LINEBURG


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Earth's Surface
FIGURE 3-22: Relationship of a vertical (E) plane and a horizontal (H) plane.
79
Chapter 3 ” Building a Directional Tin Can Antenna




FIGURE 3-23: A three-dimensional simulation of the waveguide can antenna.


This antenna works great in one general direction. Also, it has about the same gain vertically as
it does horizontally.



Summary
In this chapter, you learned about waveguides and the types of can antennas you can build on your
own. The waveguide antenna is very easy to build, but you need to know where to drill. You also
learned about antenna simulation and what antenna radiation patterns are and how to read them.
With the knowledge from this chapter, you can go on to build and experiment with many dif-
ferent types of antennas. And understanding antenna radiation patterns puts you in the driver™s
seat when choosing what antenna would work best for any situation.
Next up, let™s add a high-gain antenna to your wireless access point. You™ll learn how to choose the
antenna type: omni versus directional. And you™ll find out how to stay within the spirit of the law
when it comes to RF power. Read on to get the maximum range out of your wireless network.
chapter
Modifying Your
Access Point with a
High-Gain Antenna
Y
ou 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
in this chapter
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
Selecting the right
house, there are significant areas where your laptop can™t maintain a consistent
antenna
connection and downloads take forever when you do have a connection. In
other words, in technical terms, “coverage stinks.”
Following
What do you do? One likely answer is to use a high-gain antenna for the
government
access point. A high-gain antenna multiplies the access point™s range for
regulations
both transmission and reception. That is, it boosts both receiver sensitivity
and transmitter output. Increased signal strength means faster transmis-
Surveying the site
sions, 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, Attaching an
(an outdoor model would cost even more), it will be worth it if it makes the antenna to your
difference between a pokey, limited, unreliable network and one that is fast,
access point
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 Using an amplifier
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 interfer-
ence 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.
82 Part I ” Building Antennas




FIGURE 4-1: System using a Linksys WAP11 with a high-gain external antenna.



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
83
Chapter 4 ” Modifying Your Access Point


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 spot-
light. 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.)




FIGURE 4-2: Omni antenna (left) and directional antenna (right).
84 Part I ” Building Antennas




FIGURE 4-3: A directional antenna (left) in the corner of the coverage area.



With both omni and directional antennas, there is an inverse relationship between gain and
coverage. For an omni antenna, the higher the gain, the flatter its radiation pattern. So if you
want one access point to cover two floors of your house, for example, you may be better off
with a medium-gain omni than a high-gain one. Similarly, higher gain directional antennas
usually have more focused beams.
If you need both broad coverage and high gain, you may need to get two antennas and “aim”
them in different directions. For example, you could set up two 120 degree sector antennas for
240 degree coverage. The usual approach for this kind of setup, however, is to attach the anten-
nas to two different access points and run the access points on two different channels to mini-
mize any possible interference.



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 intro-
duce just one unit of measurement here that wasn™t mentioned in Chapter 1: dBi, decibels ref-
erenced to an isotropic radiator.
85
Chapter 4 ” Modifying Your Access Point


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 com-
mon practice. Thus, omni antennas are treated as point-to-multipoint, while directional anten-
nas 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.)




FIGURE 4-4: A single path point-to-point system and multiple path multipoint system.
86 Part I ” Building Antennas


30 dBm means you take 1 mW and double it ten times (because 30 is 10“3, and 3 dB is a dou-
bling). 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 maxi-
mum 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, depend-
ing 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 for-
ward, 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 with-
out increasing EIRP. The main advantage of doing this is the increased receive sensitivity of the
more powerful antenna.
87
Chapter 4 ” Modifying Your Access Point


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 gener-
ate 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 dou-
ble the distance for each 6 dBi of added antenna gain.




3 6
Feet Feet

Antenna




FIGURE 4-5: Diagram showing relative RF exposure.
88 Part I ” Building Antennas


The Site Survey
To help you pick the right antenna for the job, first survey the site to get an idea what coverage
pattern you want, and what degree of signal loss you are currently encountering. Start by map-
ping out the site on a piece of graph paper, noting possible locations for the high-gain antenna.
Then place the access point (with the existing antenna) in a possible location, and walk around
with your laptop to the areas you want to cover. If you have NetStumbler on your laptop, you
may be able to use it to measure the signal-to-noise ratio (SNR) in each desired coverage loca-
tion. The higher the SNR, the better. Lacking that, just note whether the laptop can connect to
the network, and if it can, how long it takes to perform some common tasks, such as transfer-
ring files. See the basic site survey document in Figure 4-6.
If nothing else, this will give you a point of comparison, to determine how much boost you get
from your high-gain antenna. In addition, you™ll probably get a better idea what kinds of trans-
mission losses you™re experiencing. Typically, you may see three kinds of losses: propagation
losses, multipath losses (signal fading), and interference.


Propagation Losses
Propagation loss represents the “resistance” of whatever the RF has to pass through. Every
medium, including air, has a particular loss at a given frequency. The loss is proportional to



Signal Quality Excellent: Good: Fair/Poor:

AP

Kitchen
Conference
Room




Visitor
Lounge




AP


FIGURE 4-6: A site survey document created from a hand-drawing on graph paper.
89
Chapter 4 ” Modifying Your Access Point




FIGURE 4-7: The Fresnel Zone is like a football.



the distance that the signal has to travel through the medium. For example, the signal will
probably lose about 6 dB going through a wall with 2-by-4 wood studs and sheetrock on
both sides.
Beyond about 20 feet from the access point, propagation losses can increase at up to 30 dB per
100 feet, depending on building construction and layout. You™ll get losses not only from walls,
ceilings and floors, but even from furniture and people.
Visualize the signal as half a football, with the antenna at the point. Beyond 20 feet or so from
the access point, anything that pokes into that area (called the Fresnel Zone) will degrade the
signal. See Figure 4-7 for an example.
Fresnel Zone incursions are a secondary factor when it comes to interference. The most criti-
cal factor is objects that are in the direct line-of-sight (LOS) between the access point
antenna and clients. However, objects below the LOS but in the Fresnel Zone can have a sig-
nificant effect, as well. That™s why, in long-distance outdoor installations, antennas are usually
positioned higher than is necessary just for LOS. Outdoors, it™s standard practice to calculate
the size of the Fresnel Zone and use that to determine how high the antenna should be.
Indoors, antennas may be installed in the attic or on the ceiling, in order to avoid Fresnel
Zone incursions. If that™s not possible, you just have to get a more powerful antenna, in order
to overcome the losses.
Whether you can clear the obstacles, or whether you just have to compensate for them, it™s good
to know what your Fresnel Zone Clearance is. There are simple calculators available on the Web
that will allow you to calculate this very quickly. (For instance, www.thirdheight.
com/support/downloads/wireless/SOM_calcs.xls.)
This calculator will help you not only with Fresnel Zones, but with your overall “link budget,”
a calculation to determine whether the signal transmitted from your antenna is powerful
enough to reach the intended receivers. A link budget calculation takes into account a variety
of factors, including TPO, transmit antenna gain, losses in the transmission antenna cable, free
space propagation losses, Fresnel Zone losses, losses in the receive antenna cable, receive
antenna gain, and receiver sensitivity. Other factors such as connectors and lightning arresters
in the cables can also be taken into account. (Figure 4-8 shows the variables involved in a typi-
cal link budget calculation.)
A common rule of thumb is that the signal should be 10 dB stronger than the minimum
required for communication. This 10 dB is referred to as the System Operating Margin
(SOM).
90 Part I ” Building Antennas



Free Space
Propagation Losses
Receive
Antenna Gain


Transmit

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