The unit of the RF signal. dB and dBm

By | December 30, 2016

When you are viewing RF-related documentation or logs, you often see a unit called dB and dBm. In fact, I did not use them differently until now. But I find that there are different meanings.

Received Signal Strength Indicator: -49dBm

The above shows the signal strength of Wi-Fi as part of the log when you grab a dumpstate from Android.
Let’s look at which case we use dB and in which case we use dBm.

[Source : pixabay.com]

dB

First of all, dB is a unit called a decibel and is not a unit for measurement. But simply a unit that is used as a mathematical concept. This dB is associated with log(used for math, not for computer).
[10 * log x]
10 * log 100 = 20 dB
10 * log 1000 = 30 dB
10 * log 10000 = 40 dB

 

It is displayed as above.
That is, it increases by 10 dB every time x increases by 10 times.
If the displayed dB is displayed as an RF result, it will be denoted as dBm.

 

dBm

Normally, RF uses a small power, usually in mW. The power that is expressed in dB is dBm.

1 mW = 0 dBm is the standard. (10 * log 1 = 0)
10 mW = 10 dBm (10 * log 10 = 10)
100 mW = 20 dBm (10 * log 100 = 20)
1000 mW = 30 dBm (10 * log 1000 = 30)

If so, the Wi-Fi signal strength extracted from Android above -49 dBm is how big it is in terms of mW.

-10 dBm = 1/10 mW
-20 dBm = 1/100 mW
-30 dBm = 1/1000 mW
-40 dBm = 1/10000 mW
-50 dBm = 1/100000 mW

Although it may be a little different due to the log scale, it is about -50 dBm, so it can be regarded as a very small power of 1 / 100000mW.

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