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bottoms out at the first stop of the pin. Also, ensure the pin rests within 1“2 mm of the foam
dielectric.
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Chapter 1 ” Building Your Own Wi-Fi Antenna Cable




FIGURE 1-19: Placing the center pin onto the conductor.




Step 9. Crimping the Core
This is the first of two crimps for the connector. Ensure your crimp tool has the correct
die for the type of cable and connector being used. For LMR-400, the crimp tool should
have a die with hex sizes of 0.429 for the outer ring, and 0.128 and 0.100 for the center
pins.
The center pin for an N-Male connector is crimped using size 0.128.

When you crimp coax cables, press all the way down once only. The hex design of the crimp tool
die ensures the pin will grip the core properly in six places. If another crimp is applied to “make
it tighter,” it could misshape the pin.

Place the pin into the crimp tool as shown in Figure 1-20. The bottom edge of the pin usually
will have a small ridge to help line it up and keep the pin seated on the core.
Crimp down with even, strong pressure. If your crimp tool has the ratcheting feature, it will
apply only the necessary amount of pressure before releasing.
26 Part I ” Building Antennas




FIGURE 1-20: About to crimp the center pin onto the center conductor.



Figure 1-21 shows a properly crimped center pin. Notice the marking around the edges where
the crimp actually clamped the pin to the center conductor.


Step 10: Placing the Connector Body
It™s time to place the connector body over the pin. Figure 1-21 shows the connector about to
slide onto the pin.
Before continuing, be very sure that the crimp ring from Step 2 is still waiting for you
down the cable behind the splayed out shield before you place the connector body onto the
cable.

Ensure the crimp ring is on the cable before snapping the connector shell into place. Once the
shell is snapped into place, it will be difficult to remove. Also, removing and replacing the shell
would degrade the cable performance. If the ring is not in place, you™ll need to cut the connec-
tor off and rebuild the cable with a new connector.

Line up the connector, and begin to slide the connector over the pin, over the dielectric foam,
and butt it up against the shield strands. If all goes well, there may be an audible click when the
27
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable




FIGURE 1-21: Moving the connector into position.



connector mates with the pin. This mating is meant to hold the connector on the pin until the
last crimp.
Tug lightly at the connector like you are going to pull it back off. It should stay in place under
light pressure. If forced or yanked, it may come off, so be gentle.
Figure 1-22 shows the connector fully inserted with the shield still pulled back. Notice that the
center pin does not extend past the inner ring of the connector.


Step 11: Shields Up!
Fan out the shield strands and trim down with scissors, as shown in Figure 1-23. To help
cleanup, hold the cable over a wastebasket. The goal is to trim down the shield but still have
enough to fit under the crimp ring. Trim the shield down to about a quarter of an inch.

Shield strands are made of steel. The thin wires can pierce the skin like a needle in some circum-
stances. Make sure to handle the waste strands with care, and clean up the area to minimize the
chance of accidents.
28 Part I ” Building Antennas




FIGURE 1-22: Connector and pin are in position.




FIGURE 1-23: Trimming the shield with scissors.
29
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable




FIGURE 1-24: The shield splayed out under the crimp ring.




Step 12: Placing the Crimp Ring
Now pull up the crimp ring you placed in Step 2. The shield will slip under the crimp ring and
should be splayed out evenly around the connector body, as shown in Figure 1-24.
If the shield is still too long, move the crimp ring out of the way and trim a little more of the
shield with the scissors. Try to get just enough shield under the crimp ring, but not sticking out
past the ring.



Step 13: Crimping the Ring
Finally, it™s time to crimp the crimp ring onto the cable. This is the second of the two crimps
needed to make the cable. As in Step 9, use the crimping tool. But this time crimp with the
larger diameter hex size of 0.429.
Place the tool at the upper edge of the crimp ring, butted against the connector body as
shown in Figure 1-25. Crimp with strong, even pressure, and only crimp once, just like in
Step 9.
30 Part I ” Building Antennas




FIGURE 1-25: Crimping the crimp ring onto the cable.


Step 14: Inspecting the Finished Product
Now that the cable is complete, it™s time for a visual inspection. Check the back of the connec-
tor at the seam of the crimp ring. If there are any shield strands sticking out, cut them off with
the razor blade, as shown in Figure 1-26.




FIGURE 1-26: Cutting off the pokey bits.
31
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable


Clipping off the loose strands at the back of the connector reduces the chance of injury when you™re
screwing on the cable. Loose strands are like splinters that may pierce the skin of unwary fingers.




Visually check the front of the connector for any loose bits of metal that may have
dropped into the connector during construction. If you find any, remove them to prevent
shorts.
That™s it! Now repeat steps 1 to 12 for the other end of the cable. After doing a few of these,
it will become second nature. With practice, building a cable connector can take just a few
minutes.


Choosing a Wi-Fi Pigtail
A pigtail acts as a converter between large diameter cables and small connecters commonly
used on Wi-Fi cards.
Because of the very small connector sizes, pigtails are difficult to build and require highly
skilled soldering techniques. We recommend that you purchase pigtails for use in your projects.
Several online stores sell pre-built pigtails in specific lengths.
To purchase a pigtail, the vendor will need to know a few things:

Length of pigtail: should be less than 2 feet to keep signal loss low
Cable connector: the type of connector to plug into the larger cable (usually male)
Device connector: the type of connector to plug into the Wi-Fi device (usually female)

The device connector is specific to the type of Wi-Fi card or access point being used.



Table 1-5 Connector Types for Common Wi-Fi Products
Connector Type of Product Wi-Fi Product

MMCX PCMCIA Card Cisco, Engenius, Proxim, Senao,
Symbol
MC-Card PCMCIA Card Apple Airport, Avaya, Orinoco
RP-SMA Access Point, PCI Card, Wi-Fi Belkin, D-Link, Linksys, Netgear,
Camera, Wireless Media Adapter SMC, U. S. Robotics
RP-TNC Access point, Bridge, Wireless Linksys
Booster
MCX Base Station, Adapter Card Apple Airport Extreme
Note: RP in the connector designation refers to “Reverse Polarity.”
32 Part I ” Building Antennas




FIGURE 1-27: Various Wi-Fi connector types.


Connector Types for Wi-Fi Cards
There are almost as many connector types as there are Wi-Fi device manufacturers. Table 1-5
lists some of the most popular connectors.
The MMCX, MC-Card, RP-SMA, and RP-TNC male connectors and some of the female
devices to which they attach are shown in Figure 1-27.



Finding Pigtails
Pigtails are not available in stores. They must be purchased from vendors that construct them
on a regular basis. Sometimes you can find them locally at swap meets or user group meetings.
Usually it™s easier to buy them online. Here are some popular sites:

www.ecwest.com
www.fab-corp.com
www.hyperlinktech.com
www.wlanparts.com
www.ydi.com

These stores generally sell antennas, wireless devices, and cables as well as pigtails.
33
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable


Cheap Cable Testing
When a transmission problem arises in a Wi-Fi system, the first place to look is at the cables
and connectors. Connectors generally take the most physical stress in a system, and also can be
the first piece to break down while operating in poor conditions. The middle of the cable or the
inside of an antenna is less likely to sustain damage if stressed when compared to the cable
ends and connectors.
This is where simple cable testing can be of great value to troubleshoot a system. To check for
continuity and for shorts, use the ohm-meter function on a multimeter. Test the entire length
of the cable through each connector.

1. Check for continuity from center pin to center pin. This should be a short or zero ohms.
2. Check from connector body to connector body. This should be a short or zero ohms.
3. Check from center pin to connector body. This should be open or infinite ohms.

Often when you™re testing a cable, it™s already installed on-site, which limits access to the cable
ends. To get around this, disconnect both ends of the cable and short the center pin to the con-
nector body on one end only. Then measure resistance of the pin to the body on the other end.
The resistance should still be zero ohms (or very close).

For the unlimited budget, products like a time domain reflectometer (TDR), spectrum ana-
lyzer, RF Power meter, and network analyzer can be used to test entire transmission systems,
including the cable. These usually cost several thousand dollars to buy and hundreds to rent.
If the connector is presumed bad, replacing it is often much less costly than extensive testing.
And very often, the only way to fix a bad connector is to replace it and start over.


Summary
Wi-Fi is radio at microwave frequencies. Transmission lines at 2.4 GHz are more prone to sig-
nal loss and must therefore be considered an important part of the entire Wi-Fi system.
A low-loss, large diameter cable and a pigtail adapter makes it easy to position the antenna for
the best radiation pattern and signal strength.
Building connectors on-the-fly opens up a new realm of independence. By obtaining the cable
in bulk, and the various types of connectors, it becomes a simple process to build your own cus-
tom cables tailored to each application. And the cable will be exactly as long as necessary.
Read on to the next chapter to explore antennas: how antennas are defined, antenna radiation
patterns, choosing an antenna, and pros and cons of high-gain antennas. You™ll even see how to
build a simple omni antenna that will boost your range by up to 200 feet.
34 Part I ” Building Antennas
chapter
Building a Classic
Paperclip Antenna
H
ave you ever had this survivor fantasy? You™re stranded on a desert
island (with your laptop of course!) and desperately need to con-
nect to the outside world. You empty your pockets on the ground,
and find nothing but commonplace objects. But then... in a frenzy of
creative brilliance you whip all the ordinary pocket-objects together to
in this chapter
come up with a jerry-rigged wireless antenna. Within minutes you are
e-mailing potential rescuers from the beach, and surfing your favorite
sites to kill time. Understanding
antennas
Of course, that is only a fantasy. But, this chapter offers you a project
which comes pretty close to the fantasy in both results and ingenuity.
Types of antennas
This chapter will show you how to put together the ultimate “homebrew”
antenna”a working Yagi antenna for 2.4 GHz Wi-Fi out of little more
Antenna power and
than paperclips stuck together. This model is commonly called the Frisko
direction
antenna, after the French Frisko brand of ice cream cups whose wooden
spoons were used in the first prototypes. Figure 2-1 shows a completed
Antenna
paperclip antenna.
polarization
The current designs of most external Wi-Fi cards put the antenna in a
flawed position, with the antenna very close to the computer. This means
Making your own
that the pattern of emissions is often blocked by the computer itself. Not
paperclip antenna
only that, the small packaging of wireless cards prevents an optimal design
for the internal antenna to pick up wireless network devices more than a
couple of 100 feet away.
This is one of the reasons that attaching even a small external antenna like
the one in this chapter can greatly improve signal strength, especially if it
is oriented properly.


