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
➤
Scissors
➤
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.