What do you do? One likely answer is to use a high-gain antenna for the access point. A high-gain antenna multiplies the access point’s range for both transmission and reception. That is, it boosts both receiver sensitivity and transmitter output. Increased signal strength means faster transmissions, too, since most access points are configured to drop back to a lower data rate when the quality of the connection deteriorates. Although an indoor high-gain antenna may cost about as much as the access point itself, (an outdoor model would cost even more), it will be worth it if it makes the difference between a pokey, limited, unreliable network and one that is fast, far-reaching, and robust. A sample setup is shown in Figure 4-1.

The mechanics of attaching the antenna can be really easy, especially if you have a Linksys access point. You do have to make sure that you get (or build) the right pigtail cable for connecting the access point to the antenna cable. You should also choose an antenna that will cover the intended area while minimizing interference. Positioning and aiming the antenna for best results can take a bit of experimentation, too.

See Chapter 1 for instructions on building the antenna cable. In fact, even if you don’t want to make your own antenna cable, Chapter 1 has good background information for many of the topics in this chapter.

Although the 2.4 GHz technology is relatively uniform worldwide, the rules about who can use it and how it can be used vary from country to country. If you are located outside the United States, manufacturers and governmental agencies may be good sources of information on what is allowed in your region.

There are two kinds of high-gain antennas: omni and directional. An omni antenna transmits and receives in all directions, though usually more horizontally than vertically: Its radiation pattern looks like a doughnut, with the antenna at the doughnut hole.

Chapter 5 has a section on Picking the Right Antenna that provides some more information on this topic. The focused beam of a directional antenna provides three advantages: First, by focusing your beam, you get a more powerful signal for the same transmission power. Second, you’re less likely to cause or experience interference problems, because you can aim your transmission beam and focus your receptivity. Third, directional antennas, due to their reduced potential for interference, typically can have higher gain than omnis. You should use a directional antenna if possible. If you need broader coverage than a highly directional antenna can provide, you may be able to compromise on a sector antenna, and put it in one corner of the desired coverage area. (See Figure 4-3.)

Staying Legal

FCC regulations specify three things:

Maximum permitted transmitter power output (TPO) of the radio in the access point, before the signal reaches the antenna Maximum permitted antenna gain without requiring a reduction in TPO Required reduction in TPO for every decibel (dB) of antenna gain above that maximum For an introduction to decibels, see Measuring Line Loss in Decibels in Chapter 1.We will introduce just one unit of measurement here that wasn’t mentioned in Chapter 1: dBi, decibels referenced to an isotropic radiator.

An isotropic radiator transmits equally in all directions. The radiation pattern of a perfect isotropic antenna would look like a beach ball, with the antenna in the center of the ball. The term “isotropic” basically refers to an ideal omni antenna. For instance, since each 3 dB represents a doubling of power, 6 dBi describes an omni antenna that doubles power twice—that is, one that multiplies power by a factor of four.

We are not attorneys. Our interpretation of FCC regulations and practices is not authoritative. In fact, much of this is under review by the FCC and industry. Regulations or legal definitions may change any time.

The radio in the access point can have up to 30 dBm TPO. You can have a 6 dBi antenna without reducing TPO. Assuming 30 dBm TPO, that’s a 36 dBm signal from the antenna. The TPO needs to be reduced 1 dB for every dB of antenna gain over 6 dBi. (In other words, for every step forward, you have to take one step backward.)

The radio in the Linksys BEFW11S4 access point, for instance, puts out 68–78 mW, depending on the channel. Even assuming 100 mW TPO, a 9 dBi antenna would bring that up to just 800 mW EIRP. You’d have to go over 15 dBi before you might be in danger of exceeding the 4 W EIRP limit (100 2(15/3)  3200).With a 78 mW TPO, a 17 dBi antenna is still under the limit (though just barely). (78 2(17/3)  3962)

You can look up the maximum output of your access point radio on the FCC Web site, if you have the FCC ID of the radio, which should be provided on the access point. For instance, on our Linksys BEFW11S4, the FCC ID of the radio is MXF-C901114. You can go to www.fcc.gov/oet/fccid/ and search on the FCC ID. One place that TPO information should be listed is under RF Exposure Info.

Some wireless devices let you reduce TPO, allowing you to use a more powerful antenna without increasing EIRP. The main advantage of doing this is the increased receive sensitivity of the more powerful antenna. In fact, the rated Linksys TPO of 78 mW is actually about 11 dBm below 1 W. Therefore, if you wanted to push the limits, you could use a 39 dBi antenna—33 dB above 6 dBi—because 33 dB divided by three is 11. (33 steps forward, 11 steps back.)

The 2.4 GHz frequency band is used in microwave ovens, because RF in this band tends to generate a lot of heat when it hits something. This heat can be dangerous to the human body. For this reason, the FCC has specified Maximum Permissible Exposure (MPE) limits for 2.4 GHz signals. The FCC has also issued a bulletin, OET Bulletin 65, “Evaluating Compliance with FCC Specified Guidelines for Human Exposure to Radio Frequency Radiation” that spells out the guidelines. (It’s at http://ftp.fcc.gov/oet/info/documents/bulletins/#65.)