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Antenna
Gain Losses
in Receive
Antenna
Cable




Losses in Transmit
Antenna Cable Receiver
Radio TPO
Sensitivity
Fresnel Zone Losses

FIGURE 4-8: Factors to take into account in a link budget calculation.


Multipath Losses and Interference
Multipath losses occur when signals take different paths to a receiver. If one path is significantly
longer than the other, the peaks and troughs of the two signals will be significantly out of sync
with one another (“out of phase”) when they get to the receiver. Signals that are totally out of phase
(180 degrees) will completely cancel one another, as the troughs of one signal cancel the peaks of
the other (see Figure 4-9). In practice, the phases of the two signals will usually drift constantly, so
the signal cancellation changes, resulting in a constantly fluctuating signal (“signal fade”).
Having two receiving antennas usually helps substantially. Phase cancellation will be different
for the two antennas, because of the differences in the length of the two paths required to reach
the antennas. With properly spaced antennas, the signal at one antenna can be strong when the
other is weak.




Multipath Signals




Net Received Signal

FIGURE 4-9: Phase cancellation caused by multipath interference.
91
Chapter 4 ” Modifying Your Access Point


When you attach a high-gain antenna to an access point, you lose the advantage of two anten-
nas, and you tend to increase multipath losses. The high-gain antenna should more than make
up for this, but you may want to take this effect into account in calculating your antenna gain
requirements. If your connection seems to continuously fade in and out, suspect multipath
losses.
Interference may come from any other equipment running at 2.4 GHz (including other Wi-Fi
networks), particularly if they™re using the same channel as you are, or equipment generating
broad spectrum RF. Interference is often “bursty” in nature, as the interfering equipment is
seldom operating full-out all the time. If your network experiences sudden and seemingly
random attacks of deteriorating performance, look for a source of interference. You may be able
to change channels or locate your network equipment and antenna to get away from the
offending source. Otherwise, you™ll have to up your power to overcome the interference.



Attaching a High-Gain Antenna
To attach the high-gain antenna to the access point, you basically build a “chain” with four
links in it: the access point, the pigtail, the antenna cable, and the antenna. (See Figure 4-10.)
To attach the antenna to the access point, follow these steps. Assuming the access point is
already working correctly, and you have good cables with the right connectors, these three steps
should take just a few minutes. (We™ll talk more about the cables in a minute.)

1. Configure the access point to use only one of its two antennas (optional, but recom-
mended if your access point supports it).
2. Attach the pigtail cable to the access point.
3. Run the antenna cable from the pigtail to the antenna.
4. Position and install the antenna.

And you™re done!




FIGURE 4-10: The chain of components in your antenna project.
92 Part I ” Building Antennas


Try using the Cantenna you built in Chapter 3 to boost gain on an access point. This antenna
works as well on an access point as it does on a laptop computer.




Step 1: Configure the Access Point to Use Just
One Antenna
Since you™re going to all the trouble of installing a high-gain antenna, you want to make sure
that the access point uses it consistently. You configure this using the access point™s firmware. It
takes just a minute or two. Here is a guideline for the process on a Linksys WAP11 or
BEFW11S4 access point:

1. Access the access point using your browser. If you haven™t changed the IP address, it™s
192.168.1.1. Enter your username and password. If you haven™t changed it yet, the
default is a blank username, and a password of “admin”.
2. When you get to the Setup screen, click the Advanced tab in the upper-right corner.
3. In the Advanced section, click on the Wireless tab.
4. Go to the Antenna Selection drop-down menu, and select either Right Spread or Left
Spread, depending on which antenna output you are using. (You look at the access point
while viewing the front panel when determining right and left.)
5. Click the Apply button at the bottom of the screen.

Figure 4-11 shows the Wireless tab in the Advanced section of the Linksys configuration
program.
In case you™re curious, the Default setting is a “diversity” setting, in which both antennas are
active. Diversity Spread should automatically select the antenna with the strongest signal.
However, since you know which one is always going to be the high-gain antenna, we™re choos-
ing not to depend on the automated function.

