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.
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Chapter 1 — Building Your Own Wi-Fi Antenna Cable
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.”
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 pigtailacts 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.
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32 Part I — Building Antennas
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.
FIGURE1-27: Various Wi-Fi connector types.
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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.
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34 Part I — Building Antennas
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Building a Classic Paperclip Antenna
Have 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 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.
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.
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 than paperclips stuck together. This model is commonly called the Frisko 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 paperclip antenna.
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 that the pattern of emissions is often blocked by the computer itself. Not 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 antennatransmits its information in a single direction, while an omni antennatransmits the information in all horizontal directions.
Understanding antennas
Types of antennas Antenna power and
direction Antenna
polarization Making your own
paperclip antenna
in this chapter
chapter
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36 Part I — Building Antennas
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 FIGURE2-1: A finished paperclip antenna.
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Chapter 2 — Building a Classic Paperclip 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 antennais 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 FIGURE2-2: Top view of coverage for a directional and an omni antenna.
FIGURE2-3: Side-view of an omnidirectional antenna signal.
Directional Antenna
Omni Antenna
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38 Part I — Building Antennas
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 armof 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- 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!
FIGURE2-4: A dipole antenna made out of paperclips.
5.15 cm 5.2 cm 5.35 cm 5.8 cm
Platform can be any length
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Chapter 2 — Building a Classic Paperclip Antenna
FIGURE2-5: A cut-away interior of a popular coaxial antenna.
Vertical Driven Array Antenna
The driven array antennais 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.
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40 Part I — Building Antennas
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.
FIGURE2-6: Magnetic-mount driven array antenna.
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Chapter 2 — Building a Classic Paperclip Antenna
Yagi Antenna
A Yagi antennais 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.
FIGURE2-7: Horizontal and vertical beam patterns for a directional antenna.
Back
Back
Front
Front Left
Right
Top
Bottom Azimuth
Horizontal Top-View
Elevation Vertical Side-View
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42 Part I — Building Antennas
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 antennais 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.
FIGURE2-8: A high-gain Wi-Fi Yagi antenna.
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