I always see db, dbi and dbm in documentation about wireless but I've never seen, what those are doing exactly.
I would understand, What does mean db, dbi and dbm?
Decibels – If a receiver is very sensitive to RF signals, it may be able to pick up signals as small as 0.000000001 Watts.
• A logarithm is the exponent to which the number 10 must be raised to reach some given value. If we are given the number 1000 and asked to find the logarithm (log), we find that log 1000 = 3 because 10^3 = 1000. Notice that our logarithm, 3, is the exponent. An important thing to note about logarithms is that the logarithm of a negative number or of zero does not exist.
• Log (-100) = undefined!
• Log (0) = undefined!
• Decibels are a relative measurement unit unlike the absolute measurement of milliwatts.
• 1 mW = 0 dBm
• The m in dBm refers simply to the fact that the reference is 1 milliwatt (1 mW) and therefore a dBm measurement is a measurement of absolute power.
• The relationship between the decibels scale and the watt scale can be estimated using the following rules of thumb:
• +3 dB will double the watt value:
• (10 mW + 3dB ≈ 20 mW)
• Likewise, -3 dB will halve the watt value:
• (100 mW - 3dB ≈ 50 mW)
• +10 dB will increase the watt value by ten-fold:
• (10 mW + 10dB ≈ 100 mW)
• Conversely, -10 dB will decrease the watt value to one tenth of that value:
• (300 mW - 10dB ≈ 30 mW)
• The unit of measurement dBi refers only to the gain of an antenna. The “i” stands for “isotropic”, which means that the change in power is referenced against an isotropic radiator.
• An isotropic radiator is a theoretical ideal transmitter that produces useful electromagnetic field output in all directions with equal intensity, and at 100-percent efficiency, in three-dimensional space. One example of an isotropic radiator is the sun.