[RFI] Disconnecting from Internet?

Keith Blackburn blackburn at qnet.com
Tue Apr 29 22:01:08 EDT 2003

A couple more comments.  Don't forget that a single clamp-on ferrite core
probably won't have enough impedance at HF to do much good.  Try winding
several turns through the core, or use a number of cores (a'la bead balun,
which is actually a common-mode choke).

There are a couple of ways RF could be getting into your computer.  Common-mode
currents coming in on antenna leads are a common source of problems with RF in
the shack.  Your bead baluns on the feedline should help this situation.  You
might also check & make sure your tuner actually has a balanced output to feed
the ladder line - I'm not familiar with the SGC tuner.  I ran a G5RV for a
while which was fed via coax, isolated at the coax to ladder line transition
with a common mode choke consisting of about 10 turns of coax, and fed with an
unbalanced tuner in the shack.  Had to find the right feedline length to
present an acceptable impedance to the transmatch, but had no problems with RF
getting into the shack.

Another possibility in your case is that the field generated by the antenna
itself is strong enough to induce circulating currents into the computer
interface wiring.  The combination of power and phone lines could be acting
like an antenna, and forcing RF to flow through the computer itself.   A good
common mode choke on the computer power cord would help in this situation (a
typical RF filter won't, since the fault protection ground lead provides a path
around the filter for common-mode currents).

I'll also second the recommendation to make sure all your radio and computer
equipment is bonded together with low impedance connections.  Providing a low
impedance path for RF to flow will reduce RF voltages between the various
components and provide an alternate path for currents that may otherwise be
flowing through the computer and modem.

Good luck,


Pete Smith wrote:

> In my limited experience with RFI in the shack computer (I used to crash
> RecAll Pro on 15 meters routinely) good low resistance common grounding
> between the transmitter, the PC case and the shack bulkhead produced a cure
> after extensive ferrite treatment of lines into and out of the computer
> failed to help.  Not coincidentally, this also cured a common mode AC hum
> problem that had plagued my efforts to use my sound card as a contest voice
> keyer.
> 73, Pete N4ZR
> _______________________________________________
> RFI mailing list
> RFI at contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi

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