Section 10.4. Playing the Fields


10.4. Playing the Fields

One of the cardinal rules for installing a wireless system is to overcome the notion that antenna wire is simply a path for electricity. Coaxial cable is a mechanically precise system that must be gently handled if it is to perform well over time. Consider: the wire from your battery to the starter in your car is designed to move electrons. The power cord from the wall to your computer is designed to move electrons as well. At frequencies below a hundred cycles per second or so (hertz, or Hz), the efficiency with which cables pass their signals is determined by the thickness of the cable. Literally, the more copper, the more current the wire can carry. Above a couple of hundred Hz, however, the electrons begin to travel closer to the outer edge of the cable, due to a process called skin effect. By the time the frequencies approach that of LANs, most of the current is traveling on the surface of the wire, not the center. In the range of microwaves and radars, (which is roughly the same frequency as 802.11x) so much of the current travels on the surface that sometimes signals are routed through pipes called waveguides instead of wires.

What this means is that you must be gentle while installing cables and mount them in a way that supports them without stressing them. The condition of the surface of the wire is the highway the electric fields travel over. Don't create potholes by mishandling the cable. The condition of the cable once it is in place, and the ability for it to resist damage as it sags and settles over the years, is what separates a good installation from a mediocre one. Good installers never nick a cable, never bend it tighter than four times the cable radiuseven while installing itand do not exert a lot of pressure while pulling it into place. This is even more important with the relatively low-power transmitters used in wireless.

10.4.1. Keeping the Waves Inside

To further explore the importance of electrical fields as they apply to security requires us to get a little more familiar with this invisible phenomenon. The patterns of the waves that emanate from antennas create a field pattern that can be measured and plotted. If we choose the length and orientation of our antenna carefully, we can make the fields that radiate from it interact in ways that shape that antenna's sensitivity and pattern of coverage.

For instance, if we energize a simple vertical antenna, we create a pattern of energy that extends around it like a doughnut. (Well, maybe a flattened doughnut.) Interestingly, if we lengthen the vertical antenna, we flatten the doughnut some more; this is because the fields tend to interact (see Figure 10-6).

Figure 10-6. A vertical antenna radiation pattern where the doughnut's width depends on the antenna length


Besides just squishing doughnuts, this interaction can be used to make an antenna directional. Let's add a second vertical antenna of identical dimension and distance from the first (that is, we will keep the wires to each antenna the same length). If we space the two antennas at some multiple of one wavelength from each other (wavelength is the physical length of the one complete cycle of our fields), something interesting happens. The waves from the first antenna will arrive at the second antenna in phase, that is, at the same part of the electrical cycle. For this reason they will add together in their travels, nearly doubling the strength along the line of the two antennas. Perpendicular to that line, however, the waves conflict with each other, nulling out the signal. The resulting antenna pattern, therefore, is highly directional (see Figure 10-7). For the record, it doubles the strength along the main axis, providing an apparent six-decibel (6 db) gain, or roughly four times the power.

Figure 10-7. Spacing two vertical antennas one wavelength apart provides a signal gain along the line of the antennas


On the other hand, if we space the antennas at some multiple of a half wavelength apart, energy from one antenna will arrive at the other antenna out of phase. Instead of adding, the signals in the line of the two antennas cancel. The signals are increased, however, along the line formed between the antennas. The resulting pattern is wider than the pattern at the full-wave spacing, but not as long, because the antennas interact over a broader area. This half wave separation forms a pattern that is about half as strong, or about 3 db.

We can back-stop the two vertical antennas with a large piece of metal to form a panel antenna. This antenna uses the backing plate to reflect some of the energy back toward the two antennas and also prevents energy from passing through the barrier presented by the plate. The result is a pattern that has a large forward pattern, but has a dead spot behind. This kind of antenna would probably produce a gain of about 5 db. Panel antennas (see Figure 10-8) are usually wall mounted, and are used to project a signal forward into the work area, not backward into the parking lot, where it can be intercepted by sneaks.

Figure 10-8. A panel antenna offers a wide, compressed unidirectional pattern


Adding an extra antenna element behind the radiating element that is slightly longer than the radiator creates a reflector. Adding elements in front of the radiator that are slightly shorter than the radiator creates a director. A reflector and one or more directors creates a very directional antenna called a Yagi, after the last name of its inventor. Yagis are very popular in wireless today. The gain of a Yagi increases with the number of elements, but 15 dB is not uncommon.

Finally, placing the radiator in front of a parabolic dish creates an antenna called a parabolic reflector or dish. The dish antenna is likely the most highly directional antenna available today, with the exception of antennas designed specifically for scanning and hacking. A dish can produce a gain of about 21 dB.

Using the manufacturer's data sheets (see Figure 10-9), it is possible to tailor an antenna system that will fully cover or illuminate the desired areas of a floor or department, while minimizing the amount of energy leaving the building.

Figure 10-9. Antenna data sheets include charts that outline the antenna pattern





Computer Security Basics
Computer Security Basics
ISBN: 0596006691
EAN: 2147483647
Year: 2004
Pages: 121

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