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Building Your Own Wi-Fi Antenna Cable

Think back to the olden days, say three or four years ago, when computers were tied to the desk with a phone line or network cord. Surfing the Web, reading e-mail, or checking your PetCam meant plugging in, jacking in, or getting wired. Now just about any device can be “unwired” to use a wireless network. You still need electricity though, so batteries or power cords are still in the picture. At least for a little while.

Ironically, wireless seems to use twice as many cables as wired connections. This wireless paradox arrives in the form of extra power cords, antenna cables, pigtail jumper cables, and Ethernet patch cables. One critical component to a successful wireless project is the antenna cable, used to extend the reach of the radio to the antenna. This chapter will show how to build an antenna cable for use with many of the projects in this book. You can purchase this type of cable in pre-defined lengths from online sources. However, building your own antenna cable is easy and can take less than 5 minutes.

The instructions in this chapter apply to a Wi-Fi coaxial antenna cable (also called coax). The steps in this chapter can be adjusted to apply to any type of coaxial cable, like that used in cable televisions.

You will need the following items:

Wi-Fi network device with an external connector (client adapter or access point)

Wi-Fi pigtail cable, if using a wireless client adapter

Coaxial cable, preferably Times Microwave LMR-400

Coaxial cable cutters

Crimp tool, ratcheting style

Crimp tool “die” with hex sizes .429, .128, and .100

Long-nosed pliers

Small wire cutters

Single-sided razor blade


Type-N connectors, reverse-polarity male

Digital multimeter or electrical continuity tester

Known-good coax cable for comparison testing

Some of these items are specific to building an antenna cable (crimp tools, connectors, and so on). Don’t worry if they are unfamiliar to you. All will become clear as the chapter progresses.

About Wi-Fi

If you want to understand what is going on with a wireless network, you first need to know some of the basics of wireless communication and radio transmission. Wireless networking is accomplished by sending a signal from one computer to another over radio waves. The most common form of wireless computing today uses the IEEE 802.11b standard. This popular standard, also called Wi-Fi or Wireless Fidelity, is now supported directly by newer laptops and PDAs, and most computer accessory manufacturers. It’s so popular that “big box” electronics chain stores carry widely used wireless hardware and networking products.

Wi-Fi is the root of a logo and branding program created by the Wi-Fi Alliance. A product that uses the Wi-Fi logo has been certified by the Wi-Fi Alliance to fulfill certain guidelines for interoperability. Logo certification programs like this one are created and promoted to assure users that products will work together in the marketplace. So, if you buy a Proxim wireless client adapter with the Wi-Fi logo branding, and a Linksys access point with the same logo on the product, they should work together.

The IEEE 802.11b Wi-Fi standard supports a maximum speed of 11 megabits per second (Mbps). The true throughput is actually something more like 6 Mbps, and can drop to less than 3 Mbps with encryption enabled.Newer standards like 802.11a and the increasingly popular 802.11g support higher speeds up to 54 Mbps. So why is 802.11b so popular? Because it was first and it was cheap. Even 3 Mbps is still much faster than you normally need to use the Internet.

A megabit is one million binary digits (bits) of data. Network speed is almost always measured in bits per second (bps). It takes 8 bits to make a byte. Bytes are used mostly to measure file size (as in files on a hard disk). A megabyte is about 8 million bits of data. Don’t confuse the term megabyte for megabit or you will come out 8 million bits ahead.

The 802.11a standard, which operates in the 5 GHz frequency band, is much faster than 802.11b, but never caught on, partly because of the high cost initially and partly because of the actual throughput in the real-world conditions of a deployed wireless network.

The fast and inexpensive 802.11g standard (which uses the same 2.4 GHz band as 802.11b) is rapidly moving to unseat 802.11b from the top of the heap. The very cool thing about “g” is the built-in backwards compatibility with 802.11b. That means any “b” product can connect to a “g” access point. This compatibility makes 802.11g an easy upgrade without tossing out yourold client hardware.

Because of the compatibility with 802.11b and 802.11g, there is no great hurry to push the myriad of funky wireless products to the new “g” standard. Most manufacturers have support for basic wireless infrastructure using 802.11b and 802.11g with access points and client adapter. Wi-Fi 802.11b really shines when you look at the host of wireless products available. Not only are there the basic wireless networking devices, like adapters, base stations, and bridges, there are also new products that were unthinkable a few years ago.Wireless disk drive arrays, presentation gateways, audiovisual media adapters, printer adapters,Wi-Fi cameras, hotspot controllers, and wireless broadband and video phones dominate the consumer arena. And the enterprise market is not far behind.

We’ve been tossing out the terms wireless, gigahertz (GHz), and frequency. Next, we’ll discuss how Wi-Fi uses wireless radio waves, also called RF, to communicate amongst the devices in a wireless network.

About RF

Entire books, libraries, and people’s careers are devoted to understanding more about radio frequencies (RF) and electromagnetism. The basics are covered here to help make your projects a success. Wi-Fi wireless products use microwave radio frequencies for over-the-air transmissions. Microwave RF is very similar to the radio used in your car, only at much higher frequencies. For a downloadable PDF of the spectrum assignments in the United States, visit www.ntia.doc.gov and look under “Publications” for the “Spectrum Wall Chart.” The chart is a few years old, but most of the information is accurate. And it’s suitable for framing.

For frequency spectrum assignments covering most of Europe, check out the European Radiocommunications Office at www.ero.dk and look under the CEPT National Frequency Tables. The ERO “Report 25” document also covers much of this information in a single report file. To find this deeply buried document, search the Web for ERO Report 25.

Visualizing the radio frequency signals helps to understand the behavior of the electromagnetic (EM) spectrum. Imagine dropping a rock in a pond.Waves are created in concentric circles coming from the point where the rock was dropped. These waves are just like radio waves, except at a very low frequency of perhaps 10 waves per second, which are called cycles per second or hertz. Now imagine a cross-section of those waves. Perhaps the rock was dropped in a fish tank and the waves are visible from the side. The wave would look similar to that shown in Figure 1-1.

The electromagnetic spectrum spans frequencies from subaudible sound of 1 hertz all the way through radio and visible light to beyond X-rays and cosmic rays at a frequency of 10 followed by


24 zeros. The frequency of an FM car radio operates at about 100 million hertz, or 1 megahertz (MHz). For example, 103.1 MHz FM is a radio station in Los Angeles.Wi-Fi operates at about 2,400 MHz or 2.4 GHz.Table 1-1 shows a frequency chart to help you understand the scale. Microwave ovens also operate at 2.4 GHz, but at much higher power than Wi-Fi gear. Onetenth of a watt (0.1 W) is typical for a Wi-Fi device, versus 1,000 watt for a microwave oven. That’s a difference of over 10,000 times the power! Still, to be safe, always observe caution and minimize unnecessary exposure when working with RF.

Frequency versus Wavelength

Frequency and wavelength are inseparably related to each other. As frequency increases, wavelength decreases and vice versa. Frequency: The rate at which a radio signal oscillates from positive to negative. Wavelength: The length of a complete cycle of the radio signal oscillation.

Wavelength is, of course, a length measurement, usually represented in metric (meters, centimeters, and so on). And frequency is a count of the number of waves occurring during a set time, usually per second. Cycles per second is represented as Hertz (Hz).

Figure 1-2 shows a Wi-Fi radio wave for channel 6 (2.437 GHz). The dimensions are important to note, because the physical properties of the wave define antenna, cable, and power requirements.Wavelength is critical for antenna design and selection as we will cover in the next chapter.