Some access points transmit through only one antenna. Make sure you choose the correct
antenna. Vendors tend to refrain from listing these engineering anomalies, so your best bet is to
test the antenna after installation to make sure which transmit antenna works best.



Step 2: Attach the Pigtail Cable to the Access
Point
Connecting a pigtail to an access point can be extremely easy, if you have a pigtail with the
proper connector, and if the access point manufacturer designed the antenna for easy removal.
Linksys access points, for example, have easy-to-remove antennas. The difficulty of finding the
proper connectors argues for buying rather than building pigtails.
93
Chapter 4 ” Modifying Your Access Point




FIGURE 4-11: The Linksys configuration program: Advanced section, Wireless tab,
Antenna Selection menu.



Linksys Makes It Easy
If you have a Linksys WAP11 or BEFW11S4 access point, connecting your high-gain antenna
to the access point is a snap, because these Linksys access points have detachable antennas.
(Here™s where your opposable thumb comes in). Just grip the antenna base firmly between
thumb and forefinger and turn counter-clockwise to remove. (See Figure 4-12.)
Other access points may not be so easy. We recommend an access point with external connec-
tors and detachable antennas, because otherwise you may have to open up the access point, dis-
connect the existing antenna wires, and solder a new connector onto the wires”undoubtedly
voiding any warranty in the process. Figure 4-13 shows a hard-wired antenna in a Cisco access
point.

The FCC Makes It Hard
The other area where you might run into a slight hiccup is in building a pigtail to connect the
access point to the antenna cable. The antenna side of the pigtail is no problem: Your antenna
cable probably has standard female N-Connectors. So, to make the connection, the pigtail
needs to have a standard, easy-to-find male N-Connector.
94 Part I ” Building Antennas




FIGURE 4-12: Removing the original antenna.




FIGURE 4-13: Cisco access point with a soldered antenna connector.
95
Chapter 4 ” Modifying Your Access Point




FIGURE 4-14: Standard male connector (left) and standard female connector (right).



The male connector has a pin in the middle. The female connector has a socket. In addition, male
connectors have threads on the inside of the connector body, while female connectors are
threaded on the outside. (See Figure 4-14.)



The problem stems from the connector on the access point. The FCC mandated weird connec-
tors for Wi-Fi antennas, because they were afraid that hobbyists would do things like connect-
ing high-gain antennas to access points and possibly jack-up the power of their wireless
networks beyond the legal limit. (Why would anybody want to do that?) They didn™t make it
illegal to hook up a high-gain antenna, but they did try to ensure that you would use only
antennas specifically designed for the wireless system. They did this by specifying connectors
that you couldn™t find in your local Radio Shack, and which are incompatible with standard
antenna connectors.
Linksys, for example, satisfies the weirdness requirement with reverse polarity (RP) TNC con-
nectors. At first glance, Reverse Polarity (also called “Reverse Gender”) connectors look like
normal male and female connectors. On closer inspection, however, you™ll find a major differ-
ence: The male connector has a socket, and the female connector has a pin. The Linksys access
points, for instance, have Reverse Polarity female connectors (female body with a pin), so the
96 Part I ” Building Antennas




FIGURE 4-15: Reverse Polarity TNC connectors.


pigtail has to have a Reverse Polarity male (male body with a socket). Figure 4-15 shows
RP-TNC connectors.
Another common example is the reverse polarity SMA connector used on many D-Link prod-
ucts. Figure 4-16 shows RP-SMA connectors.
Don™t confuse RP with Reverse Threading (RT), which apparently also satisfies the weirdness
requirement, and which refers to male bodies threaded to turn counter-clockwise to tighten,
unlike normal male bodies, which turn clockwise to tighten.
The FCC™s desire to thwart users wishing to attach standard antennas to Wi-Fi equipment has
been less than entirely successful. However, the weirdness requirement has created a thriving
pigtail industry, which you will now be supporting with a modest purchase (assuming you fol-
low the advice in the next paragraph).
In Chapter 1, we recommended purchasing pre-configured pigtails for wireless client adapters,
because of the hassle of soldering the tiny connectors needed to fit PC card connections.
Buying pigtails is the easy way to go for access points as well. There aren™t any tiny PC card
connectors to deal with. But you would probably have to special-order the RP connector for
the pigtail anyway, so why not just go ahead and special-order the pigtail itself?


