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Amplifiers

So you have installed some type of RF system, and you are not getting the range you wanted. How do you increase range? One common thought is just to add an amplifier, which would increase the transmitter power to a level high enough to enable communication at the desired range. In many cases, however, that is not legal for 802.11 devices.

As you have learned, certain rules limit EIRP power. In a good portion of the world, this limit is either 20 dBm or 10 mW/MHz, and that includes the antenna gain. So adding an amplifier is not typically legal in those locations. In an NA domain location, however, you are allowed to have 1 watt of transmitter power and a 6-dBi antenna.

As mentioned earlier in this chapter in the section "Early Spread Spectrum," one of the reasons for the rules and restrictions is to provide coexistence among WLAN users located in the same vicinity. This is the exact reason for establishing antenna and transmitter power limits. The FCC has a specific clause for amplifiers.

The Code of Federal Regulations (the source of the FCC rules) Section 15.204 provides the requirements for amplifiers in the ISM bands. An amplifier may only be marketed with the system configuration in which it was approved, and not separately. It must also be designed using nonstandard connectors or a method of identifying that the amplifier is attached to a qualified transmitter. This is to prevent improper or illegal installation with unapproved transmitters. In plain English, unless the amplifier manufacturer submits the amplifier for testing with a given transmitter, the amplifier cannot legally be sold in the U.S. for use with that particular transmitter. If the amplifier has been certified, it must be labeled with an FCC identification number citing its certification testing.

If you are using a system that includes a legal amplifier, remember that the rules concerning power still apply. If the amplifier is 1/2 watt (27 dBm), this means in a multipoint system the maximum antenna gain is only 9 dBi, and in a point-to-point system it is only 15 dBi. (27 dBm is 3 dB below 30 dBm, so the gain of a multipoint antenna can be increased from 6 dBi, by 3 dB, for a total of 9 dBi. For a point-to-point system, it can be increased by a total of 9 dBi, for a maximum of 15 dBi.)

In the ETSI regulations, there is a maximum EIRP limit of 20 dBm. Because most amplifiers start off well above this level, they are usually not permitted.