LINEBURG


<< . .

 4
( 36)



. . >>


Transmission Lines
When you work with Wi-Fi products, you will find that the transmission line is nearly
always a coaxial cable. Internal transmission lines may be of very small diameter, high loss
cable. But usually the cable run is less than a few inches, so line loss is not much of a factor.
See Figure 1-4 for an internal view of a transmission line for the Linksys WAP11, a popular
802.11b wireless access point.
An RF transmission line transfers RF energy from the transmitter to the antenna while both
losing and radiating as little as possible. Radiation should be left to the antenna system. It also
transfers RF energy from the antenna to the receiver in the same fashion.

Antenna System
The antenna system is where the rubber hits the road, so to speak. The antenna emits the elec-
tromagnetic radio frequency signal out of the Wi-Fi device. Antenna systems will be covered in
Chapter 2 while building a simple antenna for a laptop PC card.




FIGURE 1-4: Internal RF transmission line on a Linksys WAP11.
12 Part I ” Building Antennas


At this point, what you need to know is that the antenna is where you want to send as much
signal as possible. The transmission line should be designed to be as short as possible with the
least line loss to pass power to the antenna.
Once the RF signal leaves the antenna, it immediately begins to lose power. (Really, as soon as
it leaves the transceiver it begins to lose power.) The design of the antenna can redirect the
amount of power available to shape the beam pattern as needed, much like a flashlight reflect-
ing a tiny light bulb into a bright light.
Now that you know more about Wi-Fi projects in general, we can start to focus on the project
for this chapter: building an antenna cable. Before you pick up your tools, though, you need to
understand how coaxial cable works, which is the subject of the next section.


Understanding Coaxial Cables
Coaxial cables (commonly called coax) are used as the transmission line in a Wi-Fi system.
There are probably instances of Wi-Fi systems using a different transmission line, but the most
common is coax.
A coax cable is built in layers of the following materials (see Figure 1-5):

Core: A center of electrically conducting material like copper (solid or stranded)
Dielectric: A nonconducting material surrounding the core
Shield: An outer layer of conducting material like steel (solid and/or stranded)
Jacket: A nonconducting protective surface like rubber or plastic

The RF signal is created or received and then placed (or injected) onto the core of the cable. In
theory, the signal is meant to travel along the core of the cable, while the shield prevents the
signal from emanating outside the cable. In reality, some signal is radiated outside the cable,
while electrical resistance in the cable reduces the signal within the cable.
Coax cables come in two flavors when used with Wi-Fi:
Coax jumper
Coax pigtail
A coax jumper is a larger diameter cable with low loss, meant for runs between larger diameter
connectors. A common use of a jumper would be from a wireless access point antenna jack
directly to an antenna.




FIGURE 1-5: Diagram of the layers of a coaxial cable.
13
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable


A coax pigtail is used as an interface between larger diameter cables and the very small connec-
tors commonly used on PC cards. A common use of a pigtail would be to connect a PC card to
a coax jumper to an antenna.
Constructing pigtails takes much skill and patience in soldering the tiny connectors to the
small diameter cable necessary for PC card connectors. For best results, purchasing a pre-
configured pigtail is the way to go. Selecting a pigtail is covered in detail later in the chapter.

What Sizes of Coax Are Available
Cables come in many forms from different manufacturers. We have found the optimum cable
for ease-of-use and low-loss performance is the LMR-400 cable from Times Microwave. This
cable has become the popular choice in building wireless networks.
Table 1-3 shows various cable sizes from Time Microwave. These represent the most com-
monly available cables for use with 2.4 GHz Wi-Fi gear. The larger diameter cables are harder
to work with than the smaller cable because of their rigidity and bulkiness. However, the larger
cables have lower signal loss. It™s a trade-off between ease of use, performance, and cost. LMR-
400 is a good balance and costs about half the price of LMR-600.


