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tion is slightly different though. The antenna feed comes up through the bottom with a metal
casing around the shield-connected arm. Coaxial antennas are usually a total of half a wave-
length with each arm being one quarter wavelength of the frequency.
The antenna in Figure 2-5 is from a common access point with two antennas. The plastic on
one of the antennas is removed to show you the actual antenna element. You can see that the
cable runs through the base of the antenna. The center conductor extends to the top while the
shield is soldered to the metal cylinder that becomes the base radiator.

Antennas are not really affected by plastic, rubber, and other nonconductors of electricity.
When determining antenna shape, you can sometimes get an idea from the outer covering.
However, you will need to check the antenna specifications to be sure of the design. Or build
it yourself!
39
Chapter 2 ” Building a Classic Paperclip Antenna




FIGURE 2-5: A cut-away interior of a popular coaxial antenna.


Vertical Driven Array Antenna
The driven array antenna is often used for mobile applications. This is a vertical antenna with
gain created by multiple segments of half-wavelength elements arranged vertically end-to-end
to achieve gain. An array is simply more than one element working together.
The driven array means that each element has an electrical connection with the one next to it.
The signal is driven into each radiating element via an antenna coil that maximizes the transfer
of energy between adjacent elements.
Figure 2-6 shows a magnetically mounted driven array antenna with one quarter-wavelength
element on the bottom and one half-wavelenegth element on top separated by a coil. The coil
is used to match the antenna elements to each other. If an antenna has a coil on it™s structure, it
is most likely separating antenna elements.

A driven array connects elements directly and electrically. A parasitic array connects passively
without a direct electrical connection to the driven element.
40 Part I ” Building Antennas




FIGURE 2-6: Magnetic-mount driven array antenna.




Directional Antennas
You will build a directional antenna in this chapter. A directional antenna increases gain in
one direction. By becoming sensitive in a single direction, the directional antenna is a good
choice for setting up links between distant objects with a known location. Directional anten-
nas are well suited for the corner of a room, side of a building, or in a hand-held mobile
environment.

Directional antennas generally only work well in one direction. The design of the antenna
determines the field of view, or beam pattern, for the antenna. Antenna beam width is mea-
sured in degrees of a circle, as viewed from the top or the side. The top view is measured as ver-
tical beam width. The side view is measured as horizontal beam elevation. Figure 2-7 shows
these measurements for a directional antenna.

Directional antennas are very helpful in pinpointing a signal location, or for establishing a long-
distance link. The antenna you build in this chapter and in Chapter 3 will help you later in the
book.
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Chapter 2 ” Building a Classic Paperclip Antenna


Azimuth
Horizontal Left
Top-View




Front
Back




Right




Top




Front
Back




Elevation
Vertical
Bottom
Side-View

FIGURE 2-7: Horizontal and vertical beam patterns for a directional antenna.



Yagi Antenna
A Yagi antenna is a highly directional parasitic array antenna. The shaping elements are not
electrically connected to the driven element. The Yagi basically consists of a driven element, a
reflector, and two or more directional elements. Figure 2-8 shows a common Yagi antenna with
14 directional elements and one reflector.
In a very basic sense, the radiating element of the Yagi is the only part that actually receives a signal.
The other components bend and shape the pattern of RF energy for that single element. It works
something like this: A transmitted signal comes up the cable and leaves the driven element. It hits
the reflector and bounces toward the front of the antenna. Each directional element then carries that
signal further while making it stronger. When the signal leaves the last element of the antenna, it™s
focused in a single direction. The reverse is true for signals being received by the antenna.
The antenna you will build in this chapter is a Yagi antenna. There is a driven element, a reflec-
tor, and two directional elements. While the Yagi in Figure 2-8 uses aluminum and fiberglass,
you will construct yours of steel and wood.
42 Part I ” Building Antennas




FIGURE 2-8: A high-gain Wi-Fi Yagi antenna.




Parabolic Antenna
A parabolic antenna is very intuitive when you see one, like the one shown in Figure 2-9. The
rear portion of the antenna is a curved reflector that bounces incoming signals into the focal
point of the curve. A small antenna is placed at the focal point and becomes the antenna feed
point. The feed point usually has a half-wave dipole or other basic antenna. For transmitted sig-
nals, the reverse is true as signals bounce off the reflector out into the distance.
Parabolic antennas have very high gain and are very directional. They are most often used for
direct links from one station to another. When deploying a parabolic antenna you need precise
physical aim.


Panel Antenna
A panel antenna is an array of rectangular flattened dipole antennas arranged in a pattern
on a panel. These flattened dipole antennas are sometimes called patch antennas. Because
the patches are laid out in an array, the shape of the radiation pattern is aligned and
focused in one direction. The more patches in the array, the more focused the antennna
and the higher the gain. Figure 2-10 shows the inside array of a very high-gain panel
antenna.
43
Chapter 2 ” Building a Classic Paperclip Antenna




FIGURE 2-9: Picture of a parabolic grid antenna.




