3 Ways to Rein in Your Wireless Signals

Narrow the Field
The third method of limiting wireless signals is limiting the direction in which your wireless antennas transmit signals. Nearly every AP sold today comes with omni-directional antennas. An omni-directional antenna broadcasts signals in a 360-degree pattern. The signal pattern looks like a toroid, which is similar to the shape of a donut. To modify an AP so that it broadcasts its signal in a specific direction (instead of every direction), you can either buy directional antennas or modify existing antennas by adding signal reflectors. Adding signal reflectors is undoubtedly the cheaper way to go, as you'll soon learn.

Reflectors cause a signal to travel primarily in the direction the reflector faces. So, for example, if you place an AP in the corner of a room and place reflectors behind the AP antennas, the radio signal will propagate out into the room and very little of the signal will propagate through the wall behind the antenna.

To capture information, a potential intruder needs to be within the path of the radio signal; by using a reflector, you can limit the area where the intruder must be to pick up your AP signals. Using reflectors not only prevents rogue intruders on the streets from accessing your data but can also be useful in buildings shared by many companies. You can use reflectors to limit your signals' propagation into neighboring office suites, thereby reducing potential security problems.

Designing efficient reflectors involves antenna-design engineering—a complicated process that requires considerable knowledge. It's safe to say that most network administrators don't have the right skills to design an efficient reflector. Fortunately, some network administrators who are well versed in radio and antenna design have made their reflector templates available online for anybody to use.

One such person is Michael Erskine, who offers reflector designs that users can retrofit to the omni-directional antennas that are standard equipment on most APs. Erskine's three designs—the Corner Reflector (http://www.freeantennas.com/projects/Ez-10), the Parabolic Reflector (http://www.freeantennas.com/projects/template2), and the Deep Dish Cylindrical Parabolic Reflector (http://www.freeantennas.com/projects/template)—throw an AP's output signal in a specific direction. All three reflectors both increase power in the direction of the signal and severely limit signal power behind the reflectors. Each reflector is incredibly easy to use: You simply construct the reflector and position it on your antenna.

A wonderful benefit of Erskine's designs is that you can make the reflectors yourself by using readily accessible parts that you probably have on hand or that you can obtain from your local grocery store or convenience store. You can make both the Corner Reflector and Parabolic Reflector from either a thin piece of cardboard, a thick piece of paper, or acetate (typically used for printing transparencies). You'll also need some household aluminum foil and glue, such as rubber cement. You can make the Deep Dish Cylindrical Parabolic Reflector (a variation of the Parabolic Reflector), from those same components, or you might consider using a Pringles potato chip can (which is already lined with foil), some wire screen, or a thin piece of flat flexible metal. If your AP has dual antennas, you can simply construct two reflectors and place one on each antenna. Templates are available at Erskine's Web site, so you can print them out to make sure you construct the designs in the proper proportions, which is crucial for optimum performance.

You might wonder what your signal patterns will look like if you use these reflector designs. Radio signals are invisible to the unaided eye, but an antenna-design software package can produce fascimile images that provide a good representation of the signal pattern. In Figures 1 and 2, the mesh is the reflector, the black line is the antenna, and the donut-shaped area is the signal pattern.

Figure 1 shows the signal pattern for both the Parabolic Reflector and the Deep Dish Cylindrical Parabolic Reflector, both of which produce very little signal behind the antenna. Figure 2 shows that the Corner Reflector produces a bit more signal behind the antenna than the other two reflector designs, but it's still an effective design.

These solutions are extremely cost effective. Commercial directional antennas can costs hundreds of dollars each, whereas these reflectors will cost you only pennies, plus a minimal amount of time to build.

Your best bet for limiting your wireless exposure is probably to combine reduced AP output power with the use of antenna reflectors to confine the signals to a limited area. Of course, none of these solutions eliminates the need for the usual wireless network security measures.