dBi
dBi is the unit expressing an antenna's gain relative to a theoretical isotropic antenna that radiates equally in all directions. It is used as standard in antenna catalogues: a half-wave dipole is 2.15 dBi, a typical short whip antenna is 0–2 dBi, and a directional panel antenna is on the order of 8–15 dBi. The value describes not extra power produced but the direction in which the energy is concentrated.
An isotropic antenna cannot be physically built; it is a mathematical reference. Real antennas radiate energy more than this reference in some directions and less in others. The dBi value written in a catalogue is the gain in the antenna's strongest direction.
- Gain does not come for free: the dB gained in one direction is lost in others. A 10 dBi antenna produces a narrow beam.
- A directional antenna is not always good: if the remote control moves with the operator, a wide-angle, low gain antenna is more reliable.
- Do not confuse it with dBd: dBi = dBd + 2.15.
In AXI's ST and SF sets the receiver is supplied with an external antenna and cable gland; the critical decision here is not the gain figure but taking the antenna out of the metal enclosure into a free position. A high gain antenna left inside the enclosure gives a far worse result than an ordinary antenna taken outside.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety