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strange pc monitor problem

I'm running a Flex 6400 on a fast  Windows 10 Core I7 system with 3 monitors, one of which is a Samsung 40" 4K TV monitor.  The setup is great.

But while I am using FT8 on 40m, when I transmit, the Samsung monitor turns itself off (goes blank) for the duration of the transmission cycle.  This happens independent of TX power level.  But it only happens on 40m, not 80m or 30m or 20m.

I have the PC and monitors connected to the same AC outlet as the Flex 6400 and its power supply.  I am using less than 600 watts on that circuit even when I am transmitting on the Flex at 100 watts.

Has anyone encountered a problem like this or can anyone suggest a way of fixing it?

Michael -- KM6LDH

Completed · Last Updated


  • George Molnar, KF2TGeorge Molnar, KF2T Member ✭✭✭
    edited February 2019
    You are experiencing RFI to the monitor.
  • Mike-VA3MWMike-VA3MW FlexRadio Employee, Community Manager admin
    edited April 2019
    Hi Mike

    The issue is that your antenna is working well, but RF is getting into the monitor.  Every lead you have going to the monitor is a possible antenna.

    Does the problem happen at lower power levels?

    The Fix is adding RF chokes to the power leads and the video lead going to the monitor.  If that doesn't fix it, adding chokes to the PC power cords, and every other cord headed to the computer.  

    Essentially, your monitor is affected by 40M RF overload even though it is not happening on other bands.

    Mike va3mw

  • Kevin CondeKevin Conde Member
    edited April 2018
    I had that issue on 17 meters.  I run 4 monitors, and had an inordinate number of USB devices.  Prior to replacing the CPU three of the monitors were connected via USB.  Now only 1 is connected via USB.

    I pulled the station apart, rerouted cables.  removed cables that were no longer in use, and generally simplified.  The issue then went away.  My suspicion was the RFI was was affecting the USB system more than the monitor itself.
  • Jim GilliamJim Gilliam Member
    edited April 2018

    The first line of RFI defense for those using a balanced antenna like a dipole is a current choke BALUN located at the feed point of the antenna. It is possible to get a lot of radiation from the outside of the braid of  coxial cable due to common mode currents. These currents without a BALUN can be very low or very high depending on the length of the feed line running to the transmitter.

    Jim, K6QE

  • EA4GLIEA4GLI Salvador Member ✭✭
    edited April 2018
    Before you go crazy adding ferrite cores to every cable do this that Jim advices. Dipoles are unbalanced antennas by nature, a choke is absolutely essential to avoid rf returning through the braid of the feedline. Adding rf cores to all the computer cables is putting band aids but not fixing the problem, which is the rf getting back into the shack.
  • WayneWayne Member ✭✭
    edited June 2018

    I use an LG 29" 4 plex monitor sitting above the flex and my KPA500 is about a foot to the right of the monitor on a metal rack. Anytime I use the KPA500 at any power and I am displaying the Flex6400M screen on the monitor or if I am using Smartsdr for windows and its showing on that monitor when I transmit and key the KPA500 the monitor goes blank for the transmit duration then returns to normal. First time I seen it I panicked but then noticed the 480watts and my transmission was still normal from the Amp so I was happy.

    Didn't fix it yet, I just accept it for now since its the nature of the beast with my setup. If I don't use the KPA500 and just use 100 watts it does not happen so I know whats causing it and since I don't use the amp that often I haven't sought to fix it, I just live with it.

    Maybe its just Samsung/LG monitors I don't know since that is what I use almost all the time.

    My station has an MFJ-915 Isolator immediately after the antenna switch as it enters the shack. So its the power on its way out to the antenna that is the issue. It might even resolve itself if I moved the KPA500 away from the monitor but with my setup that's not an option for now.
  • Mike W9OJMike W9OJ Member ✭✭
    edited April 2018
    But while I am using FT8 on 40m, when I transmit, the Samsung monitor turns itself off (goes blank) for the duration of the transmission cycle.
    Is the monitor shutting off completely or is the video just blanking for the duration? I had this problem while running a KW and the monitor would shut off completely and turn itself back on with the ACER logo and the boot up which took about ten seconds. I put a snap on ferrite on the power lead and it seems to have fixed the problem although it was somewhat intermittent anyway so it could possibly happen again.
  • edited June 23
    Thank you to all! 

    It was indeed RFI on the HDMI cable to the monitor.  I diagnosed it by lowering the TX power to a point where the monitor would not go blank, and then gradually increased TX power until the problem occurred again.  Then I put another snap-on ferrite and tried it again.  I repeated this process adding ferrites until I reached full TX power.  I have 5 ferrites on the HDMI cable now and the monitor no longer goes blank.

  • Ken - NM9PKen - NM9P Member ✭✭
    edited April 2018
    If the cores are big enough, you can pass the cable through it more than once, and you get roughly 4x the isolation for each turn... at some frequencies...better than simply adding another core.

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