Tuesday, June 22, 2010

Do you have any cures for HF interference caused by the computer in my shack?

Q Richard Bauer, K5RB, asks, “Do you have any cures for HF interference caused by the computer in my shack?”

A Computers are notorious RF generators. Even computers with grounded metal cases can leak RF by various routes. The first thing to do is to narrow down the list of suspects. Turn on your computer system and your radio. Listen to an interference signal, turn off your PC, unplug a peripheral cable, and turn the PC back on. (Peripheral cables include those to your monitor, printer, scanner, mouse and even your keyboard.) If the interference suddenly disappears or drops sharply, investigate either the cable you just disconnected or the device it is attached to. A ferrite choke on the offending cable may help. Or, relocate the “leaky” device.

If you’ve disconnected all the cables but the interference remains, suspect either the computer’s switching power supply or RF coming from bus cards or the motherboard itself. Fixing a noisy power supply would entail installing a device that replaces the existing female ac socket with one that has a built-in filter. You may find these in computer-supply stores. This is done to try to prevent the RF generated in the power supply from getting to the ac line and using the cord as an antenna.

But if the noise is coming from the internal circuitry, you have few options left. Some hams have gone as far as covering the outsides of their PC cabinets with grounded copper mesh, but this is a bit extreme! The other option is to move your antenna if possible. Most computer interference is received at the antenna, so relocating the antenna may help.

Take a look at Chapter 18 in The ARRL Handbook, or pick up a copy of The ARRL RFI Book for more suggestions.

From QST January 1999