Unintentional radiator
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An unintentional radiator is any device which creates radio frequency energy within itself, of which a significant amount does not normally leave the device or its housing, unless it is damaged or malfunctioning.
A computer would be an excellent example, as all spurious emissions should normally be contained within the case. Likewise, radio receivers use an intermediate frequency which is detectable outside the radio—the concept behind at least one audience measurement concept for roadside detection of radio stations which passing motorists are listening to.
[edit] Regulatory overview
In the US, devices that are characterized as "unintentional radiators" are governed by Part 15 of the FCC regulations. Generally, this means the device leaks a signal at some level. Microprocessor-controlled appliances, anything with a clock, and harmonics from switching supplies all make some kind of noise. In most countries, government agencies regulate how much leakage is tolerated. This prevents leakage from cable television systems, for example, from interfering with radio communications between aircraft and control towers.
Because it costs money to filter out noise, there is always a balance struck between regulatory compliance and perfect filtering in these devices. Microwave ovens or devices with microprocessors may leak within allowable limits but may generate an undesired signal that interferes with something in the way of a licensed communications device. It also generally means that users who intentionally radiate signals (TV stations and cell phone companies) can order the device turned off if it interferes with their licensed operations.
There is an entire industry based on regulatory compliance: manufacturers shipping a product to a foreign country must comply with each country's limitations on leakage of interfering signals. For example, in Germany the TÜV issues regulatory rules for unintentional radiators. The big cylindrical bumps on the cable to monitors and laptop chargers are ferrite cores which reduce undesired signals.
