Wireless LAN Antennas Gain (dBm)

FCC Wireless Rules and Regulations

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How much power can I transmit on a 10 dBi omni and still be legal?

The FCC regulations for Point to Multi-Point allows only 36 dBm (4 watts) EIRP. This is 30 dBm (1 watt) into a 6 dBi antenna. If you use a 10 dBi wireless antenna, you must limit your transmitter (or amplifier) to 26 dBm (10 + 26 = 36 dBm). For a 14 dBi panel wireless antenna, this allows a 22 dBm transmitter (or amplifier). Power is measured at the antenna connector, so subtract any cable loss between the amplifier and the antenna. Refer to the following table:
หมายเหตุ : E.I.R.P. = กำลังส่งออกอากาศสมมูลแบบไอโซทรอปิก

Power at Antenna (dBm/Watts)

Antenna Gain (dBi)

EIRP (dBm)

EIRP (watts)

30 dBm (1 W)

6

36

4

27 dBm (500 mW)

9

36

4

24 dBm (250 mW)

12

36

4

21 dBm (125 mW)

15

36

4

18 dBm (62 mW)

18

36

4

15 dBm (31 mW)

21

36

4

12 dBm (15 mW)

24

36

4

 

How much power can I transmit with in my Point-to-Point system?

According to FCC regulations, 2.4 GHz Part 15.247 point-to-point transmitters may use a 30 dBm transmitter with a 6 dBi antenna. For a 3 dB increase in antenna gain, the transmitter power output must be reduced by 1 dB. Power is measured at the antenna connector, so subtract any cable loss between the amplifier and the antenna. Refer to the following table. (also see Canadian Rules)

Power at antenna (dBm/watts)

Max Antenna Gain (dBi)

EIRP (dBm)

EIRP (watts)
30 dBm (1 W)

6

36

4.0
29 dBm

9

38

6.3
28 dBm

12

40

10.0
27 dBm (500 mW)

15

42

16.0
26 dBm

18

44

25.0
25 dBm

21

46

39.8
24 dBm (250 mW)

24

48

63.0
23 dBm

27

50

100.0
22 

30

52

158.0

dBm to Watt Conversion Table

from http://www.cpcstech.com/dbm-to-watt-conversion-information.htm

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

Remember when calculating your total output that the FCC only allows 36dBm EIRP (4 watts)!

Transmitted power must be configured correctly to prevent potential interference problems due to the effective isotropic radiated power exceeding the limits as defined in FCC part 15.247(i).

As defined in FCC part 15.247(i), the power transmitted by the transmitter can only have a maximum power level of 1 watt or 30dBm.

The antenna can only have a maximum gain of 6dBi.

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 transmitter versus antenna gain 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