Gain refers to the ratio of the power density of the signal generated by the actual antenna and the ideal radiating unit at the same point in space under the condition of equal input power. It quantitatively describes the degree to which an antenna concentrates the input power and radiates it. The gain obviously has a close relationship with the antenna pattern. The narrower the main lobe of the pattern and the smaller the side lobe, the higher the gain.
The physical meaning of gain can be understood in this way ------ In order to generate a signal of a certain size at a certain distance and a certain point, if an ideal non-directional point source is used as the transmitting antenna, 100W is required When using a directional antenna with a gain of G = 13 dB = 20 as the transmitting antenna, the input power is only 100/20 = 5W . In other words, the gain of a certain antenna, in terms of the radiation effect in the direction of its maximum radiation, is the multiple by which the input power is amplified compared with an ideal point source without directivity.
The gain of the half-wave symmetric oscillator is G=2.15dBi. The 4 half-wave symmetrical oscillators are arranged up and down along the vertical line to form a vertical four-element array with a gain of approximately G=8.15dBi( dBi This unit indicates that the comparison object is an ideal point source with uniform radiation in all directions).
If a half-wave symmetrical oscillator is used as the comparison object, the unit of its gain is dBd.
The gain of the half-wave symmetric oscillator is G=0dBd (because it is compared with itself, the ratio is 1, and the logarithm is zero.) The gain of the vertical quaternion array is about G=8.15-2.15=6dBd.