1, dBm

dBm is an absolute value of the reference power, and the calculation formula is: 10 LGP (power value / 1mw).
[example 1] if the transmit power P is 1mw, it will be 0dbm after converted to dBm.
[example 2] for 40W power, the converted value in dBm unit shall be:
10lg（40W/1mw)=10lg（40000）=10lg4+10lg10+10lg1000=46dBm。

2. dBi and dBd

dBi and dBd are the values of the eigengain (power gain), both of which are relative values,
But the reference datum is not the same. The reference of dBi is omnidirectional antenna, and the reference of dBd is dipole,
So the two are slightly different. It is generally believed that the same gain is represented by dBi rather than dBd
It's big 2.15.
[example 3] for an antenna with a gain of 16dBd, when the gain is converted to dBi, it is 18.15dBi
(decimal places are generally ignored, which is 18dBi).
[example 4] 0dbd = 2.15dBi.
[example 5] the gain of GSM900 antenna can be 13dbd (15dBi), and the gain of GSM1800 antenna can be
15dBd（17dBi)。

3, dB

dB is a value representing the relative value. When considering how much dB the power of a is larger or smaller than that of B,
According to the following formula: 10lg (a power / B power)
[example 6] if the power of a is twice that of B, then 10lg (a power / B power) = 10lg2 = 3dB.
That is to say, the power of a is 3 dB higher than that of B.
[Example 7] the 100m transmission loss of 7 / 8 inch GSM900 feeder is about 3.9dB.
[example 8] if the power of a is 46dbm and the power of B is 40dbm, then it can be said that a is 6 dB larger than B.
[Example 9] if antenna a is 12dBd and antenna B is 14dBd, it can be said that antenna a is 2 dB smaller than antenna B.

4, dBc

Sometimes you will see dBc, which is also a unit representing the relative value of power, exactly the same as the calculation method of dB.
Generally speaking, dBc is relative to carrier power. In many cases, dBc is used to measure and
Relative value of carrier power, such as to measure interference (co frequency interference, intermodulation interference, intermodulation interference, out of band interference, etc.)
And the relative values of coupling, spurious, etc.
Where dBc is used, dB can also be used instead in principle.
The dB on the upper floor is just a ratio, not a unit of power gain!!!