Also known as: antenna gain
Antenna gain measures how strongly an antenna concentrates energy in a preferred direction compared with a reference. It is given in decibels: dBi relative to an isotropic radiator, or dBd relative to a dipole.1
How it works
Gain does not create energy; it focuses it. A high-gain antenna trades coverage for reach — an omnidirectional vertical hears all directions, while a directional Yagi adds gain toward where it points at the expense of the sides.
Relevance to SDR
For general scanning, an omnidirectional antenna is usually best; a directional, high-gain antenna helps pull in one specific distant system.
Sources
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Antenna gain — Wikipedia, for the definition of gain and the dBi/dBd reference units. ↩