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Re: [RFI] Advice for LCD cmptr RFI

To: rfi@contesting.com
Subject: Re: [RFI] Advice for LCD cmptr RFI
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sun, 26 Dec 2010 22:01:25 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On 12/26/2010 1:49 PM, Dirk Hacker wrote:
> I have put "clamp-on -type" ferrites on the LCD cable (one on each end), 
> however they don't supress the RFI

Of course they don't -- they're designed for use at VHF, not at 28 MHz.

Study http://audiosystemsgroup.com/RFI-Ham.pdf

The executive summary is that the trash you hear can be both radiated 
and conducted by cables connected to the LCD, or to the computer, or to 
the power supply. You need to kill that current with a common mode choke 
tuned to 28 MHz (and to other frequencies where you hear RFI).  That 
requires a MULTI-TURN choke through a ferrite core. Study the tutorial 
for guidance on winding the chokes.

It is also possible for RFI to be radiated by wiring inside the LCD 
monitor because it is poorly designed and/or poorly shielded.  Chokes 
will NOT fix that part of the problem -- for that you need to either 
shield the LCD (usually impractical), or do major surgery inside, or 
toss it into the trash.  The good news is that radiation directly from 
the unit is usually much weaker than that radiated or conducted by wires 
connected to the monitor.  An LCD monitor I bought a year or two ago was 
noisy. I put chokes on both the AC cord and the video cable and killed 
the noise.

Another important point.  This RF trash can be conducted into radios 
that are not properly bonded together and to ground, and by cable 
shields that are not properly bonded to the chassis at each end. That's 
called "the Pin One Problem," and it's a major cause of hum, buzz, and 
RFI.  Study the material on my website about this.

73, Jim K9YC
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