June 6, 2022

dbm to watts conversion for radio, radar and microwaves

Decibels and dbm

Decibels are used to represent the ratio of two quantities of power: db = 10 log P₁/P₀ . In the case of dbm the reference power P₀ is 1 milliwatt : dbm = 10 log P₁/1mw.

Inverting this equation gives the power in terms of the reference power: P₁= P₀10^db/10. In the case of dbm the equation is P₁= 1mw·10^dbm/10.
Why? It is not unusual in a radio, radar, or microwave system to have 100,000 watts of transmit power, 1 microwatt of receive power, and various gains and losses from antennas, distance, and absorbing media. Adding and subtracting db is more convenient than multiplying, particularly if you are using a slide rule and trying to keep track of the decimal point.

For signals on a wire it is much easier to measure volts or amps with a voltmeter or current probe, but dbm can still be used. Since Watts are proportional to Amps squared or Volts squared, db (in power) = 20 log V₁/V₀ or db = 20 log I₁/I₀ . This is only useful in cases where the voltage or current is measured at the same impedance or resistance, which allows the ratio of two powers I²R or V²/R to cancel out the R.

dbm	milliwatts
-10	0.100
-9	0.126
-8	0.158
-7	0.200
-6	0.251
-5	0.316
-4	0.398
-3	0.501
-2	0.631
-1	0.794
0	1.000
1	1.259
2	1.585
3	1.995
4	2.512
5	3.162
6	3.981
7	5.012
8	6.310
9	7.943
10	10.00
11	12.59
12	15.85
13	19.95
14	25.12
15	31.62
16	39.81
17	50.12
18	63.10
19	79.43
20	100.00

dbm	Watts
20	0.1
21	0.125
22	0.158
23	0.200
24	0.251
25	0.316
26	0.398
27	0.501
28	0.630
29	0.794
30	1.00
31	1.259
32	1.585
33	1.995
34	2.511
35	3.162
36	3.981
37	5.012
38	6.310
39	7.943
40	10.00
41	12.69
42	15.85
43	19.95
44	25.12
45	31.62
46	39.81
47	50.12
48	63.10
49	79.43
50	100.0

We hope that this information is helpful. Now go out and design something that needs a charger, power supply, or battery pack!

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