The ratio of the signal size generated by the actual antenna and an undirected ideal radiation point source at the same point in space, under the condition of equal input power, is more precisely the ratio of power density. It quantitatively describes the degree to which an antenna concentrates input power for radiation. Obviously, there is a close relationship between gain and antenna directionality, and the narrower the main lobe of the pattern, the higher the gain.

　　The gain of the half wave symmetric oscillator is G=2.15 dBi;

　　Four half wave symmetric oscillators are arranged vertically up and down, forming a vertical quaternion array with a gain of approximately G=8.15 dBi.

　　The unit dBi represents the ideal point source of isotropic and uniform radiation as the comparison object.

　　If a half wave symmetric oscillator is used as a comparison object, then the gain of the half wave symmetric oscillator is G=0 dBd; (Because it is a comparison between oneself and oneself, the ratio is 1, taking the logarithm yields zero.)

　　The vertical quaternion array has a gain of approximately G=8.15-2.15=6 dBd.

　　Note that the current unit is dBd.

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