Insert the remaining three wires in their appropriate locations as “elements.” The prongs don’t need to be centered on the platform, but it is best to center them in relationship to each other. (See Figure 2-21.)
Touch each wire with a drop of glue where it passes through the platform, just to hold the wires in place. (See Figure 2-22.)
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Chapter 2 — Building a Classic Paperclip Antenna
FIGURE2-21: Inserting the reflector and focus elements.
FIGURE2-22: Applying glue to hold things in place.
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56 Part I — Building Antennas
Mounting and Testing Your Paperclip Antenna
Now let’s take this baby out for a spin!
1.After you have soldered the pigtail to the antenna and secured it with tape, connect the pigtail to your wireless card.
2.Mount the antenna and try it out.
3.If you have glued a clothespin or clamp to the wooden platform, you can clip it to vari- ous objects, so that the antenna itself is either vertically or horizontally polarized.
4.Position and aim the antenna in search of the strongest signal.
5.Observe (and learn about) the link quality differences with the antenna in each position.
Wireless networking software should come with some program or component used to measure signal strength on your computer. In Windows XP, the Wireless Network Connection Status dialog displays a Signal Strength bar graph. The more green bars that light up in the display, the stronger the signal.
FIGURE2-23: Signal strength survey in progress.
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Chapter 2 — Building a Classic Paperclip Antenna
You can also use the software that came with the wireless adapter. This software will have some form of signal strength meter. Figure 2-23 shows the signal strength meter for an Engenius Wi-Fi adapter.
Use your “signal strength meter” to see what happens to the signal strength as you vary where the antenna is positioned and how it is oriented. You can adjust and re-orient the antenna for the best connection.
Other software can be used to measure signal strength. See Chapter 6 for details on using NetStumbler as a signal measurement tool.
Hitting the Road with Your Paperclip Antenna
Paperclip antennas are the ultimate cheap tool for connecting to Wi-Fi signals. They’re not the most efficient, but they might be the most fun.
Your new, lightweight, budget-conscious paperclip antenna will expand the receptivity of the antenna that comes with your wireless network adapter. As noted in the beginning of this chapter, you can expect a gain of up to 9 dBi, which is probably two to three times better than your laptop card.
With this type of antenna, your laptop should work better in fringe coverage areas. You can probably use your laptop an additional 100 to 200 feet from the wireless access point.
Experiment with this new range by taking your laptop to a part of your network that doesn’t usually have good signal quality. Attach your new paperclip Yagi and see how the signal strength changes. By pointing away from the access point, it should go down. Also, you can hold the antenna sideways or upright to change antenna polarization. This simple act can change your signal strength by 20 percent or more.
Summary
You now have a nice little gadget that will boost your Wi-Fi signal when the wimpy internal antenna just won’t bring in the signal. By walking around with your new antenna, you should be starting to understand how radio waves move through the air. You saw that just by rotating the antenna from horizontal to vertical and pointing it in different directions drastically affects the signal strength.
By creating this simple antenna, you have entered the realm of microwave RF engineering.
The device you built in this chapter would have been unthinkable a short time ago when these microwave radio frequencies were reserved for military and scientific use.
Read on to the next chapter. We expand on the concept of homebrew by introducing the waveguide antenna—a very powerful, highly focused antenna that can be built using an empty coffee can and some ingenuity. (Plus knowing where to drill!)
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Building a
Directional Tin Can Antenna
In Chapter 1, you learned how to make a Wi-Fi antenna cable that can be used to connect an external antenna to your Wi-Fi card or access point. It is now time to build an antenna and put the cable from Chapter 1 to good use. While there are many commercial antennas available on the market today, they can be expensive. And hey, let’s face it, attaching a commercial antenna to your Wi-Fi network will not turn heads like making your own will.
There are several different types of antennas that you can build. The most famous Wi-Fi antennas are made from either a coffee can or a Pringles potato chip can. In this chapter, you will learn how to build your own antenna from a regular, metal coffee can. You will be able to build it quickly and cheaply. As an added bonus, you will have lots of coffee, which will come in handy in staying awake for the other projects in this book.
Here are the items you will need for this chapter’s project:
➤The coaxial cable you built in Chapter 1
➤Metal can about 4 inches in diameter and 51⁄2inches long (100 mm–135 mm)
➤Type N-Connector
➤Long-nosed pliers
➤Small wire cutters
➤Single-sided razor blade
➤Scissors
➤Hammer
➤Drill
➤Soldering iron and solder
➤Copper embossing material (optional)
Understanding waveguides Finding and
preparing the can Building the
radiating element Constructing the
can antenna Using antenna
simulation software
chapter
in this chapter
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60 Part I — Building Antennas
Types of Can Antennas
There are two popular types of homebrew Wi-Fi can antennas, the Pringles can antenna and the tin can antenna. They both have the same means to an end—increase signal strength in one direction—but they differ radically in operation and construction.
The Pringles can antenna is actually a Yagi antenna with a Pringles can covering used to mount the antenna components. You may recall from Chapter 2 that a Yagi antenna uses a single ele- ment as a radiator, with additional metallic elements. A single reflector element and multiple director elements help to shape the beam into a directional pattern.
