Most computers have an EMI filter built into their body where the AC line cord enters the power supply. This filter is generally insufficient to prevent substantial re-radiation of EMI voltages back into the power wiring of your house and neighborhood. To reduce the power retransmitted down the AC power cords of your equipment, plug them in to special EMI filters that are in turn plugged into the wall socket. This particular filter from ICE is specified to reduce retransmission of EMI by a factor of at least 1000 in its high-frequency design range. Although ideally every computer component using an AC line cord ought to be filtered, it is especially important for the monitor and computer CPU to be filtered in this manner as the most useful information available to opponents is believed to come from these sources.
Telephone line is generally very poorly shielded.
To prevent direct retransmission, EMI filters are specifically designed for modular telephone jacks to mount at the telephone or modem, and for mounting directly at the service entrance to the house. Your phone company or telephone manufacturer may be able to supply you with free modular filters, although the design frequencies of these filters may not be high enough to be effective through much of the EMI spectrum of interest. Ferrites are magnetic materials that, for certain ranges of EMI frequencies, attenuate the EMI by causing it to spend itself in heat in the material rather than continuing down the cable. Every cable leaving your monitor, computer, mouse, keyboard, and other computer peripherals should have at least one ferrite core attentuator. Don't forget the telephone lines from your fax, modem, telephone or the unshielded DC power cord to your modem.