Gain refers to the input power equal conditions, the actual antenna radiating elements and the ideal of the premises the same point in space generated by the signal power density ratio.
It quantitatively describes an antenna input power to focus on the radiation level. Gain and antenna pattern is clearly closely related, the more narrow pattern main lobe, side lobe smaller, the higher the gain.
(1) dB, which is a purely relative value, that value of A then B, the logarithm. A frequently used to say how much higher or lower than B dB, but the individual that the number of A’s gain is dB, is unreasonable because we do not know what B is. Some people describe only to save the industry has become accustomed to, it defaults to the dbi
(2) dBd, which is a standard reference values, namely B defined as free-space half-wave dipole antenna, so the value of A and B have compared to a unified frame of reference, an antenna for the 10dBd, that is The half-wave dipole antenna than the antenna in the direction of the primary radiation can gather 10 times more energy.
(3) dBi, the meaning of this unit is relatively complicated point, but it is the most expresses the ratio of units of the physical environment, where B is the point source reference oscillator (this thing actually does not exist, can be seen as a relatively short wavelength of a short dipole, or call it micro-segmentation), the standard definition of the point source oscillator is ideal spherical omni-directional radiation, this time with dBd have a certain mathematical relationship, and that 1dBd = 2.15dBi.
However, in practice the vast majority of the antenna installation height will be affected by the impact of one of the most important thing is the ground effect, mirror effect because the ground is often the beam shape changes, often in some directions than 2-5dB. Here you should understand 19dBi the bar.
Many formal dBi antenna manufacturers often prefer to mark the antenna gain, they usually show marked mounting height or numerical calculation, or his production is that we usually know the installation environment, such as the roof antenna set, often omit the description.
(4) dbm, the decibel milliwatts. Is a unit of absolute power that is calculated as: 10lg power value / 1mW. For example: If the transmit power of 1mW, according to dBm units after the conversion value should be: 10 lg 1mW/1mW = 0dBm; for 40W of power, then 10 lg (40W/1mW) = 46dBm.
Half-wave symmetric oscillator gain G = 2.15dBi = 0dBd (because it is myself and their ratio, the ratio is 1, the logarithmic zero value) For example: for a gain of 16 dBd antenna gain is converted into units of its dBi when compared 18.15dBi, generally ignore the decimal places for 18dBi.
3DB is doubled for each additional power of MW, MW is the power is reduced by half 3DB In addition, each additional 10DB is to increase the rate of ten times the co-MW; 10DB is to reduce the power per MW divided by ten,
for example, if we There are several anti-push from 23DBM MW of energy, as much as possible into the DBM values 10 and 3 combinations: 23DBM = 0 +10 +10 +3, as 0DB = 1MW, each additional 10DBM is the power increased tenfold increase 3DBM = 200MW can be simply speculation.
Seen in the online discussion 0dBm +0 dBm =? In fact, a direct write is meaningless. Need to be converted to the original power do addition and subtraction.
Also need to consider the signal frequency, amplitude and phase. When two signals with amplitude, same frequency, same phase, can be equivalent to write:
1mW +1 mW (= is equivalent to) 0dBm +0 dBm = 0dBm +3 dB = 3dBm
Yagi antenna for short-range communications, low gain; usually seven to 15 dB.