dBm

---

Although the Radio Frequency (RF) power level at either transmitter output or receiver input is normally expressed in Watts, electrical engineers often express it in dBm, or "decibel, milliwatts," which is a value referenced to 1 milliwatt (mw) in a 600 Ohm (or in the case of some radio calculations, 50 Ohm) system. The power relationship between dBm and Watts can be expressed as follows: PdBm = 10 x Log PmW, so it is quite easy to convert Watts to dBm.

For example:

1 Watt = 1000 mW, so

10 * Log 1000 = 30 dBm

therefore, 1 Watt = 30 dBm

likewise:

.000000001 = -90 dBm

.000001 Watt = -30 dBm

.001 Watt = 0 dBm

.01 Watt = 10 dBm

.1 Watt = 20 dBm

100 Watts = 50 dBm

To convert dBm to Watts, you could us the following formula:

PdBm = 10 log (P/0.001), where P equals Watts.

Thus, 36 dBm = 10 log (P/0.001)

P = 3981 * 0.001, therefore

P = 3.981 Watts.

Wi-Fi and dBm

Since the dBm convention is found to be more convenient than the Watts convention, in the Wi-Fi world, dBm is also useful for "link budget" calculations. These are calculations of what environmental and equipment factors that will add and subtract from the signal power along the transmission path.

For example: specifications for equipment that operates in the Industrial Scientific and Medical (ISM) band generally allow 100 mW (20 dBm) of EIRP, which is the final power of the radiated signal after any gain produced by the antenna. This yields a use-able link of about 1 kilometer (0.62 miles) in length, depending on atmospheric conditions. Maximum permissible transmitter output power in the licensed bands can vary throughout the world. Some units with dish antennas are adjustable to 50 watts (47 dBm) EIRP, including the gain of the antenna. The practical range at these levels can be up to 15 kilometers (9.32 miles) or more, again depending on local conditions. In other circumstances, e.g. the tropics, that range may come down to 5 km (3.1 miles). Higher power options (+57 dBm and +67 dBm EIRP) are now available (longer link paths may be achievable using repeaters).

Furthermore, Wi-Fi equipment is usually specified in decibels compared to known values. For example, Transmit Power (Tx) and Receive Sensitivity (Rx) are specified in "dBm," where m means 1 milliWatt (mW). So, 0 dBm is equal to 1 mW; 3 dBm is equal to 2 mW; 6 dBm is equal to 4 mW, and so on. For example, a Cisco Aironet 350 Series Access Point at 100 mW transmit power is equal to 20 dBm. dBw is occasionally used for the same purpose, but as a comparison against 1 Watt (1000 mW).

Common mW values to dBm values are shown below in the table below.

Common mW Values to dBm Value

dBm	mW	dBm	mW
0 dBm	1 mW	0 dBm	1 mW
1 dBm	1.25 mW	-1 dBm	0.8 mW
3 dBm	2 mW	-3 dBm	0.5 mW
6 dBm	4 mW	-6 dBm	0.25 mW
7 dBm	5 mW	-7 dBm	0.20 mW
10 dBm	10 mW	-10 dBm	0.10 mW
12 dBm	16 mW	-12 dBm	0.06 mW
13 dBm	20 mW	-13 dBm	0.05 mW
15 dBm	32 mW	-15 dBm	0.03 mW
17 dBm	50 mW	-17 dBm	0.02 mw
20 dBm	100 mW	-20 dBm	0.01 mW
30 dBm	1000 mW (1 W)	-30 dBm	0.001 mW
40 dBm	10,000 mW (10 W)	-40 dBm	0.0001 mW

---