LED Screen RFI - Or is it?

Yesterday, in my continuing attempt to clear the kitchen table of radio clutter, I took delivery of an LG LED monitor.  This was to allow a notebook (remember those?) to be taken off the table and permit easier reading for tired, ageing eyes.  The Samsung notebook and its PSU, now five years old, has never caused a whisper of RFI.


Whilst my station now looks a bit like those 'big guns' on QRZ, with their screens in the air looking like a NASA control room, the LED screen initially looked like a major regressive step and a total waste of £91.

The reason?  RFI, of course!

A clear table and easier viewing.  But the RFI is bad...initially.

Grounding the almost impossible to access chassis of the screen by using one of the VESA mounting screws to make the connection reduced the RFI by a small amount.  But it wasn't possible to reduce it further.  It was difficult to identify the exact source of the RFI, but it seemed initially that the switch-mode PSU might not be the culprit.

I opened up the switch mode PSU to see if any components had been left out.  It seemed the PSU, whilst as basic as they come, was nevertheless reasonably filtered.  I wrapped the mains and outlet cables around ferrite cores, but this didn't make any noticeable difference.

Just add foam!  Whilst it doesn't fill me with confidence, this 19V, 2A SMPSU seems to be very well filtered.

I read online that VGA cables, even when they have ferrites at both ends, can be powerful emitters of RFI when active.  It was clear that, when there was no signal but the LED screen was nevertheless on (e.g. showing menu displays, etc.) , the RFI diminished drastically.


So, I took to wrapping my VGA cable in - wait for it - aluminium foil!  I connected this to ground by means of an existing connection to the RF ground, keeping it in place with a simple cable tie. 

Whilst this initially seemed to work spectacularly well, with no RFI resulting, that RFI soon came back.


The problem turns out to be related to the display resolution.  If the native laptop resolution is used, the RFI is diminished.  Or, rather, it shifts along in frequency such that the RFI might then occur outside of your current operating band.  It doesn't disappear altogether.

The wrapping of the power line to the monitor, nearest to the screen itself, through a large ferrite collar several times made a drastic reduction in intensity of the RFI.  But, again, it doesn't remove it altogether, and it can be strong on some frequencies.

I don't know why this problem occurs, so if you have more technical expertise or insight than me - and, let's face it, that isn't difficult - then please submit a reply to this post.

As a final precaution, I kept the monitor chassis grounded.  I also soldered a small wire to the shield of the VGA cable and connected this to the station RF ground.  I doubt it makes any difference at all, given the coaxial nature of the cable, and that the chassis ground should be taking care of the whole system.

After a few days, I decided, though, that this monitor produced far too much RFI for a situation where there was previously none.  I switched the monitor for a 10-year old HP w22 monitor, which looks to be well shielded internally.  Other than one or two very weak peaks on 12m, this monitor is RFI-quiet, and entirely acceptable.  It also has a pleasing screen ratio, unlike today's letterbox screens!

So, out with the new, in with the old!

Update: Even the HP monitor produces too much RFI in a station where there is hardly any man-made noise.  So, I'm back to the old laptop, which has no detectable interference issues.