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. AC and other filters mentioned in this note are available from a wide variety of sources including, for example, Radio Shack. Some filters reduce retransmission of EMI by a factor of at least 1000 in the high-frequency 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 usually comes 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. Use ferrite toroids and split beads to prevent EMI from escaping on the surface of your cables. 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 attenuator.