Modifying Your Access Point with a High-Gain Antenna

You buy a wireless access point and a laptop card with visions of working or surfing the net in the kitchen, in the bedroom, on the deck, or in a hammock in the backyard.No longer will you be chained to a desk or a table! Only one problem:No matter where you place the access point in your house, there are significant areas where your laptop can’t maintain a consistent connection and downloads take forever when you do have a connection. In other words, in technical terms, “coverage stinks.”

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.

You should be aware that it is possible to make your Wi-Fi network too powerful. As wireless networks extend their reach, they are more likely to “jostle” one another. Your neighbors could start picking up your signals, or you might start picking up theirs. The FCC has rules to keep such interference to reasonable levels. There are also FCC safety rules about how much RF energy human beings should be exposed to over how long a period of time. By keeping interference and safety guidelines in mind when you design your system, you’ll protect yourself and others, and you’ll have the added comfort of knowing that you’re legal.

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The chances of getting busted for going over the limit for an indoor network are about the same as for removing one of those little “do not remove on pain of death” tags on a mattress. Basically, someone has to complain before the FCC does anything. However, if you are counting on this system, say for your business, it’s best to stay within the rules.

If you get decent performance when the access point and the client are close, but things get flakier the farther away you get, a high-gain antenna on the access point could be just the thing to pump up your signal.

Here’s what you will need for this project:

1. Wireless access point with an external antenna connector and detachable antenna

2. High-gain antenna

3. Pigtail cable

4. Antenna cable

5. Hardware and tools (such as screws and screwdriver) for mounting the antenna

6. Opposable thumbs

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.

Choosing an Antenna

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.

A directional antenna transmits and receives in a narrow beam, usually within a 30 to 60 degree “slice” of a full circle (see Figure 4-2.). You can envision its radiation pattern as a spotlight. A directional antenna with a fairly broad beam, such as 120 degrees, is called a “sector” antenna.

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.

The FCC regulations do not talk in terms of omni and directional antennas. Instead, they talk about “point-to-multipoint” and “point-to-point” networks. Strictly speaking, every network in which an access point is accessed by clients is point-to-multipoint in its design. However, the apparent intention of the regulations is to permit more gain for more focused transmissions, because they are less likely to cause interference. It is this intention which is followed in common practice. Thus, omni antennas are treated as point-to-multipoint, while directional antennas are treated as point-to-point. Figure 4-4 shows a point-to-point versus multipoint network.

In addition, the regulations state that each specific antenna model must be certified with each specific access point model, before they can legally be used together. However, we are not aware of any effort to enforce this at the end user level, and common practice seems to be: to stay within certification guidelines, as opposed to actually certifying in every case.

 

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.

FCC Point-to-Multipoint Rules

Here’s a summary of the FCC rules for point-to-multipoint transmissions:

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.)

30 dBm means you take 1 mW and double it ten times (because 30 is 10–3, and 3 dB is a doubling). This works out to a number that is so close to 1 W (1.024 W, to be exact) that everyone just calls it 1 W. Because each 3 dB represents a doubling of wattage, 6 dBi means the maximum permissible transmitted signal from the antenna is 4 W Equivalent Isotropically Radiated Power (EIRP). (1 W 2 2) The bottom line: If you’re using an omni antenna, design your system so that it doesn’t radiate more than 4 W EIRP.

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.

The omni antennas that come with access points are generally 3 dBi or so. Therefore, anything much less than 6 dBi would be only marginally better than the manufacturer’s antenna. Most of the add-on omni antennas on the market are in the 6 to 15 dBi range, which is the sweet spot for equipment like Linksys access points—safely legal yet definitely worthwhile.

FCC Point-to-Point Rules

Point-to-point rules are the same as point-to-multipoint rules, except that you need to reduce TPO 1 dB for every 3 dBi of antenna gain over 6 dBi. In other words, three steps forward, one step backwards: A big improvement over the corresponding point-to-multipoint rule! For example, a 24 dBi antenna is 18 dB over a 6 dBi antenna. So, to use a 24 dBi antenna, you would have to lower a 1 W (30 dBm) radio 18/3 or 6 dB to 24 dBm or 1/4 W. (18 steps forward, 6 steps back.)

In practice, many access points don’t allow you to adjust the TPO. However, you can take into account the fact that the existing TPO of the access point is less than 1 W. For instance, 125 mW is 9 dB less than 1 W. Therefore, we would be very safe in using the 24 dBi antenna in the previous example with our Linksys BEFW11S4, because its radio comes “pre-lowered” more than the required 6 dBm.

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.)

In practice, most of the available directional antennas for Linksys access points are in the 12 to 27 dBi range, keeping them within the intention of the FCC regulations.

 

FCC Safety Rules

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.)

These guidelines deal with how much radiation hits people, not how much the antenna puts out. As with a microwave oven, the size of the heated body is also important. Specifically, the FCC guideline states that potentially hazardous exposures may occur at levels over 4 watts per kilogram (4 W/kg) averaged over the entire body. Even continuous exposure to 4 W/kg or less should be okay.

We’re not going to get into the mathematics of calculating exposure levels. Also, we are not doctors, nor are we dispensing medical advice. But we will note that the exposure is inversely proportional to the square of the distance (double the distance, one fourth the exposure). Therefore, the easy way to minimize health hazards is to avoid spending extended periods of time near high-gain antennas. Figure 4-5 shows the relative exposure from someone standing 3 feet and 6 feet from the antenna.We would not stay within two feet of an ordinary Linksys access point with 3 dBi antennas for more than five minutes on a regular basis. And we’d double the distance for each 6 dBi of added antenna gain.