Radio Frequency Interference (RFI), at the heart of the matter is the interception of unwanted radio signals. For any given case of RFI, there are two possibilities for who is at fault: the transmitter or the receiver.

To begin with, RFI from licensed radio transmitters (TV, and radio stations for example) because of a defect is extremely rare. Most of the time the RFI from the transmitter is caused by from cheap consumer electronics products. Improperly shielded computers are much often to blame for RFI than TV's and radios.

Within a computer, there are many different frequencies within the HF and VHF bands that are possible culprits for interference. Computers also use square waves in their systems which may causeharmonics all over the place. Video cards updating the screen and monitors are also paticularly good at generating RFI. Monitors are especially bad with their poorly shield plastic case. Interestingly, the innocent keyboard also generates RFI with its microprocessor and keyboard cable that act together to form a fairly good loop antenna that sends the signal and harmonics all about.

RFI on telephones is always the receiver's fault. Radio transmitters are never transmitting at frequencies that are in the audible range of humans. This often occurs when a radio station is located near telephone facilities and the radio signal is picked up by the wire acting as an antenna. This signal is then demodulated by the circuitry in the telephone itself. RFI in telephone lines is a very common reason for reduced modem performance.