Computer Monitor EMC by G4OBK

(This article originally appeared in CDXC Digest)

Since installing a computer in my shack in 1991 I have found that my computer monitor radiates significant RF interference into my HF antennae. The usual, but often impractical, piece of advice is to move the antenna further away from the source of interference – the shack. However, even with antennae as far as 100 feet away the problem still arises. I have found that virtually all conventional computer monitors that use switched mode power supplies will radiate harmonics at between 30 and 50 KHz intervals throughout the HF spectrum. The radiation eases above 30 MHz. Over the years the only “clean” monitor I’ve had was an old Phillips VGA model that I sold in 1998 to Bill Hawthorn G3MCS. I’ve used the following makes of 15 inch SVGA monitors with similar results: AOC, IBM, COMPAQ, MAG, PHILLIPS and GOLDSTAR. Most of these monitors were TCO95, TCO99 and MPRII compliant, but this didn’t make one iota of difference. There are several recommended fixes to prevent a monitor radiating, most of them work to some extent, and some of them make matters worse! Ferrites on both ends of mains leads and on graphics leads, earthing the monitor chassis to your station earth, removing the monitor from its case and treating the inside of the case to a coat of electro-conductive paint before earthing it are examples. You can also try covering the outside of the case, where there are no vents, with foil and then earthing it. All these potential fixes were tried over the years with little success. The sure fix is to switch the monitor off when the signal of interest happens to be centred on one of the sproggies. As I became interested in RTTY and Data this option was a non-starter as it is impossible to hold a QSO without the monitor being turned on. Another dodge is to alter the refresh rate of the monitor in windows by going into –

Control panel>settings>display>settings>refresh frequency. By altering the refresh rate you will find that the interference can be moved around the band and the distance in frequency between sproggies can be changed.

It’s a fact that laptops radiate very little – that was one option but due to the cost, lack of sufficient comms ports and user comfort I ruled this out.

Tired of shooting myself in the foot I decided this year to sort the problem out once and for all. I did a straw poll on the Packet Cluster. The only monitor that was reported “clean” was the Sony Trinitron. So I purchased one last month from PC World – the A200 17 inch Sony, on offer at £199. THIS MONITOR RADIATED MORE INTERFERENCE THAN ANY I HAVE PREVIOUSLY USED! (Maybe older models were better.) With my HF beam antenna 100 feet away from the shack I was picking up between S4 and S7 of noise at 40-50 KHz intervals throughout HF, and that was with a beam antenna 100 feet distant. The problem was at it’s worst on 24/28 MHz. In desperation I returned the monitor to PC World and looked for an alternative. Under their returns procedure any item can be returned within 14 days for an exchange or refund without giving a reason. I opted to exchange the Sony for a Mitac LCD TFT 15” EX15 model. This unit comes complete with an external 12v 4A switched mode PSU (not the “wall wart” type but with input/output fly leads). It also has built in speakers for audio from the soundcard. Not only is the screen clarity far superior and sharper than any SVGA monitor I have seen, there is no RF radiation anywhere to be found!

If you don’t go down the LCD route and want to reduce the amount of RFI emissions from your existing monitor I would refer you to The RSGB Guide to EMC, Appendix 6 by G3JWI.

Phil Catterall G4OBK

E-mail: g4obk@philcatterall.plus.com

Website: http://www.qsl.net/g4obk