FIGURE 2-10: The internal components of a common high-gain panel antenna.
44 Part I ” Building Antennas


Panel antennas work well in one direction off the face of the antenna. The metal backing sur-
face is employed as a reflector and mounting point for the panel antenna. The face of the
antenna is often covered in nonconducting plastic for weather proofing and to help prevent
damage.

Waveguide Antenna
A waveguide antenna is very strange indeed. This type of antenna is actually formed by the
space and the surface surrounding an electrically conductive enclosure.
The size and shape of the waveguide determines the frequency at which the waveguide will
operate best. A small driven element in a precise location near the rear of the waveguide creates
the signal that is shaped by the metal surface of the enclosure. The shape of the enclosure
directs the beam pattern outward, away from the opening. Figure 2-11 shows a basic home-
made waveguide antenna made from a tin can.
Waveguide antennas are often built from aluminum. However, a tin can is a very good
conductor, and it™s the perfect size and shape for the waveguide antenna you will build in
Chapter 3.




FIGURE 2-11: A waveguide antenna you can build.
45
Chapter 2 ” Building a Classic Paperclip Antenna


Understanding Antenna Polarization
Antenna polarization stems from how an antenna radiates energy. The design of an antenna
forces certain physical and electrical characteristics. As radio frequency (RF) energy is shaped
and radiated by the antenna, the antenna changes the shape and beam pattern of the RF.
Antennas are usually designated with vertical or horizontal polarization. At this point, the
important thing for you to know is that polarizations like each other. For example, a vertically
polarized antenna will work best receiving signals from another vertically polarized antenna
(vertical-to-vertical). In fact, the signal strength can be 100 times less if you use mismatched
polarization (vertical-to-horizontal).
You can use mismatched polarization to your advantage when working with closely placed
antennas. The solar-powered repeater you will buld in Chapter 9 uses two antennas mounted
on the same pole. To help keep them from interefering with one another, they are placed in
opposing polarizations (horizontal and vertical).
The end-result of all this polarization talk is that you will want to hold your antenna vertically
upright (vertically polarized) to pick up a typical store-bought access point (which is also verti-
cally polarized). When we say “vertical” we mean that the paperclips are sticking straight up
and down. “Horizontal” is when it is flat as compared to the ground.


Before You Start
Before you start this journey, you should understand one thing: You don™t end up with a power-
ful, durable antenna for long-term use. For that you want to build, or buy, a more rugged, com-
mercial-grade unit. In Chapter 3, we will show you how to make the famous coffee can
waveguide antenna, which can be made quite a bit more powerful and sturdy than this
lightweight paperclip model.
But, with that cautionary note, this paperclip antenna will work. It will extend the range of the
antenna built into your wireless Network Interface Cards (NICs), and can produce a gain of up
to 9 dBi. And you will be able to point to it with pride as your entry in that unofficial scavenger
contest to push more and more bits for less and less cost.
There are more efficient and probably more effective ways to build a Wi-Fi antenna than using
a paperclip. But are there any more fun? What could be better than using the humble paper
clip as the central ingredient for your next antenna?
For many reasons, the paperclip appears over and over as the ultimate MacGyver tool. It is dirt
cheap, found anywhere and easily hidden in the palm of the hero™s hand even while his captors
pat him down. How many times have you seen the paperclip open handcuffs and locks in the
movies?

As with many of the projects in this book, local laws may regulate the usage of such an antenna.
You should familiarize yourself with the local rules and regulations before you dive in.
46 Part I ” Building Antennas


What You Need
Use the following list to collect and prepare the items you™ll need to build your antenna. I™ve
listed dimensions in both metric and foot/inches units where possible.

I™ve included eye protection in the list of items you need to have before you start. Don™t skip this,
or other safety precautions”please!




Here™s what you need:

Four large paperclips (the largest has to be at least 11.49 cm (4.52 inches) when straightened)
A flat wooden spoon, the kind that comes with ice cream cups”or some other suitable
platform for supporting your antenna prongs (perhaps a floppy disk)
Wi-Fi pigtail cable connected to your laptop NIC (wireless PCMCIA card with an
external connector).
Solder iron and solder. An iron in the 15 W to 30 W range will work fine. Thin rosin
core solder (0.75 mm is a good size) is preferred for electronic work because acid core
solder will corrode components.
Small bottle or tube of craft glue (virtually any kind will do)
Small wire cutters
Needle-nosed pliers
A ruler that marks tenths of an inch (such as a drafting ruler from a craft store) or a met-
ric ruler with markings for millimeters
A pen for marking hole locations
Eye protection for cutting wires and soldering
A pair of vise grips or (even better) a small tabletop vise (the kind used for making fish-
ing flies) for holding your antenna securely while working on it
A drill with a bit slightly smaller than the diameter of the paperclips or else a thin wire
brad and a light hammer for tapping it through the wood.

