dBd
dBd expresses antenna gain relative to a half-wave dipole reference. Since the dipole itself has 2.15 dBi of gain relative to an isotropic antenna, the conversion is fixed: dBi = dBd + 2.15. If the same antenna is listed as 3 dBd in a catalogue, that means 5.15 dBi; a gain figure given without stating the unit is not enough for comparison.
The joint use of two references is a practical source of confusion, because dBi always looks 2.15 units larger than dBd. For the same antenna, the value given in dBi looks more impressive; that is why the unit must be verified when making comparisons.
- dBd is common in the radio-communications and professional radio world.
- dBi is standard in short range device and Wi-Fi catalogues.
- On the regulatory side the distinction matters even more: ERP is dipole-referenced, EIRP is isotropic-referenced, and the difference between them is again 2.15 dB.
In the field this distinction is useful when you are thinking about replacing one antenna with another. If you compare two catalogues in different units, you will see a 2.15 dB difference that does not actually exist. Since 2.15 dB alone can change range by roughly 30%, this is not a small mistake.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety