[RFI] Keyboard RF Susceptibility

Michael Tope W4EF at dellroy.com
Thu Aug 9 18:27:38 EDT 2007

----- Original Message ----- 
From: "Michael Tope" <W4EF at dellroy.com>
To: <rfi at contesting.com>
Sent: Thursday, August 09, 2007 11:08 AM
Subject: [RFI] Keyboard RF Susceptibility

>I am trying to clean up a few problems at my station. Right now
> I can transmit on 160 meters at 1.5KW with the stereo turned 
> and the TV's on and I get no hint of RF interference. The video 
> display on my computer is clean, the internet doesn't get 
> blocked, and my mouse works fine. The one area where I do
> have trouble is my computer keyboard. It won't recognize button
> closures when I am transmitting until I drop my output power 
> down to around 300 watts. I've tried two (admittedly cheap) 
> keyboards and both of them exhibit this same RFI susceptibility 
> problem despite the fact that in both cases I had the cable 
> wrapped around a #31 ferrite core.  
> Any suggestions out there for a keyboard that might be more
> RFI proof than the ones I've tried so far (one was an $8 Inland
> brand and the other the OEM keyboard from my wife's 
> E-Machine)?
> Thanks,
> Mike W4EF.................................................

Thanks to everyone who sent me suggestions. I did some more 
experiments and it looks like its direct radiation into the keyboard
as opposed to common-mode action on the cable. With 8 or so
turns of the cord wrapped around a 2.4" O.D. #31 core (ferrite 
placed close to the keyboard), the threshold for keystroke interruption
is around 300 watts for two hands on the keyboard and around 500 
watts for one hand on the keyboard. If I remove the ferrite core from 
the cord, the threshold actually goes up to around 500 watts with both 
hands on the KB and around 700 watts with just one hand. The fact that 
the threshold is a function of hand placement leads me to believe its 
direct coupling to the key buttons. In fact, if I use a wooden dowel rod 
instead of my hand to press the keys, the threshold increases to around 
1200 watts. 

Looks like another keyboard might be in order. Another possibility
might be a metal shield underneath and an overshield for the cord: 


Anyhow thanks again, folks. 

73, Mike W4EF..................................

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