FCC Regulations on WiFi

The Federal Communications Commission (FCC) has which regulate power for Wi-Fi networks. From the viewpoint of the FCC regulations, the power of a Wi-Fi broadcast is measured in units of equivalent isotropically radiated power (EIRP). EIRP represents the total effective transmitting power of the radio in a Wi-Fi card or access point, including adding gains from an antenna and subtracting losses from an antenna cable. When using an omnidirectional antenna (see "Different Kinds of Antennas" earlier in this chapter for more information about types of antennas) with fewer than 6 decibels (dB) gain, the FCC requires EIRP to be under one watt.

• antenna gain counts
• cable signal loss counts
• One Watt (1000mw) is the FCC limit on WiFi devices

You cannot get a more powerful wireless device without an FCC license than 1 watt for 802.11x protocols.

The FCC Regs say:

(b) The maximum peak output power of the intentional radiator 
shall not exceed the following:
(1) For frequency hopping systems in the 2400-2483.5 MHz band 
employing at least 75 hopping channels, and all frequency hopping 
systems in the 5725-5850 MHz band: 1 Watt. For all other frequency 
hopping systems in the 2400-2483.5 band: 0.125 Watt.

(4) Except as shown in paragraphs (b)(3) (i), (ii) and (iii) of this 
section, if transmitting antennas of directional gain greater than 6 dBi 
are used the peak output power from the intentional radiator shall be 
reduced below the stated values in paragraphs (b)(1) or (b)(2) of this 
section, as appropriate, by the amount in dB that the directional gain 
of the antenna exceeds 6 dBi.

Which may indicate with a high gain antenna, up to 4 watts is permitted. Furthermore, there seems to be an exemption for point to point communications.

Sec. 15.247 Operation within the bands 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz.

(i) Systems operating in the 2400-2483.5 MHz band that are used 
exclusively for fixed, point-to-point operations may employ transmitting 
antennas with directional gain greater than 6 dBi provided the maximum 
peak output power of the intentional radiator is reduced by 1 dB for 
every 3 dB that the directional gain of the antenna exceeds 6 dBi. 

This is known as the "3 to 1 Exemption." This appears to allow someone to exceed 4 watts if the transmission is focused for a high gain directional signal if you use a low gain antenna that's direction might have a 45 degree beamwidth.

Neither exception is clear.

Understand what the power output of the radio is (in dBm) and the gain of your antennas (dBi).

A 15dBm radio + 2dBi Antenna makes for an output of 17dBm.

To convert milliwatts to dBm:

dBm = 10 * (log (1000 * P)) P = Power in Watts 1000mW = 1 Watt

To convert dBm to milliwatts:

P = 0.001 * (10**(dBm/10)) P = Power in Watts

Watts and dBm are both units of measure of power; either can be used. 0.001W = 0dBm; 3dBm = 0.002mW; 10dBm = 0.01W for example.

Simply put, FCC regulations have a 1 WATT (30dBm) limit on omni directional antennas and 2 WATT (33dBm) limit on directional antennas.

The most popular WiFi standards today, 802.11b and 802.11g, both operate in the 2.4 GHz (Giga Hertz) frequency band and are susceptible to interference from other products operating in the 2.4 GHz band including microwave ovens, Bluetooth devices, baby monitors and cordless telephones. 802.11n, currently in Draft 2.0 standard (with projected standard approval in the fall of 2008), can use both 2.4 GHz and 5 GHz frequencies.

In the US, the FCC limits the radiated power in 2.4GHz. The limit is a function of antenna horizontal beamwidth. More power with narrow beamwidth. This tends to reduce interference and benefit spectrum sharing.

The 802.11g specs for OFDM require linearity in the standard and add-on amplifiers such that antenna gain is far more prudent than transmitter power. Most WiFi chipsets cannot produce an honest, high quality signal at more than about 36mw. The same chipsets can produce about 100mW in the lowest speeds of 11g and all 11b speeds, since 11b is not OFDM.

Increasing antenna gain is a lot more cost effective than increasing transmit power.