Topics

FCC Rules Dictate Antenna Use

In the United States, the Federal Communications Commission (FCC) regulates the use of antennas through FCC Part 15.247, which defines power limitations for Wi-Fi devices. The key to applying these rules is to understand EIRP (Equivalent Isotropically Radiated Power), which represents the total effective transmit power of the radio, including gains that the antenna provides (and losses from the antenna cable). When using omni-directional antennas less than 6dBi gain, the FCC rules require EIRP to be 1 Watt or less.

With higher gain directive antennas, the FCC relaxes EIRP limitations. When using antennas having a gain of at least 6dBi gain, the FCC allows operation up to 4 watts EIRP, which is 1 watt device output power plus 6dBi of gain. The reason higher EIRPs are acceptable is that the higher gain antennas are more directive, which reduces the possibility of RF interference with other systems.

For antennas with gain greater than 6dBi, the FCC requires you to reduce the transmitter output power if the EIRP is already at the maximum of 36dBm (4W). This means that as antenna gain goes up, you must decrease the transmitter power. Higher gain antennas would mostly apply to point-to-point solutions having very long range requirements, which is not used for multipoint Wi-Fi hotspot access applications (point-to-multipoint).

FCC 2.4 GHZ Band Rules (Point-To-Multipoint)

Max Equivalent Isotropically Radiated Power (EIRP) = 36dBm (4 Watts)
Antenna
Gain (dBi)		 Device Output
Power Allowed		 EIRP
(dBm)		 EIRP
(Watts)
6 30dBm or 1000mW 36 4
9 27dBm or 500mW 36 4
12 24dBm or 250mW 36 4
15 21dBm or 125mW 36 4


As can be seen, increasing the gain of the antenna requires a proportional decrease in the device output power. The maximum power point and largest radiation pattern occurs with a 6dBi antenna. For an omni-directional, increasing antenna gain and decreasing output power is a negating effect that only results in a narrower signal dispersion pattern and minimal increase (if any) in range. It's main advantage is to reduce source power requirements.

        

Why shouldn't I just crank the power up and use the highest gain antenna I can find?

Most countries follow the FCC regulations. While one can take a 1000mW (30dBm) device and connect a 15dBi gain antenna resulting in an EIRP of device 30dBm + 15dBi antenna gain = 45dBm (32W), there are a number of very good reasons not to:

1) Ruining the airwaves for everybody
Imagine being at a party. Everyone is speaking in a normal voice. Along comes one person who uses a megaphone when talking. Whenever he speaks, everyone is disrupted. Nobody else will be heard. Using another example: one VHF handheld radio outputs 5W while another fixed station operates at 25W. At distance, the handheld can hear the base station but the base station cannot hear the handheld due to the handheld's lower transmit power. If the handheld added a booster to 25W, they would be on an even level, each hearing the other. If the handheld boosts to 100W, everyone would hear it, but the 100W handheld would only be able to hear the 25W base station that would be at a distance suitable for 25W transmit power. Therefore, the 100W station would not need to transmit at 100W! Transmitting with more power than needed walks over the communications of everyone that would normally not be within range, essentially polluting the airwaves for everyone. 

2) Level playing field
The 36dBm limit is to create a communications interchange that's fair to all. Commercial hotspot installations will be no more than 36dBm (4W). A laptop puts out about 17dBm (50mW), so using a booster of 36dBm (4W) brings it up to the same power level as the hotspot. Putting out 45dBm (32W!) will result in the hotspot being able to hear you at great distance, but you won't be able to hear their 8x lower 36dBm (4W) reply.

3) Too flat a signal radiation pattern
Professional hotspot installations put their antenna(s) up high (roofs and towers)and point them downward. This is done to provide the best clear line-of-sight. Using a flat pancake like pattern as from a 15dBi gain antenna can result in the signal being below the intended hotspot when mounted lower, such as on a rail. Often, this Wi-Fi booster will connect with a distant hotstpot and not even see the much stronger one nearby. A rocking platform can cause the signal to be lost as the beam horizontally arcs through a distant hotspot's antenna.

4) Antenna size
A 12-15dBi gain omni-directional antenna will be 6+ feet (183+cm) in length. This makes it difficult to handle and prone to being snagged by sheets and dock lines when mounted low. Its size is not practical for masthead installation. By contrast, 6dBi antenna is only 13-14 inches (33-36cm) in length and can easily be mounted at an out of the way location.

5) Typical distance from hotspot
In the real world, 99% of the time, the distance from a hotspot will be less than 1 mile. When in close, the excess power from a 12-15dBi gain antenna can overdrive the hotspot receiver rendering it mostly unusable. Your best all round antenna will be a 6dBi with an 8.5-9dBi as max. Both are easily capable of doing 3-5 miles and more based on having a clear line of sight to a suitably capable hotspot.

6) It breaks the law!
Severe penalties can be levied including seizure of equipment (or more!). The penalty severity varies from country to country. Can you be detected? The operator of a hotspot that all of a sudden finds its Wi-Fi performance degraded will investigate when the issue persists. They may discover they're being overdriven. For connections of short duration, you likely won't get detected since the problem will be corrected when you disconnect. If you're there for a longer time, they may have access to and use a highly directional antenna/instrument and sweep the area to find the offender. Bottom line is that you CAN be detected as the source of a problem.


Copyright © 2009-2011 Bitstorm Inc. All rights reserved.