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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.

A coax pigtail is used as an interface between larger diameter cables and the very small connectors 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 preconfigured 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 commonly 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 signal 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 creating 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 possible coming out of the other end of the cable, so either keep it short or use a larger diameter cable.

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 numbers 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 dBm38.9 dB 18.9 dBm (about 0.001 mW)

2. LMR-400 (6.6 dB loss): 20 dBm6.6 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 fraction of the original signal. LMR-400, on the other hand, while inefficient, still has a usable signal. 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!

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.

 

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.