Antenna gain measures how much stronger the signal is in an antenna's best direction than it would be from a reference antenna fed the same power. It is a redistribution, not an amplifier: whatever is gained in one direction is taken from others.
Where gain comes from
Same total power, different shape: more gain means a narrower beam. Idealised single-lobe pattern, field-strength scale.
Beamwidth64°
Power ratio10×
A lossless antenna radiates all the power you feed it, whatever its shape. An isotropic radiator spreads that power over a whole sphere. An antenna that squeezes the same power into a narrower beam must have a stronger signal inside the beam, and weaker signal outside it. Narrower beam, more gain.
Two words are worth separating:
Directivity describes only the shape of the pattern: how concentrated the beam is.
Gain is directivity minus the antenna's own losses. A lossless antenna has gain equal to its directivity. A lossy one has less; see Bandwidth and efficiency.
RememberGain does not add power. It points the power you already have.
dBi and dBd
Gain is quoted in decibels against one of two references. dBi is relative to an isotropic radiator. dBd is relative to a half-wave dipole. A dipole is already 2.15 dB better than isotropic, so the same antenna has a number 2.15 larger in dBi.
A half-wave dipole is the dBd zero point, and it already has 2.15 dBi against an isotropic radiator.
Same gain over isotropic8.15 dBi
Antenna
dBd
dBi
Isotropic radiator (hypothetical)
−2.15
0
Half-wave dipole
0
2.15
Antenna with 6 dBi
3.85
6
Antenna with 10 dBd
10
12.15
Always check which reference a gain figure uses. The same antenna reads 2.15 higher in dBi than in dBd, so two gain figures can only be compared when they share a reference.
In practice
Effective radiated power. Add the gain and subtract the losses, then convert once to a power ratio. Example: 100 W, a 10 dBi antenna and 2 dB of feed line loss leaves a net +8 dB, a ratio of 6.31, so about 631 W EIRP. ERP is measured against a dipole instead, so subtract 2.15 dB more: about 385 W ERP.
Gain is directional. The quoted figure is at the peak. Gain in the direction you do not need is wasted. A colinear vertical gains by flattening its pattern toward the horizon, which helps for local VHF contacts but sends less upward.
Gain works both ways. It helps you hear in the favoured direction as well as be heard, and it makes pointing matter: see Rotators and controllers and The Yagi-Uda beam.
Higher gain means more careful safety. The beam concentrates RF exposure along its axis; see RF exposure.