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Re: hf noise from monitor



At 04:58 AM 2/8/2003, nick pugh wrote:
>I have ask on the bb before but no joy. I am getting from interference on
>20m from my monitor and have been unable to get it out.

I've found it very difficult to retrofit computer monitors for reduced EMI.

Here are some ideas:

Shielding is almost impossible after the thing is built.  I tried spraying 
the inside of a monitor case with conductive paint once.  It did not work 
out well.  First I got the wrong kind of paint that didn't stick well to 
the plastic.  Flakes of conductive paint inside a monitor with all the high 
voltage wiring running around is not a good thing.  After I recovered from 
that with some different paint, I found that the shielding wasn't very 
effective, because I was unable to get good connectivity at the various 
joints, connector exits, etc.  It is just really hard to put in shielding 
after the fact.  The effect of a "crack" between two pieces of shield is 
inversely proportional to the length of the crack.  Fingerstock is the 
correct solution for such seams, but retrofitting fingerstock to a molded 
plastic case is nearly impossible.

Ferrite beads are your friend.  Buy some of those split beads that are 
designed for use over large diameter cables.  Try to get the kind of 
ferrite that is active in the frequency range where you've got the 
interference problem.  Some are designed for VHF, others for HF.

Use a sniffer to locate the big sources of RFI.  I suggest using your HF 
receiver as the sniffer.  Tune to the frequency of the nasty 
interference.  Make a probe by soldering a little loop of wire to a coax 
connector.  Attach this loop to a cable which goes to the antenna input of 
your HF receiver.  Now "sniff" around the monitor with this probe.  Use the 
sound level out of the HF receiver, or the S-meter to tell you what's hot.

Cables are often hot.  If so, put beads on the cable.  Note that a bead's 
location on the cable may matter.  Slide the sniffer down the cable, 
starting at the monitor.  If you see different signal levels at different 
places on the cable, try putting the bead where the signal level is 
highest.  The loop pickup senses current, and the bead inhibits current, so 
the bead should logically have maximum impact where the probe senses most 
current, all other things being equal.  All other things are seldom equal, 
but its a good theory.

Open up the monitor.  (Taking precautions to avoid electrocuting 
yourself.)  Sniff around inside.  There is usually a loose bundle of about 
5-10 wires going to a socket on the tail end of the CRT.  In my experience, 
this bundle is a hot RF source.  I have had good results from putting a 
split bead around this bundle.

Your mileage may vary.

Someone else suggested changing scan rate.  That is also a good idea.  It 
will mostly just move the interference, but if it moves out of your 
favorite band, that may be all that is needed.

My experience is that newer computer equipment generally generates less EMI 
than older stuff.  Again, you mileage may vary a lot.  The FCC standards 
are nowhere good enough to prevent the kinds of interference issues that 
hams have in the shack.  The FCC standards can be logically derived from a 
set of assumptions which are appropriate for the problems they were trying 
to solve: ordinary consumer has a computer and a radio and a TV.  The user 
is in a city where the FCC has mandated that the radio and TV station have 
excellent coverage (ie pretty high signal level).  The appliances in the 
house can be moved away from one another.  The ham on the other hand is 
listening often to very weak signals from far away, and he wants to use the 
computer on the same desktop as the sensitive radio.  These assumptions 
logically produce EMI requirements about 30dB tighter!  Because stuff that 
meets the FCC standard can easily be not good enough for ham use, results 
are kind of random.  Depends on the particular brand and model.

Also consider that you may be able to improve things by modifying your 
radio setup.  Consider the paths that currents take to get into your 
radio.  Is the signal from the monitor being received by your antenna?  If 
so, you have little choice but to reduce the emissions or move the antenna 
farther away.  However often the signal is received by your coax antenna 
cable.  This can happen inside the shack, where the monitor is only a few 
feet away from the antenna cable.  The monitor induces currents in the coax 
shield that ride up the cable to the antenna.  To reduce this effect, you 
need to inhibit currents on the outside of the coax.  One fix is ferrite 
beads on the antenna cable.  That works on VHF, but it is difficult to get 
enough series impedance at HF.  Better yet some people make "shield 
breaker" gizmos specifically designed to do this.  I don't remember brand 
names, but I've seen 'em on ham web sites.  An old ARRL handbook suggested 
taking a cardboard tube from the inside of a roll of paper towels, and 
sliding your antenna coax thru it.  Then stuff the inside with steel 
wool.  As current flows up the outside of the coax, the field generated 
induces currents in the fibers of the steel wool, where it is dissipated by 
the lossy steel.  Something like that anyway.  Don't know if this 
works.  I've never tried it.  Another handbook somewhere suggested making a 
6" coil of several turns in your coax somewhere in the feedline, preferably 
not near your computer.  This works by making the current on the outside of 
the coax see the impedance of the coil.  (Like a balun.)  That impedance 
then acts to inhibit current on the outside of the coil.

Good luck.


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