Step 3: Run the Antenna Cable From the Pigtail to the Antenna
Ultimately, you™ll probably install the antenna, and then run the cable to it. However, before
you do that, we suggest that you put everything together in one room. That way, if it doesn™t
97
Chapter 4 ” Modifying Your Access Point




FIGURE 4-16: Reverse Polarity SMA connectors.


seem to be working correctly, you™ll easily be able to do tests (such as cable continuity tests)
without running back and forth between the access point and the antenna.
When you do install the cable in its final position, avoid crimping and excessive bending of the
cable. Also avoid possible sources of interference, which would primarily be other equipment
operating in the 2.4 GHz band (like cordless phones and microwave ovens), but could include
sources of broad spectrum RF such as fluorescent lights.
The shorter the cable, the better. Consider locating the access point and the antenna within a
few feet of one another, as this will give you the highest power output for your antenna. In out-
door installations, this may not be possible, since you may need to put the antenna in a location
where power cannot easily be run. However, indoors, you should be able to get power almost
anywhere fairly easily, even if it means running a long extension cord.


Step 4: Position and Install the Antenna
Positioning the antenna may be an iterative process. If you have multiple possible locations for the
antenna, you™ll want to do some testing in each location before settling on the best one. Mounting
the antenna higher up, like on a wall or on top of a file cabinet, is usually better. Think light bulb or
flashlight. Does the room light up better when the light is near the ceiling away from obstructions
or down on the floor behind a desk? Figure 4-17 shows a typical install of a high-gain antenna.
98 Part I ” Building Antennas




FIGURE 4-17: A wall-mounted high-gain antenna.




FIGURE 4-18: Antenna on a pole during a site survey test point.
99
Chapter 4 ” Modifying Your Access Point


Do not damage the housing of the antenna, or any metallic surfaces. Often, an antenna will use
a metal backplane as a reflector to increase the gain. If you drill through that reflector, the gain
will be adversely affected.
As a final step, perform another site survey as described earlier in this chapter. This time, use
the new configuration with the new antenna. Figure 4-18 shows a site survey test point with a
high-gain antenna.
For maximum mobility, fill a sports bag with a 12-volt battery, a DC-to-AC inverter, and an
access point. The access point is configured to transmit a beacon so the client can monitor signal
strength. This setup keeps you highly mobile for hours during the survey”no power cords.




What About Signal Amplification?
This is an appropriate place to mention the alternative of using amplifiers, as opposed to
antennas, to boost signals. For instance, Linksys sells a Wireless Signal Booster (WSB24)
that amplifies both transmit and receive signals. Tests indicate that amplifiers are effective
for improving connection reliability and throughput within your existing coverage area.
However, a high-gain antenna is still the way to go if you need to expand your coverage
(see www.smallnetbuilder.com/Reviews-38-ProdID-WSB24-1.php.).
Figure 4-19 shows a test of the Linksys WSB24 using NetStumbler in power monitor mode.




FIGURE 4-19: Linksys WSB24 adds power to a Linksys access point.
100 Part I ” Building Antennas


RF signal amplifiers work in different ways. Make sure you understand the input and output
requirements before investing in an antenna and configuring your access point. For example, the
Linksys WSB24 requires antenna diversity to be enabled on its input. It then converts the signal
to only transmit through the Right and receive only on the Left antenna jacks.


