>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
cable.
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
ARRL Lab
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