In fact, the Pringles can isn’t really a can, it’s just a cylindrical cardboard container. Figure 3-1 shows the internal components of the Pringles can antenna. The primary components are the radiator and the beam-shaping elements. All other components serve to hold the antenna together in the correct position for best efficiency.
While the Pringles can is merely a shell, the tin can is the actual antenna on a tin can antenna.
This is because the tin can antenna is a “waveguide” antenna (see Figure 3-2). That is, the size, shape, and electrical conductivity of the tin can act upon the radio frequency signals. When you place a small radiator in the right location, the dimensions of the can itself will shape the beam and light up the sky.
FIGURE3-1: The popular Pringles can Yagi antenna and the insides.
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Chapter 3 — Building a Directional Tin Can Antenna
A waveguideis a type of radio frequency (RF) transmission path. Where low-frequency systems can use copper wires, like that used in your car radio, high-frequency RF will sometimes use waveguides to route high-power, high-frequency signals. Military radar systems often use waveguide transmission lines.
Understanding Waveguides
A waveguide is a type of transmission line, like coaxial cable (see Figure 3-3). But, unlike coax- ial cables, waveguides can carry microwave frequencies with almost no loss. RF energy as high as 60 GHz or higher travel easily through a waveguide conduit. A waveguide is constructed from metal in a very specific size and shape, usually rectangular. It is also very costly to manu- facture, install, and can be difficult to maintain. Because it’s made from metal, and must be of exact dimensions, waveguide transmission lines are very rigid.
Waveguides exploit a very interesting aspect of electromagnetic RF energy: The duality of elec- tromagnetism. Electromagnetic energy is composed of an electric field and a magnetic field (hence the name). In a coaxial wire, these fields are present along the center conductor and reflected from the outer shield. In a waveguide, these two fields travel along the waveguide FIGURE3-2: The tin can waveguide antenna.
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62 Part I — Building Antennas
without the need for a center conductor. The inner surface of the waveguide essentially directs the signal through the empty space of the interior itself.
Waveguide theory breaks apart all of the elements of radio frequency transmission. The details are quite complicated and can fill volumes. For this book, the important thing to note about waveguides is that size and shape of the waveguide itself is important, and placement of the radiator inside the waveguide is important.
Constructing a waveguide transmission line is difficult. To use a waveguide antenna, however, is a snap. You only need a short portion of the waveguide path to make an antenna. And Wi-Fi frequencies dictate a size and shape that is easily available at any grocery store.
Sizing a Waveguide Antenna
As you know, a waveguide needs to be of specific dimensions. The waveguide antenna, there- fore, must be the correct size for the frequency you are working with. In this case, you are working with Wi-Fi operating in the 2.4 GHz band.
Let’s size this antenna for the middle of the band at channel 6, which has a frequency of 2.437 GHz (see the frequencies in Table 1-2 in Chapter 1). With proper construction, this antenna should operate well across all Wi-Fi frequencies from channels 1 to 14.
To ensure a can that’s sized well, it should follow the dimensions shown in Figure 3-4.
Slotted Waveguide Antenna
Rectangular Circular 90-Degree
Connector
FIGURE3-3: Examples of various waveguides.
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Chapter 3 — Building a Directional Tin Can Antenna
Radiator
Radiator Offset
100 mm
123 mm
FIGURE3-4: Dimensions for a waveguide can antenna.
The dimensions for the can antenna built in this chapter are:
Diameter: ideally 100 mm plus or minus 10 percent (90–110 mm) Length: about 123 mm or a full wavelength, plus or minus 10 percent Wedge-shaped radiating element: 24 mm (about 1/5 of a wavelength) Radiator offset: 27 mm (about 7/32 of a wavelength)
To calculate wavelength, use the formula wavelength in millimeters = 300 divided by the frequency in gigahertz. So, the wavelength for channel 6 is 300 / 2.437 = 123 mm.
Finding the Right Can
For this project, you can use just about any smallish coffee can. There are a couple of things you have to keep in mind: it has to be a metal can and it should be close to the dimensions noted in the previous section. Remarkably, Maxwell House and Folgers Coffee cans are the exact dimensions needed for this project. The ounces (or grams) measurements vary somewhat from 11.5 oz. (368 g) to 13 oz. (326 g) because they are measuring weight. But the can dimensions are identical for these two brands and probably many more.
To find the right can, go to your local grocery store with a measuring tape or ruler and measure the cans on the shelf. The store personnel might look at you funny. Just tell them you are buying some coffee to make your Internet access go further.
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64 Part I — Building Antennas
In choosing your coffee, remember that one can make many cups of coffee to drink. You really have two choices; you can dump the coffee or keep a pot of coffee ready at all times for the other projects in this book. For this chapter, we used an 11.5 oz. Maxwell House coffee can.
Preparing the Can
It’s time to get the can ready to be converted into a directional antenna. You can do this in two steps: preparation and cleaning.