An antenna cannot create energy, only concentrate it — just as a torch with a reflector shines much further than a bare bulb of the same power. That gain in one direction is called gain.
In practiceA SATCOM dish concentrates all its power towards one satellite; a whip antenna on a handheld radio radiates in all directions, so you do not have to point it. Rifle-shaped drone jammers ("drone guns") use a directional antenna to transmit strongly at the drone with little power.
An antenna cannot amplify energy, but it can concentrate it. Gain (antenna gain, G) expresses how much more strongly an antenna radiates in its main direction compared with an imaginary antenna that spreads all energy evenly in all directions (an isotropic radiator). Gain is usually expressed in dBi (decibels relative to isotropic).
Gain, beamwidth and antenna size are inextricably linked:
- The larger the antenna (relative to the wavelength), the narrower the beam and the higher the gain.
- The higher the frequency (shorter wavelength), the narrower the beam for the same antenna size.
- High gain works both ways: the antenna transmits more concentrated energy and receives more sensitively from that direction (reciprocity).
Typical values: a dipole ~2 dBi, a ship's navigation radar ~30 dBi, a large fire-control dish or phased array 35–45 dBi. In the radar equation (Inverse square law) the gain counts squared (transmit and receive) — so antenna quality matters at least as much as transmit power.
Gain is the reason a radar with "only" 100 kW of peak power can have an effective radiated power (ERP) of hundreds of megawatts in its beam. For EW receivers this means that being in a radar's main beam is something entirely different from being in a side lobe (Main lobes, side lobes and back lobes).