In the wireless communication industry, the issues of dB, dBi, dBd, dBc, dBm, and dBw are often involved, which are defined as follows:
1.dB
dB is a value that characterizes the relative value, a pure ratio, which only represents the relative magnitude relationship between two quantities, and has no unit. When considering how many dBs the power of A is greater or smaller than the power of B, the following calculation formula is used: 10log (A power/B power), if the voltage ratio of the two is used for calculation, 20log (A voltage/B voltage) is used.
[Example] The power of A is twice as large as the power of B, then 10lg (power A/power B)=10lg2=3dB. In other words, A's power is 3 dB greater than B's power. Conversely, if A’s power is half of B’s power, then A’s power is 3 dB lower than B’s power.
2. dBi and dBd
dBi and dBd are the quantities that represent the power gain of the antenna. Both are relative values, but the reference standards are different. The reference datum of dBi is an omnidirectional antenna, and the reference datum of dBd is a dipole, so the two are slightly different. It is generally believed that expressing the same gain, expressed in dBi, is 2.15 larger than expressed in dBd.
[Example] For an antenna with a gain of 16dBd on one side, when the gain is converted into dBi, it is 18.15dBi (generally ignoring decimal places, it is 18dBi).
[Example] 0dBd=2.15dBi.
3.dBc
dBc is also a unit that represents the relative value of power, which is exactly the same as the calculation method of dB. Generally speaking, dBc is relative to the carrier power. In many cases, it is used to measure the relative value of the carrier power. For example, it is used to measure interference (co-channel interference, intermodulation interference, intermodulation interference, band interference). External interference, etc.) and the relative magnitude of coupling, spurs, etc. Where dBc is used, in principle, dB can also be used instead.
4. dBm
dBm is a value representing the absolute value of power (it can also be considered a ratio based on 1mW power), and the calculation formula is: 10log (power value/1mw).
[Example] If the power P is 1mw, it is 0dBm after being converted into dBm.
[Example] For the power of 40W, the converted value in dBm unit should be:
10log(40W/1mw)=10log(40000)=10log4+10log10000=46dBm.
5. dBw
Like dBm, dBw is a unit that represents the absolute value of power (it can also be considered a ratio based on 1W power), and the calculation formula is: 10log (power value/1w). The conversion relationship between dBw and dBm is: 0 dBw = 10log1 W = 10log1000 mw = 30 dBm.
[Example] If the power P is 1w, it will be converted to dBw and then it will be 0dBw.
In short, dB, dBi, dBd, and dBc are the ratios between the two quantities, representing the relative magnitude between the two quantities, while dBm and dBw are the values representing the absolute magnitude of power. In the calculation of dB, dBm, and dBw, pay attention to the basic concepts. When one dBm (or dBw) is subtracted from another dBm (dBw), the result is dB, such as: 30dBm-0dBm = 30dB.
Generally speaking, in engineering, there is only addition and subtraction between dBm (or dBw) and dBm (or dBw), not multiplication and division. The most commonly used is subtraction: dBm minus dBm is actually the division of two powers, and the division of signal power and noise power is the signal-to-noise ratio (SNR). dBm plus dBm is actually the multiplication of two powers.
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