The dBm sometimes called dBmW or Decibel-miliwatts is an abbreviation for the power ratio in decibels (dB) the measured power referenced to one milliwatt (mW) ( see Conversion Table below ). It is used in RF and fiber optic, as a convenient measure of absolute power because of its ability to express very large or very small values in a short form. By comparison, the decibels ( dB ) is a dimensionless unit , used to quantify the relationship between two values, such as the ratio Signal to Noise. Audio and telephony, dBm is typically referenced relative to an impedance of 600 ohm, while in dBm RF is usually referenced to an impedance of relatively 50 ohms.
What is the difference between “dB”, “dBi” and “dBm”
The DB, dBi e dBm are not synonymous, they mean things quite different.
What is dB
O dB ( decibels ) is a relative measure two different power levels, is used to declare the gain or loss of a device relative to another device. In other words, dB is a logarithmic unit used to express the ratio of two values of a physical quantity, often a power or intensity. This is the basis of dBx.
What is dBi
O dBi (dB isotrópico) – Shows the gain of an antenna relative to an isotropic antenna hypothetical , that evenly distributes energy in all directions (flat land).
What is dBm
O dBm (dB mW) is power in relation to 1 milliwatt widely used in RF, audio and telephony ( and we are seeing this Article).
Examples of the Extremes dBm
-120,0 dBm The signal strength received at the ground from a satélite GPS + 80 dBm The power 100 kW broadcast of a commercial radio
A convenient form, no dimension is to measure the power:
The exact formula : P (dBm) = 10 log10 · (P (In) / 1 mW)
P (dBm) = Power expressed in dBm
P (In) = Absolute power measured in Watts
mW = miliWatts
log10 = log base 10
From this formula, power in dBm 1 Watt é de 30 dBm. As the calculation is logarithmic, each increase of 3dBm is roughly equivalent to twice the actual power of a signal.
In the table below:
Conversion Table dBm Watts x (50 Ohms) x Volts (RMS)