Summary
In this chapter, you™ve learned the basics required to successfully install a high-gain antenna, a
process that is usually not terribly difficult or expensive, and can yield highly gratifying results.
Not only do you have instructions for installing the antenna, you have a game plan for doing
the job right, determining your equipment needs based on a site survey, a link budget, and legal
and safety restrictions.
Now that your stay-at-home network is super-charged, read on to Chapter 5, “Gearing Up for
War Driving,” to learn about taking wireless networking on the road”peering into the invisi-
ble world of wireless networks operating all around you.
part
War Driving ”
Wireless Network
Discovery and
Visualization in this part
Chapter 5
Gearing Up for War
Driving

Chapter 6
War Driving with
NetStumbler

Chapter 7
Mapping Your War
Driving Results
chapter
Gearing Up for
War Driving
M
ost computer enthusiasts find the idea of seeing the invisible
radio waves of wireless networks somewhat thrilling. There is a
sort of voyeuristic interest in seeing Wi-Fi hotspots appear on
your screen. After driving a few blocks, you start to see the names others
have come up with, the type of equipment, where these networks are, and
in this chapter
so on. It™s like peering into the ether and seeing a whole new world around
you, unseen to those without the right tools. We find it quite compelling.
Selecting a wireless
It™s pretty simple to get started war driving. This chapter will show you how
adapter
to gather the components needed for a war drive. We will install the system
into a car, go on a drive and record what we find. You may be surprised at
Choosing your
what™s out there!
software
You will need the following items:
¤ Laptop computer ”To run the war driving software and record Using GPS
results
¤ Wireless network adapter ”To scan the airwaves for wireless net- Setting up your
mobile system
works
¤ External antenna and pigtail ”To increase range
Going for a war
¤ Some form of mobility, like a car, bike, boat, stroller, or even feet
drive
¤ Scanning software ”The actual program doing the scanning
¤ GPS unit ”Optional; use this to plot hotspots on a map



Overview of the War Drive
Imagine yourself driving late at night. You have a full tank of gas, it™s dark,
and a faint electronic glow illuminates the right side of your face. As each
house or building passes by, your laptop blips out another group of unusual
words like tsunami, default, dog house, taffy, 101, spock, or who knows?
Or picture yourself driving home from work, taking the scenic route ”
through the commercial district. Just to see what pops up.
104 Part II ” War Driving


Perhaps on a road trip, you are passing trucks as if they are standing still. As you approach a
weigh station, a blip pops up on your laptop. Hmm... a new access point. This one reveals the
presence of a Wi-Fi weigh station network.
You™ve started to experience the allure of war driving. Invisible waves pop up on your computer
screen revealing the unknown and unseen.
The act of driving a car equipped with a computer, a wireless card, and software designed to scan
for wireless networks has come to be known as war driving. The term war driving derives from an
idea from the early 1980s to dial many telephone numbers to find a computer modem: war dialing.
The term itself was coined from the dialing program made popular in the 1983 movie WarGames.
One of the interesting aspects of war driving is that you will find wireless access points where
you least expect it. From a deserted highway in the middle of nowhere to a rural truck stop to a
bustling cityscape, wireless networks are exploding onto the airwaves. The phenomenon is
quite remarkable.
Here are some places to visit to get you started:

Your own neighborhood
Industrial parks
Downtown
Highways and freeways
Off the beaten path
Take your rig on a family road trip!

Figure 5-1 shows the view during a rare daytime war drive.
The original term war driving has also spawned a host of derivatives applying to many situa-
tions in which people scan while not actually driving ” for example, war walking, war strolling,
war boating, and war flying. They all mean one thing: looking for wireless networks, usually
while moving. We prefer to scan while driving.

Some people have promoted the idea that the war in war driving is actually an acronym for
“Wireless Access Reconnaissance.” This is really an after-the-fact case of creating an acronym
for a simple word. But it sure sounds less ominous than war.

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