You can get more background information on this antenna design at the following Web sites:

www.xaviervl.com/Antenne/Frisko/
www.seattlewireless.net/index.cgi/MicroTVAerial
www.wifi-montauban.net/communaute/index.php/DisquettAntenna

The pioneers of the paperclip antenna speak French, and their Web sites are in the French
language, but they include translation links for English (and other) readers.
47
Chapter 2 ” Building a Classic Paperclip Antenna


Some popular Web search engines provide translation tools for dozens of languages. Visit the
language section at www.altavista.com or www.google.com and use the Language
Translation links to view the sites in other languages. Sometimes the translated text is a little
choppy, but the meaning comes through.



Choosing a Wireless Card
You will have more success in your paperclip antenna project if you are working with a good
wireless card. Wireless cards can come with or without built-in external connectors.
The cheaper cards without such connectors are mainly bought by corporations that assume their
employees will only be connecting to the nearby access points in the workplace environment.
It is possible to access the internal connectors of wireless cards (NICs) that don™t have such
external jacks”but doing that is its own separate bit of technical wizardry that involves “crack-
ing the case” of the NIC to get at its connectors.

Close-up photos of this feat are online at www.ivor.it/wireless/pigtail.htm.




However, 802.11b cards are now much cheaper than they were in the past, and it generally just
makes more sense to buy a new card that comes with built-in external connectors you can
attach the coaxial pigtail to (see Figure 2-12). These come in several varieties (“MC-Card,”




FIGURE 2-12: Card and pigtail.
48 Part I ” Building Antennas


“MMCX,” and others) so make sure that your pigtail connector matches the kind you have on
your wireless card. Increasingly, the cards seem to have standardized on using female MMCX
connectors”for which the pigtail then needs a male MMCX connector.



Choosing Platform Materials
This antenna was first made using a wooden spoon from a French ice cream cup called “Frisko”
(hence its name). It is just large enough to accommodate the four prongs, and just thick
enough to allow them to stand up well.
However, the mind boggles at all the many possible materials that can be used. These antennas
have been made with medical tongue depressors, floppy diskettes, and small cardboard tubes.
We used a jumbo-sized craft stick, also known as a popsicle stick.
Credit cards don™t work well, because they are too thin. They don™t support the prongs well
(without mounds of glue) and so the prongs tend to flop over and even touch.
You can glue a wooden clothespin to the platform you select, so that the antenna can be easily
clipped on a stable mount, like the side of your laptop screen.



Building the Paperclip Antenna
Building the antenna is a seven-step process, as follows:

1. Prepare the antenna elements
2. Get the mounting platform ready
3. Create the driving element or dipole
4. Prepare the pigtail
5. Attach the pigtail to your new antenna
6. Secure the pigtail
7. Insert the last few elements into the antenna

Figure 2-13 shows the basic components and how they come together. Note that the rounded
paperclip (piece 3) is the actual driven element of the antenna. That is, it™s the wire getting the
radio signal from the wireless card. The other paperclips (pieces 1, 2, and 4) shape the beam to
make it more directional.


Step 1: Preparing Your Wire Prongs
Each radio frequency has a specific wavelength. To function as an antenna, the dipole loop has
to be half the length of that wavelength.
49
Chapter 2 ” Building a Classic Paperclip Antenna




Platform can
be any length




5.15 cm 5.2 cm 5.35 cm 5.8 cm
FIGURE 2-13: Paperclip antenna diagram with dimensions.


Take your needle-nosed pliers and carefully straighten four large paper clips. Cut them to the
lengths indicated in Table 2-1.


Step 2: Preparing Your Antenna Platform
Carefully mark on the wooden platform the five places where the wires of your antenna will be
passing through. See Figure 2-14 for hole spacing for the paperclips.

This design is optimally tuned for reception on Wi-Fi Channel 6 (the approximate middle of the
frequency band). It will also work on the other Wi-Fi channels. But if you really want to get peak
frequencies, paperclip length and distance apart from each other will differ for each channel. See
the section How Are Dipole Dimensions Calculated later in this chapter.

Using your hand drill, drill the five holes using a drill bit slightly smaller than the paperclip
wires.



Table 2-1 Paperclip Lengths in Inches and Centimeters
Element Inches Centimeters

Piece 1 2.03 5.15
Piece 2 2.05 5.2
Wire that will be bent into your dipole 4.52 11.49
Piece 4 2.28 5.8
50 Part I ” Building Antennas




4 mm




2 cm
1.5 cm 1.5 cm

FIGURE 2-14: Diagram showing spacing between elements.


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