Thursday, 17 May 2018

LED Montor RFI: the biggest ham radio mystery?

A while ago, I wrote about my troubles with a 21" LG computer monitor (no TV reception circuitry), which produced copious amounts of interference to all bands up to at least 50MHz.

The best solution I came up with was to wind a 10m VGA cable on a wide PVC former to make a choke.  This worked to a degree, but the bands were still simply nowhere near as quiet as they were without the monitor.

I quickly decided to discontinue the use of the £100 monitor as unsuitable for radio station purposes.

During and since those days, I've read the various discussions and blogs about screen RFI.  In fact, given LED screens' popularity in 'NASA-style' ham stations, and that there are so many hams out there, it is a genuine mystery as to why there is not much more discussion about the problem.  I think a lot of it may be to do with the fact that so many people live in RFI-infested areas already.

A search for videos, perhaps to see someone showing RFI on their monitor, yielded no results.

I found a strong element of capacitive coupling with my screen's RFI.  If I brought my hand closer to the screen, or just simply stood in front of it, rather than behind, it would produce much stronger RFI.  I don't know of a potential solution to that problem.  Grounding the screen also provides a handy path for the monitor-generated RFI, which similarly increases in strength.

Most people who claimed to have resolved RFI from screens clearly hadn't.  They had simply added chokes or other measures that only moved the RFI in frequency and maybe reduced its level somewhat, thinking this 'fixed the problem'.  There's a big difference between reducing RFI and eliminating it, especially if you are interested in weak signal work.

When RFI moves in frequency, it doesn't do so to such a degree that it falls magically outside the ham bands.  It just appears in other parts of the band.  So you might find the FT8 section of 20m free of RFI, but parts of the SSB portion displaying very strong birdies.  You can often simulate the 'fixing' of RFI from screens just by changing the resolution.  You may find your WSPR portion nice and free of noise at lower resolution, only to find the RFI is now down at CW parts.

Things that do NOT reduce RFI are:

(1) The PSU. Powering the LED screen from 12V batteries yields the same RFI.
(2) Screening cables.
(3) Grounding the screen and/or cables
(4) Using HDMI instead of VGA cables

Things I found that do reduce the RFI, but not by remotely enough to return to no-screen, RFI-free conditions:

(1) Connecting a random wire to a grounding point on the screen (e.g. connector chassis), causing the chassis to detune slightly. With a laptop screen (HP), I find this detuning is effectively enough to call it a complete fix.
(2) Different positions and distances of screen from transceiver.

(3) Not having your body close to the screen.
(4) Taking cables away from the screen at right angles as far as possible.
(5) An air-wound choke of around 20cm diameter (VGA only; HDMI cables are generally too short for this).

It seems that LED screens, more especially more modern ones, produce high RFI levels across the board.  It doesn't much matter which one you buy.  I've tried our home TV, a four year-old Samsung LED unit, and it also produces the same noise, although at a significantly lower level than the LG, despite being physically larger.

I also found the same capacitive coupling RFI effect with a Nexus 7 tablet connected to a charger.  Whilst it produces no detectable RFI when simply sitting there on a desk, it produces a low but detectable (by ear) change in noise levels when I place my hand near the screen. 

If you have any ideas or example of fixes that actually work and eliminate RFI altogether, then I and many other people would like to hear from you!






 

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