Recognizing Different Antennas
If you do any research on antennas, you will notice that there are several
different types of antennas around. Two common types are directional and
omnidirectional. The difference between these two types of antenna is a
simple but important one. A directional antenna transmits its information
in a single direction, while an omni antenna transmits the information in
all horizontal directions.
36 Part I ” Building Antennas




FIGURE 2-1: A finished paperclip antenna.


In addition, you need to understand how antenna efficiency, or gain, works as related to other
antennas. Antenna gain is measured in decibels, isotropic (dBi), defined as the strength of an
antenna as related to a theoretical sphere around an imaginary antenna. dBi is a logarithmic
measurement, so every 3 dBi is a doubling of gain. What you need to know is the higher the dBi,
the more sensitive and focused the antenna.

An omni antenna sends and receives signals equally in front, behind, to the left, or to the right of
the antenna. However, when you go above or below the antenna, signal strength drops off sig-
nificantly. The trade-off you make when choosing a high-gain antenna is this focusing, or thin-
ning, of the above and below energy. The low-gain omni works better vertically than a high-gain
omni, but it won™t extend as far horizontally.

Figure 2-2 shows a diagram of two antennas viewed from the top. The directional antenna is
most sensitive in one direction, meaning signals being sent and received by the antenna will be
strongest in the direction the antenna is pointing. The omnidirectional antenna sends and
receives signals in all directions equally. This is a generalization, but it™s mostly accurate. Later
chapters will delve further into the specifics of antenna operation.

Since directional antennas direct their information at a specific target (or at least in the direc-
tion of the target), they require less power to transmit, but more precision in their placement.
Omnidirectional antennas need little precision in their placement, but require more power to
send and receive signals.
You are probably familiar with these different types of antennas, because you see them almost
every day. A satellite dish would be considered highly directional, looking up into space, while
37
Chapter 2 ” Building a Classic Paperclip Antenna




Omni
Directional
Antenna
Antenna
FIGURE 2-2: Top view of coverage for a directional and an omni antenna.


the antenna on your car is omnidirectional, listening to radio no matter which direction your
car is facing. Let™s take a closer look at these different types of antennas.


Omni Antennas
The omnidirectional antenna is probably the most common Wi-Fi antenna available. Just
about every Wi-Fi device you can buy comes with an omni antenna. This is because the omni
is so easy to set up, and generally works in consumer environments without much planning.
There are a few different types of omni antennas. Omni signals spread out sideways, but not
vertically (see Figure 2-3).

Even though an omni antenna does not work very well above and below, it is not considered a
directional antenna. Wi-Fi antennas are generally rated in two-dimensional space that assumes
it is mounted parallel to the Earth™s surface. Knowing how the beam is shaped, and that an
antenna is not truly omnidirectional will help you choose the right antenna for your Wi-Fi toys.


Dipole Antenna
The dipole antenna is just about the simplest antenna there is. The dipole is a half-wave antenna
that consists of two opposing radiating elements. It™s made up of two quarter-wavelength poles




FIGURE 2-3: Side-view of an omnidirectional antenna signal.
38 Part I ” Building Antennas




Platform can
be any length




5.2 cm 5.35 cm 5.8 cm
5.15 cm
FIGURE 2-4: A dipole antenna made out of paperclips.



that are not connected to each other and fed in the middle by the transmission line. A standard
dipole is open on each end, but it can also be folded over on itself. The dipole you will build in
this chapter is a folded dipole.
Figure 2-4 shows a simple dipole made from steel paperclips. Each arm of the dipole is 31 mm
in length, or 1/4 of a wavelength for Wi-Fi channel 6. The center conductor is soldered to the
right arm, while the shield is soldered to the left arm. It doesn™t matter to which side you solder.
The dipole antenna is unique in that it can be mounted vertically or horizontally. When stand-
ing vertically, the dipole antenna is omnidirectional. When horizontal, this antenna will radiate
outward in two directions off the sides (and slightly upwards), like turning a donut on its edge.

Coaxial Antenna
A coaxial is another common antenna used in Wi-Fi. It™s used on most wireless access points
you can buy. If the access point has a stubby little antenna on it, chances are it™s a coaxial
antenna. The coaxial antenna works in much the same way as a dipole antenna. The construc-

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