[RFI] computer keyboard

Hare, Ed, W1RFI Hare, Ed, W1RFI" <ehare@arrl.org
Wed, 28 Jan 1998 19:31:00 -0500

>Can anyone recommend a computer keyboard that is RFI resistant,
>preferably a compact type? I have applied ferrite toroids to both ends
>of the keyboard cable just in case the cable was acting as an antenna
>but this did not correct the problem. Un plugging the key board clears
>the problem. Whenever I transmit on 80 or sometimes 40 meters the
>computer genetates all kinds of strange characters, etc. on the screen.

Unfortunately, I can't tell you which keyboards are okay; hams don't call 
ARRL HQ and tell us that they are NOT intefering with their computers. :-) 
 At W1AW, they currently have Gateway 2000 computers; their keyboards seem 
immune, although I must admit that our antennas are located up in the air a 
bit. :-)

I note that you had tried ferrite material, but the single clamp-on beads 
are really not enough for 80 or 40 meters.  Get some FT-140-43 ferrite cores 
(Amidon, Palomar, etc), or other cores you KNOW to be ferrite that will work 
in the HF range and warp about 10 turns of the keyboard cable onto a ferrite 
core, at both ends of the cable.  This SHOULD make a significant difference, 
at least in the power theshold that causes the problem. If you find it goes 
from 15 watts to 95 watts, for example, you are on the right track.

If it makes no difference, it may be the wiring in the keyboard itself that 
is causing the problem. (This could also be true if it goes from 15 watts to 
50 watts, as an example: the problem could have a 15 watt threshold on the 
cable pickup and a 50-watt threshold on direct pickup.)  If it IS the 
keyboard, the easiest solution is to try another.  You could also TRY some 
of the EMI shielding sprays available, trying to get a good shield inside at 
least most of the keyboard.   You could also try a shielded keyboard cable, 
grounded at either the computer or the keyboard end, or both.  Flat 1/2-inch 
braided strap is usually hollow and can be used to shield your existing 

If you do try sprays, try to ensure that both halves of the keyboard case 
will be electrically connected to each other and to the keyboard-cable 
shield.  And remember, these sprays are conductive paint; if the surface 
being sprayed is not clean and compatible with the spray, the paint could 
flake off later, putting bits of metal flakes inside the keyboard.  The 
gibberish on the screen will probably return.

Radio Frequency Interference has a chapter on computer  RFI.

73 from ARRL HQ,
Ed Hare, W1RFI

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