Raspberry Communication
Frequencies
Intensity and Decibel scale (dbm)
dB vs. dBm
Decibel (dB) and dB relative to a milliwatt (dBm) represent two different but related concepts.
A dB is a shorthand way to express the ratio of two values. As a unit for the strength of a signal, dB expresses the ratio between two power levels. To be exact, dB = log (P1/P2).
Using the decibel allows us to contrast greatly differing power levels (a common predicament in radio link design) with a simple two- or three-digit number instead of a more burdensome nine- or 10-digit one.
For instance, instead of characterizing the difference in two power levels as 1,000,000,000 to 1, it's much simpler to use the decibel representation as 10*log (1,000,000,000/1), or 90 dB. The same goes for very small numbers: The ratio of 0.000000001 to 1 can be characterized as -90 dB. This makes keeping track of signal levels much simpler.
The unit dBm denotes an absolute power level measured in decibels and referenced to 1 milliwatt (mW). To convert from absolute power "P" (in watts) to dBm, use the formula dBm = 10*log (P/1 mW). This equation looks almost the same as that for the dB. However, now the power level "P" has been referenced to 1 mW. It turns out that in the practical radio world, 1 mW is a convenient reference point from which to measure power.
Use dB when expressing the ratio between two power values. Use dBm when expressing an absolute value of power.
reference https://www.isa.org/standards-publications/isa-publications/intech-magazine/2002/november/db-vs-dbm/
dbm
RF power is most commonly expressed and measured in decibels with a milliwatt reference, or dBm. A decibel is a logarithmic unit that is a ratio of the power of the system to some reference. A decibel value of 0 is equivalent to a ratio of 1. Decibel-milliwatt is the output power in decibels referenced to 1 mW.
Since dBm is based on a logarithmic scale, it is an absolute power measurement. For every increase of 3 dBm there is roughly twice the output power, and every increase of 10 dBm represents a tenfold increase in power. 10 dBm (10 mW) is 10 times more powerful than 0 dBm (1 mW), and 20 dBm (100 mW) is 10 times more powerful than 10 dBm.
You can convert between mW and dBm using the following formulas:
P(dBm) = 10 ยท log10(P(mW))
Range
Signal Strength Required for -30 dBm Max achievable signal strength. The client can only be a few feet from the AP to achieve this. Not typical or desirable in the real world. N/A -67 dBm Minimum signal strength for applications that require very reliable, timely packet delivery. VoIP/VoWiFi, streaming video -70 dBm Minimum signal strength for reliable packet delivery. Email, web -80 dBm Minimum signal strength for basic connectivity. Packet delivery may be unreliable. N/A -90 dBm Approaching or drowning in the noise floor. Any functionality is highly unlikely. N/A
source https://support.randomsolutions.nl/827069-Best-dBm-Values-for-Wifi
Video
Watch this great video from KhanAcademy
watch the full video