Wi-Fi signals operating at a frequency of 2.4 GHz have an average wavelength of about 12 cm. Since the wavelength is so short, antennas can be physically very small. A common design for antennas is to make them 1/4 of a wavelength or less in length, which is barely more than an inch long. That’s why Wi-Fi antennas can perform so well even though they are physically very small. As a comparison, a car radio antenna is much longer to get a decent signal because FM radio signals are an average of 10 feet long.

Wavelength and antenna length go together.To oversimplify, the longer the antenna, the more of the signal it can grab out of the air. Also, antenna length should be in whole, halves, quarters, eighths, and so on of the intended wavelength for best signal reception. The highest reception qualities come from a full wavelength antenna.

Perform this simple math formula to find wavelength: 300 / frequency in megahertz. The answer will be the wavelength in meters. So, 300 / 2437 0.12 meters or 12 cm.  

Unlicensed 2.4 GHz Wi-Fi

Wi-Fi makes use of the internationally recognized unlicensed frequency band at about 2.4 GHz. The IEEE standards body created 802.11b and defined the “channels” and frequencies for use by manufacturers worldwide. Different countries accepted the standard and allowed the use of devices in this frequency range with few restrictions.The word unlicensed as it applies to Wi-Fi specifically means that products can be installed and used without prior approval from the local governing body. That’s the Federal Communications Commission (FCC) for users in the United States. Radio systems that operate in “licensed” bands require an application and permission procedure before turning on or using a radio system. For example, FM radio stations require permission from the FCC before broadcasting.

Certain other unlicensed products have been in use for some time: CB radios, walkie-talkies or consumer two-way radios, cordless phones, and many other radio products operate in unlicensed bands. Unlicensed is not equivalent to unregulated, though. There are still rules that need to be followed to stay legal, especially regarding power output. This is covered in Chapter 2. In the United States, 802.11b usage is regulated by the FCC. The FCC laws define maximum power output, among other more specific regulations. In addition, the FCC approves products for use in the U.S. market. Manufacturers must submit their product for testing and authorization. The FCC then grants an “FCC ID” for the product. Anyone can look up an FCC ID from the Web site at www.fcc.gov (look under Search, for “FCC ID Number” searches). This can help you track down the true manufacturer of a Wi-Fi radio product, despite the label or brand.

Wi-Fi Channels

As defined in 802.11b,Wi-Fi consists of 14 channels worldwide. Only channels 1 to 11 are available in North America. Channels in other countries vary.Table 1-2 shows each channel and frequency, and the countries with approval to use that channel. (The lucky ones in Japan can use all 14!)

What is not easily shown in Table 1-2 is channel separation.To make the channel numbering scheme work with different radio technologies, the IEEE community defined these 802.11b channels with significant overlap. For example, channel 6 is centered on 2.437 GHz, but it extends in both directions by 11 MHz (0.011 GHz). That means channel 6 uses 2.426 GHz

to 2.448 GHz, which, as shown in Table 1-2, means it uses frequencies already assigned to channels 4, 5, 6, 7, and 8. Clearly,Wi-Fi devices using channels 6 and 7 would not operate together in harmony because of the interference. To ensure trouble-free operation, with little interference from any other Wi-Fi devices, the channels need to be separated.

In the United States, channels 1, 6, and 11 are the sweet-spots for maximum usage with the least interference. In Europe, the recommended channels are 1, 7, and 13, and in Japan, the channels are 1, 7, and 14. For this very reason, most products come with one of these channels as the default setting, and most Wi-Fi hotspots are set to one of these three channels.

Recently, users have been squeezing these nonoverlapping channels down to minimal-overlapping channels 1, 4, 8, and 11. This opens up significantly more options for Wi-Fi device and access point placement. There are possible downsides due to the increased interference, but it’s worth testing if your setup needs a lot of devices in a small space. Now you would have a basic understanding of how Wi-Fi works in a physical and logical sense. There’s lots more to Wi-Fi technology and specifications, but that’s all you need to know about the theory for now. Next, we’ll get down to the specifics about building your own Wi-Fi projects.