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dBm to Watt Conversion Table

dBm Watts dBm Watts dBm Watts
0 1.0 mW 16 40 mW 32 1.6 W
1 1.3 mW 17 50 mW 33 2.0 W
2 1.6 mW 18 63 mW 34 2.5 W
3 2.0 mW 19 79 mW 35 3.2 W
4 2.5 mW 20 100 mW 36 4.0 W
5 3.2 mW 21 126 mW 37 5.0 W
6 4 mW 22 158 mW 38 6.3 W
7 5 mW 23 200 mW 39 8.0 W
8 6 mW 24 250 mW 40 10 W
9 8 mW 25 316 mW 41 13 W
10 10 mW 26 398 mW 42 16 W
11 13 mW 27 500 mW 43 20 W
12 16 mW 28 630 mW 44 25 W
13 20 mW 29 800 mW 45 32 W
14 25 mW 30 1.0 W 46 40 W
15 32 mW 31 1.3 W 47 50 W

As defined in FCC part 15.247(i), the power transmitted by a Wi-Fi device to an antenna can only have a maximum power level of 30dBm (1W).

The FCC does allow a maximum of 36dBm (4W) Effective Isotropic Radiated Power (EIRP) out the antenna (i.e. 6dBi gain). Transmitted power must be configured correctly to prevent potential interference problems due to the  EIRP exceeding the limits as defined in FCC part 15.247(i).

Therefore, at maximum Wi-Fi device transmit power, the antenna can only have a maximum gain of 6dBi.

However. If the power at the transmitter is lowered by 1dB, the antenna gain can be increased by an additional 3dB. For example, if an installation reduced power at the transmitter to 29dBm, it could use an antenna having a gain of 9dBi.

In general, for every 1dB power reduction at the transmitter from 30dBm, an installation can add 3dB gain at the antenna.

A breakdown of maximum transmitter power versus maximum antenna gain allowed is as follows:

    30dBm transmit - 6dBi antenna
    29dBm transmit - 9dBi antenna
    28dBm transmit - 12dBi antenna
    27dBm transmit - 15dBi antenna
    26dBm transmit - 18dBi antenna
    25dBm transmit - 21dBi antenna
    24dBm transmit - 24dBi antenna

It can be seen therefore that as the gain of the antenna is increased, a corresponding decrease in the transmit power must be applied. For a system using an omni-directional antenna, this has an effect of negating any benefit that a higher gain antenna offers.

Additionally, for an omni-directional antenna, the radiation pattern becomes flatter as the antenna gain increases. Beyond 6dBi, the pattern becomes less useful for any installation that is not on a solid fixed platform. For example, if installed on a boat with a 9dBi or greater gain antenna, the rocking of the boat can cause the radiation pattern to alternately be above and below the line-of-sight as the distance to the access point increases. And distance is what we are trying to achieve. A 6dBi antenna has a horizontal beam width of ±30° thus assuring that the radiation pattern will always be in the line-of-sight of the access point.


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