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For some reason this didn't get posted...let's try it again.
Keith
>
> Vince,
>
> Unfortunately, a "shield" in front of the CRT will do essentially
> nothing at HF, and very little at VHF. While the placement of an RF
> "barrier" in front of a CRT may reflect some direct radiation at VHF and
> higher frequencies, any claims of RF shielding for these types of
> screens are essentially smoke & mirrors. In order for any shield to
> contain emissions, it must approximate a faraday cage, i.e., be
> electrically continuous, which is impossible to achieve by placing a
> conductive barrier in contact with a plastic CRT housing. There have
> in the past been a number of designs which took a brute force shielding
> approach to CRT's, but most of them looked like battleships and were
> basically a big metal box with a conductive screen you could look
> through to see the face of the display.
>
> There are two general sources of emssions in a computer display;
> switching noise from the power supply, and radiation of the video
> waveform itself, with all its harmonics and subharmonics relating to
> frame rate, line rate, dot rate, and what is being displayed. A lot of
> power supply noise problems could be readily resolved by the use of
> appropriate EMI filters on the power supply, along with some judicious
> application of rise/fall time control on the switching waveforms.
> Unfortunately, such things cost money and so the manufacturers tend to
> leave out everything they can get away with. Sometimes a common-mode
> choke or an RFI filter on the power cord will help.
>
> Radiation of the video waveform is much more problematic, since in order
> to obtain a sharp disply the electron beam must be rapidly switched on
> and off, which means waveforms with lots of energy and very short rise
> and fall times. Shielded video cable can sometimes help, provided there
> is a way of properly terminating the shield to provide a return path to
> the source of the energy in such a way that the return path itself
> doesn't create a radiation problem. Common-mode chokes can sometimes
> help here if the cable itself is radiating, but there isn't much you can
> do about radiation from internal wiring without getting into the guts of
> the display.
>
> There are three parts to every RFI problem; a source, a transmission
> path, and a victim. In the case of a computer monitor, you may be
> better off to attack the transmission path and victim. I've seen
> noticeable reduction in noise in my own shack by nothing more than the
> simple expedient of moving the computer to a different AC outlet than
> the one used to power the radio. Moving the monitor a little ways away
> may also be sufficient. Making sure you have a good RF reference point
> for all the station components can also sometimes help by minimizing
> susceptibility to circulating currents induced by the noisy monitor
> (Note that having a good RF reference point/plane for the station
> equipment has very little to do with a connection to physical earth).
>
> Unfortunately, there will be situations where none of these
> after-the-fact patches are adequate, in which case you either turn the
> monitor off while operating or try to find a quieter unit.
>
> Good luck,
>
> Keith
> KB6B
>
>
> VinceK6BIR@aol.com wrote:
> >
> > I have seen a lot of talk here on RFI from a computer moniter to the rigs,
> > has anyone tried a moniter screen sheild? I was looking at them in the
> > stores, and I see that there was two diferent types, the one that cost more
> > said that it prevented RFI, this make me wonder if it would help, has anyone
> > tried using one, and if so what was the results? If they did help, which one
> > did you use?
> > Tnx, Vince K6BIR
> >
> > --
> > FAQ on WWW: http://www.contesting.com/FAQ/rfi
> > Submissions: rfi@contesting.com
> > Administrative requests: rfi-REQUEST@contesting.com
> > Questions: owner-rfi@contesting.com
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Message-ID: <39810CD5.220B1D94@qnet.com>
To: <rfi@contesting.com>
Date: Thu, 27 Jul 2000 21:32:21 -0700
From: Blackburn <blackburn@qnet.com>
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To: rfi@contesting.com
CC: VinceK6BIR@aol.com
Subject: Re: [RFI] Moniter RFI and screen sheilds
References: <ca.7f41d66.26b06d72@aol.com>
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Vince,
Unfortunately, a "shield" in front of the CRT will do essentially
nothing at HF, and very little at VHF. While the placement of an RF
"barrier" in front of a CRT may reflect some direct radiation at VHF and
higher frequencies, any claims of RF shielding for these types of
screens are essentially smoke & mirrors. In order for any shield to
contain emissions, it must approximate a faraday cage, i.e., be
electrically continuous, which is impossible to achieve by placing a
conductive barrier in contact with a plastic CRT housing. There have
in the past been a number of designs which took a brute force shielding
approach to CRT's, but most of them looked like battleships and were
basically a big metal box with a conductive screen you could look
through to see the face of the display.
There are two general sources of emssions in a computer display;
switching noise from the power supply, and radiation of the video
waveform itself, with all its harmonics and subharmonics relating to
frame rate, line rate, dot rate, and what is being displayed. A lot of
power supply noise problems could be readily resolved by the use of
appropriate EMI filters on the power supply, along with some judicious
application of rise/fall time control on the switching waveforms.
Unfortunately, such things cost money and so the manufacturers tend to
leave out everything they can get away with. Sometimes a common-mode
choke or an RFI filter on the power cord will help.
Radiation of the video waveform is much more problematic, since in order
to obtain a sharp disply the electron beam must be rapidly switched on
and off, which means waveforms with lots of energy and very short rise
and fall times. Shielded video cable can sometimes help, provided there
is a way of properly terminating the shield to provide a return path to
the source of the energy in such a way that the return path itself
doesn't create a radiation problem. Common-mode chokes can sometimes
help here if the cable itself is radiating, but there isn't much you can
do about radiation from internal wiring without getting into the guts of
the display.
There are three parts to every RFI problem; a source, a transmission
path, and a victim. In the case of a computer monitor, you may be
better off to attack the transmission path and victim. I've seen
noticeable reduction in noise in my own shack by nothing more than the
simple expedient of moving the computer to a different AC outlet than
the one used to power the radio. Moving the monitor a little ways away
may also be sufficient. Making sure you have a good RF reference point
for all the station components can also sometimes help by minimizing
susceptibility to circulating currents induced by the noisy monitor
(Note that having a good RF reference point/plane for the station
equipment has very little to do with a connection to physical earth).
Unfortunately, there will be situations where none of these
after-the-fact patches are adequate, in which case you either turn the
monitor off while operating or try to find a quieter unit.
Good luck,
Keith
KB6B
VinceK6BIR@aol.com wrote:
>
> I have seen a lot of talk here on RFI from a computer moniter to the rigs,
> has anyone tried a moniter screen sheild? I was looking at them in the
> stores, and I see that there was two diferent types, the one that cost more
> said that it prevented RFI, this make me wonder if it would help, has anyone
> tried using one, and if so what was the results? If they did help, which one
> did you use?
> Tnx, Vince K6BIR
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/rfi
> Submissions: rfi@contesting.com
> Administrative requests: rfi-REQUEST@contesting.com
> Questions: owner-rfi@contesting.com
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--
FAQ on WWW: http://www.contesting.com/FAQ/rfi
Submissions: rfi@contesting.com
Administrative requests: rfi-REQUEST@contesting.com
Questions: owner-rfi@contesting.com
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