What are “dBi” and “dB” ?

Imagine an antenna that radiates energy equally in all directions, much like our sun does. In scientific lingo, this is said to be an “isotropic radiator”, because it has no preference for radiation in any direction … in other words it has no “directivity”. This type of isotropic antenna is said to have “no gain”. “No gain” can be expressed in linear terms like x1 (times 1). That simply means that all directions have the same energy radiation, and are all equal to the average energy radiation.

Antenna engineers like logarithmic terms, and we say this no-gain situation is 0 dBi (pronounced “zero dee bee eye”). Imagine a giant stellar sized mirror beside our sun. Imagine how it would change this energy distribution and give the sun directivity. With such an imaginary mirror, one half of our solar system would be dark (behind the mirror). The other half would be twice as bright (seeing the direct sun plus it’s reflection). Mirrors or lenses have the appearance of intensifying energy in some preferred directions by stealing and redirecting it from disadvantaged directions. Antennas do the same thing. Mirrors don’t create light, they only divert, direct, or concentrate it in some direction. Antennas don’t create radio energy, they also only divert, direct, or concentrate it in some direction. This is directional feature is called gain. Please remember, no new energy is created, it is simply redirected or given directionality (directivity). The amount of intensification in a preferred direction is quantified as gain. Thus a mirror can redirect half of the energy from the sun (or a candle), and make it look twice as bright (i.e. two candles). It is said to have a gain of 2x (times two) or doubling.

The Logarithmic dB Scale

Antenna engineers use a logarithmic scale to express this apparent 2x (times two) mirror power doubling as +3 dBi. It still means “doubling”. Here are some other examples of ratios or multipliers on the engineering dB log scale.