Besides heat, all electrical circuits radiate something else—electromagnetic fields. Every flow of electrical energy sets up an electromagnetic field that radiates away. Radio and television stations push kilowatts of energy through their antennae so that this energy (accompanied by programming in the form of modulation) radiates over the countryside, eventually to be hauled in by a radio or television set for your enjoyment or disgruntlement.

The electrical circuits inside all computers work the same way but on a smaller scale. The circuit board traces act as antennae and radiate electromagnetic energy whenever the computer is turned on. When the thinking gets intense, so does the radiation.

You can't see, hear, feel, taste, or smell this radiation, just as you can't detect the emissions from a radio station (at least not without a radio), so you would think there would be no reason for concern about the radiation from your computer. But even invisible signals can be dangerous, and their very invisibility makes them more worrisome—you may never know if they are there or not. The case of your computer is your primary (often only) line of defense against radiation from its electronic circuitry.

The problems of radiation are twofold: the radiation interfering with other, more desirable signals in the air, and the radiation affecting your health.

Radio Frequency Interference

The signals radiated by a computer typically fall in the microwatt range, perhaps a billion times weaker than those emitted by a broadcasting station. You would think that the broadcast signals would easily overwhelm the inadvertent emissions from your computer. But the strength of signals falls off dramatically with distance from the source. They follow the inverse-square law; therefore, a signal from a source a thousand times farther away would be a million times weaker. Radio and television stations are typically miles away, so the emissions from a computer can easily overwhelm nearby broadcast signals, turning transmissions into gibberish.

The radiation from the computer circuitry occurs at a wide variety of frequencies, including not only the range occupied by your favorite radio and television stations but also aviation navigation systems, emergency radio services, and even the eavesdropping equipment some initialed government agency may have buried in your walls. Unchecked, these untamed radiations from within your computer can compete with broadcast signals, not only for the ears of your radio but those of your neighbors. These radio-like signals emitted by the computer generate what is termed radio frequency interference (RFI), so called because they interfere with other signals in the radio spectrum.

The government agency charged with the chore of managing interference—the Federal Communications Commission—has set strict standards on the radio waves that personal computers can emit. These standards are complex, and ensuring your computer is actually in compliance would require expensive test equipment. The law does not require that you check—the burden is on the manufacturer. Moreover, at their hearts, the FCC standards simply enforce a good-neighbor policy. They require that the RFI from computers be so weak that it won't bother your neighbors, although it may garble radio signals in your own home or office.

The FCC sets two standards: Class A and Class B. Computer equipment must be verified to meet the FCC Class A standard to be legally sold for business use. Computers must be certified to conform with the more stringent FCC Class B standard to be sold for home use.

Equipment-makers, rather than users, must pass FCC muster. You are responsible, however, for ensuring that your equipment does not interfere with your neighbors. If your computer does interfere, legally you have the responsibility for eliminating the problem. While you can sneak Class A equipment into your home, you have good reasons not to. The job of interference elimination is easier with Class B certified equipment because it starts off radiating lower signal levels, so Class B machines give you a head start. Moreover, meeting the Class B standards requires better overall construction, which helps ensure that you get a better case and a better computer.

Minimizing Interference

Most television interference takes one of two forms: noise and signal interference.

Noise interference appears on the screen as lines and dots that jump randomly about. The random appearance of noise reflects its origins. Noise arises from random pulses of electrical energy. The most common source for noise is electric motors. Every spark in the brushes of an electric motor radiates a broad spectrum of radio frequency signals that your television may receive along with its normal signals. Some computer peripherals may also generate such noise.

Signal interference usually appears as a pattern of some sort on your screen. For example, a series of tilted horizontal bars or noise-like snow on the screen that stays in a fixed pattern instead of jumping madly about. Signal interference is caused by regular, periodic electrical signals.

Television interference most commonly occurs when you rely on a "rabbit ear" antenna for your television reception. Such antennae pull signals from the air in the immediate vicinity of the television set, so if your computer is nearby, its signals are more likely to be received. Moving to cable or an external antenna relocates the point your TV picks up its signals to a distant location and will likely minimize or eliminate interference from a computer near the TV set.

You can minimize the interference your computer radiates and improve your television reception by taking several preventive measures.

The first step is to make sure the lid is on your computer's case and that it and all expansion boards are firmly screwed into place. Fill all empty expansion slots with blank panels. Firmly affixing the screws is important because they ground the expansion boards or blank panels, which helps them shield your computer. This strategy also helps minimize the already small fire hazard your computer presents.

If the interference persists after you've screwed everything down in your computer, next check to see if you can locate where the interference leaks out of your computer. The most likely suspects are the various cables that trail out of your computer and link to peripherals such as your monitor, keyboard, and printer. Disconnect cabled peripherals one at a time and observe whether the disconnection reduces the interference.

Because it operates at the highest speed (and therefore the highest frequency), an external SCSI cable is most prone to radiating interference. All external SCSI cables should be shielded.

Your mouse is the most unlikely part of your computer to cause TV interference. The mouse operates at serial data rates, which are much too low to interfere even with VHF television.

If disconnecting a cable reduces onscreen TV interference, the next step is to get the offending signal out of the cable. The best way is to add a ferrite core around the cable. Many computer cables already have ferrite cores installed. They are the cylindrical lumps in the cable near one or the other connector. Install the ferrite core by putting it around the offending cable near where the cable leaves your computer. You can buy clamp-on ferrite cores from many electronic-parts stores.

Unplugging one cable—your computer's power cable—should completely eliminate the interference radiated by your computer. After all, the computer won't work without power and can't generate or radiate anything. You can reduce the interference traveling on the power line by adding a noise filter between your computer's plug and its power outlet. You can usually obtain noise filters from electronic-parts suppliers. Although a noise filter is not the same thing as a surge suppresser, most of the better surge suppressers also include noise filtering.

Health Concerns

Some radiation emitted by computers is of such low frequencies that it falls below the range used by any radio station. These very low frequency and extremely low frequency signals (often called VLF and ELF) are thought by some people to cause a variety of health problems.

Your computer's case is the first line of defense against these signals. A metal case blocks low-frequency magnetic fields, which some epidemiological studies have hinted might be dangerous, and shields against the emission of electrical fields. Plastic cases are less effective. By themselves they offer no electrical or magnetic shielding. But plain plastic cases would also flunk the FCC tests. Most manufacturers coat plastic cases with a conductive paint to contain interference. However, these coatings are largely ineffective against magnetic fields. Most modern systems now use metal cases or internal metal shielding inside plastic cases to minimize radiation.

No matter the construction of your computer, you can minimize your exposure to radiation from its case by ensuring that it is properly and securely assembled. Minimizing interference means screwing in the retaining brackets of all the expansion boards inside your computer and keeping the lid tightly screwed into the chassis. Keeping a tight computer not only helps keep you safe, it also keeps your system safe and intact.

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