Jeff Duntemanns Drive-By Wi-Fi Guide

Coaxial Cable

Making radio waves go where you want them to go is more art than science, and a lot of that art lies in the design and use of coaxial cable. Coax (as insiders call it) can be thought of conceptually as pipe for radio waves. Radio energy travels inside it without being radiated into the air. It's used to carry radio energy between an antenna and a radio; in this case, a Wi-Fi client adapter or access point.

Coax comes in a lot of different thicknesses and is made from many different materials, but the essential design is the same: An inner conductor (generally copper wire) surrounded by a layer of insulation, surrounded by an outer conductor ( generally cylindrical copper-strand braid) encased in a protective plastic sheath. You can get coax as thin as .1" in diameter, or as thick as your arm. Some is as flexible as simple copper wire, while some is as stiff as water pipe. For most ordinary Wi-Fi work, you don't need to fuss with coax. But to do certain things and fix certain problems, you're going to have to use coax, and not all coax is created equal, especially at the lofty frequencies at which microwaves operate.

What Coax Is For

When you do require coax, you're most likely going to use it to bring signal to and from external antennas. When Wi-Fi enthusiasts speak of pigtails, they mean short lengths of coax with specific coaxial connectors soldered to each end.These are used to connect client adapters and access points to external antennas. You may be used to thinking of 'external' antennas being used outdoors, or in exotic applications like linking separate networks in bridge mode, but the commonest use of external antennas is to extend the reach of a Wi-Fi client adapter that's just a little too far away from its access point to sustain a good connection. For example, I built myself a tin can antenna to allow my laptop to share my broadband Internet connection at full speed from my living room. (For more on this, including how to build your own, see Chapter 15.) The antenna connects to my laptop through a pigtail consisting of 19" of thin coax.

External antennas are sometimes used to get signal into a 'dead spot' where a Wi-Fi signal doesn't quite reach because of intervening obstruction from walls and metal objects. Sometimes getting the antenna just a few feet away from the computer is all you need, and again, a pigtail and an external antenna are just the thing. In modern corporate cube farms with metal cube walls and integrated overhead file cabinets, it sometimes takes a blade antenna tacked unobtrusively to the top of the cube wall to pull in the company Wi-Fi signal, which may be blocked by the metal in the cube structure and fittings.

Frequency, Power, and Loss

The key phrase here is 'just a few feet away.' There's a problem with coaxial cable: It eats radio signal energy unavoidably, in a predictable way at a predictable rate. The amount of signal you lose depends on both the length (and type) of the cable, and the frequency of radio energy you're using. The 2.4 GHz frequency at which Wi-Fi operates is up in the microwaves, and at microwave frequencies, coax is positively voracious in the way it eats signal.

The degree of loss present in a specific type of coax depends on its dimensions and the materials it's made of. In very broad terms, thick coax is less lossy than thin coax. It's also less physically flexible, to the extent that a lot of microwave coax should be considered 'bendable' rather than flexible.

Table 8.1 summarizes the loss exhibited by the most common types of coax used at microwave frequencies. The loss is given in dB per 100 feet. For the common LMR 240 coax, the loss is 12.7 dB per 100 feet. (I explained decibel calculations earlier in this chapter.) For the moment, consider this rule of thumb: A 3 dB increase means your power doubles; a 3dB loss means your power is cut by half.

It adds up, with every 3 dB of loss meaning your power halves again. A 6 dB loss means your power will be only one fourth of its original level. A 9 dB loss means you have one eighth left; a 12 dB loss means you have one sixteenth left, and so on. At some point you needn't bother: Pass a Wi-Fi signal through 100 feet of RG 58 (much used by CBers and amateur radio operators for far lower frequencies) and only four ten thousandths of your original signal will come out the other end, and that's almost too small to measure.

Calculating Coaxial Cable Losses

In terms of the length of the cable, the loss increases (or decreases) linearly. In other words, if 100 feet of cable loses 6 dB, 50 feet will lose 3 dB, and 25 feet will lose 1.5 dB. To calculate the loss for a shorter length of coax, multiply the stated loss in Table 8.1 by the length in feet divided by 100. For example, calculate the loss present in nine feet of LMR 195, which has a loss of 18.6 dB per 100 feet:

To calculate the power ratio associated with a dB value, divide the dB value by 10 and raise 10 to that power. If the dB value was negative (in other words, if you're calculating a loss) take the inverse of that value:

If your run of coax gives you a 1.67 dB loss, that means you'll have only 68% of your input power at the output of the cable. And that's only nine feet of cable!

Clearly, you want to keep your coax runs (if you use them at all) down to the absolute minimum. You can counteract cable loss, to some extent, with antenna gain, as I explained a little earlier in this chapter.

Other Loss Effects

Damaged cable and cable that has been exposed to water for a long time will exhibit higher loss than when new and undamaged. Bad solder joints or clumsy crimp jobs in coaxial connectors increase the loss present in a coaxial cable run. Don't kink coax or damage the braid through abrasion or pinching, and unless you're experienced with attaching connectors to cable, buy pre-made pigtails or cable sections with the connectors professionally attached. Some coax is designed to be exposed to the elements; most is not.

Even perfectly attached coaxial connectors introduce a measure of additional loss by creating an impedence bump (basically, a short section of radio turbulence) in the transmission path. To avoid this, don't use more connectors than you must.