the power, every 10 dB multiplies it by ten. Transmitter output power, received signal level and receiver sensitivity are all expressed in dBm.
- dBi is the gain of an antenna relative to the theoretical isotropic antenna that radiates equally in all directions. It is a reference that does not physically exist but is mathematically clean; the regulatory and simulation world uses it.
- dBd is the gain of an antenna relative to a real half-wave dipole. Antenna manufacturers sometimes prefer this in marketing, because the number looks 2.15 smaller.
If an antenna listing says "5 dB gain" without stating the unit, it means there is a 2.15 dB uncertainty between the two possibilities. If you recall that 6 dB doubles the range, that uncertainty corresponds to roughly 30% in range; so the unit is not a detail to be skipped.
When calculating, one rule is enough: convert everything to dBi, then add it up in the link budget.
Full article: Wireless remote control installation: pairing, relay, safety