[Elecraft] Monitor noise

[John Grebenkemper, KI6WX]

Paul;
Any CRT display generates strong magnetic fields at the harmonics of the
vertical and horizontal sweep rates.  These fields are used to sweep the
electron beam across the CRT.  On standard US television, these occur at 60
Hz and 15.75 kHz.  On a computer monitor, the vertical rate is usually 75 or
85 Hertz and the horizontal frequency is dependent on the vertical sweep
rate and the horizontal display resolution.  For instance, a 1024 by 768
pixel display with an 85 Hertz refresh rate will have a horizontal sweep
frequency of 68 kHz.

These magnetic fields are extremely difficult to shield.  Most TV's and
monitors these days have some magnetic shielding, but much of the energy
still leaks out.  In the early days of TV, there was no magnetic shielding
and most TV's would generate significant harmonics in the lower frequency
ham bands every 15.75 kHz.  There is much less of a problem these days
because there is now some magnetic shielding even though its primary purpose
is to keep magnetic fields from distorting the colors you see on the TV.

Computer monitors are a worse problem because the higher frequency of the
horizontal sweep can generate significantly higher HF energy.  The FCC rules
regarding computer radiated emissions don't start until 30 MHz, so by law
there is no requirement to limit the radiated emissions in the HF bands.  As
with TV's, the magnetic shielding that is used is designed to prevent color
contamination in the display, not to limit HF radiated emissions.

Improved grounding or removing grounding of the computer and monitor
shouldn't have much effect on the magnetic field emission.  I doubt whether
any conventional shielding would remove the problem; you have to use a
ferro-magnetic type of material to reduce these types of fields and it would
have to completely surround the monitor.

Changing the vertical refresh rate or the display size could move the
problem to a different frequency.

The one other method of attacking the problem is to determine if it is
coupling in at the antenna or the transmission line.  If you disconnect the
transmission line at the antenna end, does the level decrease or stay the
same.  If it decreases, the energy is coming in via the antenna and the only
way to reduce it is to locate the antenna further away.  If it stays the
same, disconnect the coax from the receiver.  If it decreases, the noise is
entering by the coax.  This can sometimes be reduced by putting a common
mode choke on the coax.  If it stays the same, it is coupling directly into
the receiver either via the power connection or directly through the
receiver case.  A power connection problem can usually be helped with a
common mode choke on the power feed.  If it is directly entering the
receiver, moving the receiver away from the monitor will reduce the noise
level.

Hope this provides some ideas on how to attack this type of problem.

-John
 KI6WX




> I'm nearing the end of my rope.......
> I've been trying to "noodle through" the problem of horrible noise to my
> K2(and other HF rigs) from my Compaq Presario MV500 computer monitor. I
> picked up a boatload of snap-on and varried ferrite beads of all shapes
> and sizes and installed them on every cable coming in and out of the
> computer and monitor and power cables to the radio and computer. There
> was no change in the noise which is 7khz wide and appears every 50khz or
> so on almost every band. (Also, my radio and computer equipment is
> grounded to a ground strap connected to a cold water pipe 4' from my
> operating desk. )
>
> It seems that the problem might lie in the design of the monitor, but
> I'm trying to avoid buying another one. Has anyone had any success
> solving this problem? Or, can anyone recommend a different compatible
> monitor that solved their problems?  Any thoughts on the flat-screen
> monitors?  I'm quite sure that I'm missing oodles of DX hidden behind
> these various "walls of noise" :-)
>
> Thanks!
>
> Paul    kb9yvp