dBi
Understanding dBi
dBi provides a universal standard for comparing antennas. Since a perfect isotropic radiator is physically impossible but mathematically well-defined, it serves as a convenient reference. All antenna gains are positive in dBi because any real antenna focuses energy better than an isotropic radiator in at least one direction.
dBi vs dBd
Some antenna specifications use dBd (decibels relative to a half-wave dipole). Since a dipole has 2.15 dBi gain: dBi = dBd + 2.15. Always check which reference is used when comparing antennas.
Common dBi Values
| Antenna | Gain (dBi) | Usage |
|---|---|---|
| Isotropic | 0 | Reference only |
| Half-wave dipole | 2.15 | Reference antenna |
| Quarter-wave whip | 5.15 | Mobile, handheld |
| Yagi (10 el) | 11-14 | Amateur radio |
| Standard gain horn | 15-25 | Calibration |
| 1.2m dish at 12 GHz | 38 | VSAT terminal |
| 15m dish at 6 GHz | 52 | Earth station |
G_linear = 10^(dBi/10)
dBi = dBd + 2.15
EIRP = P_tx (dBm) + G (dBi) - L_cable (dB)
Frequently Asked Questions
What does dBi mean?
dBi measures antenna gain relative to an isotropic radiator (a theoretical antenna that radiates equally in all directions). A 10 dBi antenna concentrates energy 10 times more effectively than an isotropic antenna in its main beam direction.
What is the difference between dBi and dBd?
dBi references an isotropic radiator. dBd references a half-wave dipole. Since a dipole has 2.15 dBi gain: dBi = dBd + 2.15. A '10 dBd' antenna has 12.15 dBi gain. Always verify which reference is being used.
Can antenna gain in dBi be negative?
No. All real antennas have gain of at least 0 dBi (equal to isotropic) in their direction of maximum radiation. However, in some specific directions (nulls), the gain can be below 0 dBi, meaning less radiation than an isotropic antenna in that direction.