Here are some tips you can use to fix monitor RFI in your radio room. I hope these will work for you.
Increasingly, hams and shortwave listeners are using computers in the shack. Computer monitors can produce radio interference, or RFI. What’s more, if you use SDR with a spectrum scope, this interference is visible and most annoying. And the offending monitors don’t even have to be near your radio. Monitors anywhere in the house, or even next door, can cause RFI.
The picture above shows the two types of common RFI from monitors. First are the birdies, or oscillations that spread out over a range of frequencies. Usually these are quite weak, but can often be S7 or more. Second is the switched mode power supply hash. These are often quite strong. As shown, they will vary in frequency as the free running oscillator in the power supply warms up, and then settle down. Birdies can be eliminated with a notch filter. Hash can be reduced with a noise canceller as long as you have a noise antenna near the monitor. But best to eliminate at source.
There is no such thing as a better brand that is free of this problem. Generally, you need to check out different monitors and find ones that work best in your radio environment. So, here are my three tips to fix monitor RFI.
- Stay away from monitors with external power cubes. The cable between the power supply and the monitor can act as an antenna for common mode interference. Better to use a monitor with an internal supply.
- Use DVI over VGA cables. My experience shows that DVI produces less RFI. Many monitors today offer both connectors so you have a choice.
- Experiment with VGA different cables to fix monitor RFI. Not all cables are created equal. Some are better shielded. Make sure your cable has ferrite core choking. But remember, some ferrite mixes don’t work well at HF. Fortunately, most of us have a box of old VGA cables lying around.
The RFI shown above is a screen capture of my main monitor interfering with reception on my Perseus receiver, on a different computer. This monitor runs off a laptop which only has VGA. By substituting different cables I was able to eliminate both types of interference.
Another tip is to try winding a choke into your monitor cable. Just a few small turns taped together. This can help.
Fix Monitor RFI – Kijiji or Craigslist to the Rescue
Sometimes the simplest way to fix monitor RFI is to just get a different monitor. Up here in Canada you can use Kijiji to find good quality used monitors at very reasonable prices. I use a pair of 20″ LG Flatron DVI monitors on my SDR computer. Each cost $30 on Kijiji. Once I found that this model was free of RFI, I got another one.
Similarly, I replaced my wife’s noisy externally powered VGA monitor which sent S9 birdies around the house. Her new monitor is a used BenQ 21″ model for $25. Completely RFI silent. She’s happy because it’s larger. I am happy because it has no birdies.
Incidentally, the power supply noise (hash) shown above was coming from the VGA cable, not the monitor itself.