The Difference Between dB, dBm, dBc, dBi, dBd

- Mar 13, 2020-

The Difference Between dB / dBm / dBc / dBi / dBd


dBm is a unit for assessing absolute power. Its calculation formula is: 10log (P power value / 1mW). In the communication industry, we often encounter similar units such as dB, dBm, dBc, dBi, dBd, etc. Let's compare them together :

The Difference Between dB, dBm, dBc, dBi, dBd


dB is a value that represents the relative value. When considering how much the power of A is greater or less than the power of B, this relative power unit dB is generally used.

The calculation method is as follows: 10log (A power / B power)

Example: The power of A is twice the power of B. The calculation is 10lg (A power / B power) = 10lg2 = 3dB. That is, the power of A is 3 dB greater than the power of B.

We often say that the transmission line loss and cable loss are also expressed in dB, that is, the output power of the terminal is compared with the input power of the source and then taken as the logarithmic value. Since dB is a relative unit, dB is also used to compare the relative performance on the antenna. For example, the gain of antenna A is 3dB greater than that of antenna B, which means that A (dBi) -B (dBi) = 3dB, and dBd is also understood.

2.dBi and dBd

dBi and dBd are generally used as relative power units to measure the radiation performance of the antenna. dBi and dBd are the values of the gain (power gain). Both are relative values, but the reference standards are different. The reference reference of dBi is an omnidirectional antenna, while the reference reference of dBd is a dipole antenna, so the two are slightly different.

Generally speaking, for the same gain, the result expressed in dBi is 2.15 larger than that expressed in dBd.

Example: For an antenna with a gain of 3dBd, when the gain is converted into dBi, it is 5.15dBi (the decimal point is 18dBi). Similarly, 0dBd = 2.15dBi.



dBm is a unit for assessing absolute power, and its calculation formula is: 10log (P power value / 1mW), where P unit is mW.

For example, the transmit power P = 1mW, the converted dBm is 0dBm.

For another example, for the power of 2W, the value obtained after conversion in dBm units should be:

10log (2W / 1mw) = 10log (2000) = 30 + 10log2 = 33dBm.


4, dBc

dBc is also a unit representing 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, and is used to measure the relative ratio to the carrier power, such as measuring interference (such as co-frequency interference, out-of-band interference, intermodulation interference, and intermodulation interference, etc. ) And the relative magnitude of coupling, spurs, etc. Where dBc is used, it can also be replaced by dB in principle, dB is more general, and dBc is more professional.

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