TopTechnical DictionarydBm - logarithmic power unit

dBm - logarithmic power unit

dBm – absolute level relative to 1 mW (milliwatt); according to the rules we should use dBmW, however, dBm unit was adopted.

 

The unit is commonly used in telecommunications (e.g. access point output power) or used in designing WLAN networks. Expressed in dB, it indicates, how much the power is higher or lower than 1 mW. 0 dB corresponds to 1 mW (milliwatt).

 

A commonly used power equation can be used to convert mW into dBm.

 

P(ref) – reference value, in this case 1 mW (milliwatt)

 

To simplify, the power equation (dBm) is:

 

Example
Convert 10 mW into dBm:

 

The following equation can be used to convert dBm to mW:

 

Example
Convert 20 dBm into mW (milliwatts).

 

Table 1. Converting dBm into mW - wide range 

-30 dBm 0,001 mW
-20 dBm 0,01 mW
-10 dBm 0,1 mW
0 dBm 1 mW
10 dBm 10 mW
20 dBm 100 mW
30 dBm 1000 mW
40 dBm 10000 mW
50 dBm 100000 mW
60 dBm 1000000 mW
70 dBm 10000000 mW
80 dBm 100000000 mW

Table 2. Converting dBm into mW - detailed range 

14 dBm 25,119 mW
16 dBm 39,811 mW
18 dBm 63,096 mW
20 dBm 100,000 mW
22 dBm 158,489 mW
24 dBm 251,189 mW
26 dBm 398,107 mW
28 dBm 630,957 mW
30 dBm 1000,000 mW

Power can be easily converted into voltage: dBm into dBµV and dBmV. However, the internal impedance must be known to convert power to voltage. DVB-T, cable or industrial television systems use 75 Ohm impedance. Radio systems use 50 Ohm impedance. 

Equations for those two environments are given below: 

dBmV = 46,9897 + dBm (@ 50 ohm)
dBmV = 48,7506 + dBm (@ 75 ohm) 

Example
Converting 15 dBm into dBmV (for 50 Ohm impedance) 

dBmV = 46,9897 + 15 = 61,9897 dBmV 

dBµV = 60 + dBmV 

dBµV = 106,98 + dBm (@ 50 ohm)
dBµV = 108,75 + dBm (@ 75 ohm) 

Example
Converting 20 dBm into dBµV (for 50 Ohm impedance) 

dBµV = 106,98 + 20 = 126,98 dBµV 


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