Keep It Short!
As shown in Table 1-3, cable loss is measured by distance. Therefore, to keep the strongest sig-
nal and the lowest loss, you should keep the cable as short as possible. For most of the projects
in this book, you will need cables of less than 10 feet in length. For larger projects, such as cre-
ating a free wireless hotspot, you would need a longer cable.
Also, the cable type is very important at high frequencies. For example, using 10 feet of LMR-100
cable induces a loss of 3.9 dB, while the same length of LMR-400 induces a tiny loss of 0.7 dB.
Because of the high loss factor of LMR-100, an access point should have no more than 3 feet
of LMR-100 cable between it and the antenna. On the other hand, an access point using the
more efficient LMR-400 cable could have a 20 foot“long cable and work just as well.

Manufacturers list cable line loss as measured in 100 feet of cable. This does not mean you
should, or even can, use 100 feet in your cable runs. You usually want as strong a signal as possi-
ble coming out of the other end of the cable, so either keep it short or use a larger diameter cable.



Table 1-3 Cable Sizes Commonly Used for 2.4 GHz
TM Part Number Diameter Line Loss at 2.4 GHz (Per 100 Feet)

LMR-100 1/10” “38.9 dB
LMR-240 3/16” “12.7 dB
LMR-400 3/8” “6.6 dB
LMR-600 1/2” “4.4 dB
14 Part I ” Building Antennas


Many radio enthusiasts and some manufacturers host line loss or attenuation calculators on the
Web. Search the Web for coax line loss to find some of these simple-to-use calculators.




Measuring Line Loss in Decibels
The concept of decibel measurement, or dB, is covered more in Chapter 2. But for now, it™s easy
to think of it as the higher the number, the stronger the signal. Remember that negative num-
bers descend as they get higher ( 80 is less than 30). Transmission line loss is represented as
negative dB.

Wi-Fi radio transceiver effectiveness is described as a measurement of power output and receive
sensitivity. Generally, these two measurements are expressed as power in milliwatts (expressed
as mW, meaning 1/1000 of a watt) or as “dBm” (decibels related to 1 mW).

Decibel measurement can be confusing. But there are two key concepts to make this easy to
understand:

Decibels are relationship-oriented
Decibels double by threes
Relationship-oriented means that there is no set value for a dB. The trailing letter in a dB
measurement defines the relationship. For example, dBm means decibels related to 1 mW of
power. 1 dBm equals 1 mW. When you know the value of the relationship, decibels are easy to
calculate.
Doubling by threes is due to the logarithmic nature of RF energy. When comparing a signal of
1 dBm (1 mW) to a signal of 3 dBm (2 mW) you see that it™s double the power.
This doubling nature of power measurement or line loss makes it easy to see how a cable can
quickly reduce the RF signal to almost nothing.

Calculating Line Loss
Continuing the last example (LMR-100 versus LMR-400), let™s start with a signal of 100 mW
( 20 dBm) and send it out along the 100 foot“cable, as shown in Table 1-3.
Start with the transmit power, 20 dBm or 100 mW, subtract the negative dB of line loss, and
the result is the power at the other end of the cable:

1. LMR-100 (38.9 dB loss): 20 dBm 38.9 dB 18.9 dBm (about 0.001 mW)
2. LMR-400 (6.6 dB loss): 20 dBm 6.6 dB 13.4 dBm (about 20 mW)

In each case, it™s a large drop. But look at the difference! LMR-100 drops power to a tiny frac-
tion of the original signal. LMR-400, on the other hand, while inefficient, still has a usable sig-
nal. With either cable, once the signal gets to the antenna and out into the air, there will be
even more signal loss. (See Chapter 13 for more on airspace loss and link budget.)
The significant loss in the cable makes repetition important: keep it short!
15
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable


Cable usually comes in bulk on reels of 500 feet. Bulk cable vendors will happily cut a length of
cable for your order. When ordering bulk cable, select a length of cable that is several feet longer
than required. Although it adds a few extra dollars to the order, the extra cable makes it easy to
repair construction mistakes or connector problems.


Types of Coax Connectors
Connectors, obviously, are used to connect RF components together. In Wi-Fi there are only a
few common connectors for large diameter coax. Unfortunately, the connector styles are not
commonly used outside of the Wi-Fi arena. So, picking up a connector at your local consumer
electronics store is generally out of the question. Hopefully in the future, more specialized retail
establishments will carry this type of equipment. But for now, expect to buy online or purchase
directly from distributors.

Male versus Female Coax Style
Connectors are designated as male and female, which is another way of describing them as
plug and socket. A male coax connector has a solid center pin or plug with an outer casing that
enshrouds the female connector (see Figure 1-6). A female coax connector has an open center
socket which accepts the male center pin.
In Wi-Fi coax cables there are often other components to the cable connectors, such as the
inner ring on a Male N-type connector. The male/female designation is defined by the center
conductor (plug or socket).

Reverse Polarity
Reverse polarity is another way of saying that a connector has gone from plug to socket or
socket to plug, reversing its polarity. This adds confusion to the entire male/female designation.
When using reverse polarity connectors, male and female is reversed, where a male connector is
the same design except that its center conductor is a socket. Female reverse polarity connectors
use a plug for the center conductor.
The outer casing is generally the same for normal and reverse polarity. The RP style only changes
the center conductor. So a male RP connector still enshrouds the female connector. See Figure 1-7
for a diagram of reverse polarity connectors. Hopefully that will make it a bit less confusing.




FIGURE 1-6: Diagram of male and female coax connectors.
16 Part I ” Building Antennas




FIGURE 1-7: Diagram of reverse polarity male and female connectors.


Reverse polarity is a commonly used connector type in Wi-Fi devices. The style is not commonly
used in other coax applications. The general understanding regarding reverse polarity connectors
is that it fulfills government requirements to make it more difficult for the average consumer to
modify Wi-Fi devices. Now that you know the secret, you™re not an average consumer.


Building a Coaxial Cable
That™s enough theory! Now it™s time to get your hands dirty and get started on this chapter™s
project, which is building a coax antenna cable.
Cable construction opens new freedom to creating wireless projects. With this skill, you can
order the components you need and custom-build a cable that fits your application perfectly.
And the cost of the components is usually lower than buying a pre-built cable.
N-Male is the most commonly used connector for Wi-Fi cabling, because most antennas have
N-Female connectors. And, as you know, N-Male mates to N-Female. So, these steps will
assume you have chosen LMR-400 cable with the standard N-Male connector. Please adapt
the steps to your application where needed. Figure 1-8 shows the necessary dimensions for a
Times Microwave N-Male connector.
Table 1-4 shows a list of connectors for use with LMR-400 cable. These connectors are solder-
less and each requires only two crimps. The connector types listed here are for hand-tighten-
ing. A myriad of other connector types are also available.
With the right set of tools, building a cable is a step-by-step process:

1. Prepare the cable
2. Slide the crimp ring onto the cable
3. Strip off the outer jacket
4. Pull back the inner shield
5. Strip the dielectric foam core
17
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable




FIGURE 1-8: Times Microwave N-Male Reverse Polarity connector.

6. Remove any shorting material on the foam core
7. Cut the center conductor to correct size
8. Place the center pin onto the center conductor
9. Crimp the center pin
10. Place the connecter body onto the cable
11. Replace shield over the connector body
12. Place the crimp ring over the shield and connector body
13. Crimp the crimp ring
14. Inspect your finished product


Table 1-4 Common Connector Types for LMR-400 Cable
Connector Type Application Times Microwave Part
Number

N-Male Cable jumper ends EZ-400-NMK
N-Female Antenna termination EZ-400-NF
TNC Male Cable jumper ends EZ-400-TM
TNC Female Antenna or pigtail termination EZ-400-TM-RP
18 Part I ” Building Antennas


There are a lot of steps, but it™s actually very simple. Each step is discussed in detail in the para-
graphs that follow.


Step 1: Preparing the Cable
The cable is treated as a bulk item until ready to assemble the end connectors. So the ends are
often cut into an irregular shape.
Use the cable cutters to square off the end of the cable, as shown in Figure 1-9.
After cutting the cable, the dielectric foam will become elongated. Use a set of pliers to reform
the foam into a rounded shape, as shown in Figure 1-10. This will make it easier to strip later.
Don™t worry about the shape of the shield and outer jacket.


Step 2: Placing the Crimp Ring
Before going any further, place the crimp ring onto the cable as shown in Figure 1-11. Slide it
out of the way down the length of the cable. Later, you™ll pull the crimp ring into place on the
back of the connector shell.




FIGURE 1-9: Cutting the end of the cable to make it square.
19
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable




FIGURE 1-10: Using pliers to reform the white foam core.




FIGURE 1-11: Placing the crimp ring before any other work.
20 Part I ” Building Antennas




FIGURE 1-12: Stripping using a rocking motion. Watch those fingers!



Step 3: Stripping and Removing the Outer Jacket
There are special tools for stripping all types of cables, but a razor blade works well, costs less,
and is more versatile. If you are very good at handling sharp objects, a pocket knife works too.
Check the instructions that came with the connector for exact dimensions needed. Strip off
about 1/2 inch more than necessary to leave room for trimming.


When stripping a cable with a razor blade or sharp knife, try not to nick the underlying elements
of the cable. By rocking the razor blade, you will score through the jacket without harming the
shield underneath.

Figure 1-12 shows a cut taking place around the entire circumference of the cable. It™s a little
unclear at this angle, but my fingers are being kept well out of the way!
Cut through the outer jacket just enough to be able to pull away the jacket without harming
the shield.
After stripping around the cable, make a groove along the length of the cable. Make three or
four cuts with just enough force to cut a little deeper each time, as shown in Figure 1-13. The
goal is to come as close to the shield as possible without cutting all the way through the cable.
Now grab the end of the outer jacket with a set of long-nosed pliers and pull away the jacket.
Tear along the grooves scored into the jacket and peel off the jacket with your fingers to reveal
the shield mesh underneath, as shown in Figure 1-14.
21
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable




FIGURE 1-13: Scoring along the length of cable.




FIGURE 1-14: Peeling off the outer jacket of the cable.
22 Part I ” Building Antennas




FIGURE 1-15: Fanning out and pulling back the shield.



Step 4: Pulling Back the Inner Shield
To get the next cut ready, use your fingers to carefully fan out and pull back the shield mesh
layer, as shown in Figure 1-15.


Step 5: Stripping the Dielectric
Now strip off the foam dielectric core along with the solid aluminum wrapping. This requires
much less force than the cable jacket. Be sure to apply light pressure and try not to nick the
center conductor. (See Figure 1-16.)
To remove the foam core from the center conductor, just twist and pull.


Step 6: Checking for Shorts
At this stage, you need to inspect the cable for shorts along the dielectric. Remember that the
dielectric material is a nonconductor of electricity. If there is an electrical short from the center
conductor to the outer shield, the cable will not perform well, i.e. if it works at all.
The easiest way to accomplish this is with a visual inspection. Check for any stray shielding
strands or aluminum foil material. See Figure 1-17 for an example of foil shorting the center
pin.
23
Chapter 1 ” Building Your Own Wi-Fi Antenna Cable




FIGURE 1-16: Make gentle cuts to remove the dielectric and foil.




FIGURE 1-17: Shorts must be removed at this stage.
24 Part I ” Building Antennas




FIGURE 1-18: Clipping the core conductor.


To remove a foil short, use small wire cutters to scrape away and cut the foam at an angle. You
can also use a fingernail for any smaller, more elusive bits. The corrected foam should be white
all the way around.


Step 7: Clipping the Core
Clip off the center core to the proper length for the connector being used. The connector pack-
aging or data sheet should have this specific measurement. In the case of this connector, we
clipped it to 3/16 of an inch. If the center conductor is too short or too long, the connector
shell will not seat correctly. Figure 1-18 shows the relative length for an N-Male connector.
After trimming back the core, remove any ridges or burrs around the cut edge. This will allow
the pin to seat properly.


Step 8: Inserting the Center Pin
Place the center pin onto the conductor as shown in Figure 1-19. Ensure the center conductor

<< . .

 4
( 36)



. . >>

Copyright Design by: Sunlight webdesign