Nearby radio and television stations can generate powerful electromagnetic signals. The signals propagate through the air and can be picked up by a computer. The cables that connect computers to peripheral devices such as printers and external hard drives or mass storage devices can act as excellent antennae. The keyboard and monitor cables, the printer cable, the cables to modems and other external devices all pick up the radiated signals from radio and TV stations. If computers are located close enough to these sources they can experience problems.
Most manufacturers today add traps which reduce the amount of EMI, or Radio Frequency Interference (RFI) as it is also known, radiated from and induced into many types of cables. Most good quality monitor cables, for example, have a bulge at each end which contains a radio frequency trap. These traps prevent EMI from radiating into or out of the display at the one end and the computer at the other.
Radio and TV caused EMI has the same entry pathways as for Radar and microwave.
EMI can gain access through openings in the computer case. These access points can be caused by running the computer without the side of the case in order to facilitate easy access for various reasons and cover plates left off of the bus slots in the back of the computer. EMI can infiltrate a computer even through a cover that is merely set in place and not fastened down with the appropriate screws. Cables connected at one end to the computer but left dangling at the other end while a peripheral is disconnected can act as antennae to capture signals and draw them into the computer.