Sunday, March 3, 2013

Seems doing nothng is doing something.....

RFI Friday night
My last post dealt with my PC monitors causing me some grief in the form of RFI to my K3. On Saturday I spent most of the day spinning my wheels trying to find a way to lesson or better yet cure the RFI problem. Below is what I had tried to in order to fix the problem.
1. I changed out the video cables and wrapped them with chokes as well.....did nothing.
2 Moved the monitors to other locations on the operating desk...that was loads of fun and it too did nothing.
3. Tried running the K3 just off battery power and.... nothing.
4. Ran the monitors from the 12 volt Astron power supply and again nothing.
5 Put snap on chokes on ALL cables coming out of PC and the ALL cables from each monitor.....NOTHING!
6. Even tried tin foil on the back of the monitor and grounding it...only made monitor look sci-fi.
7. Put 1:1 balun on the coax coming out of the K3 and still had problem.
8. Tried my Elecraft K2 (maybe just a K3 problem) found same issue with K2.
Sunday...not to bad

9. Plugged monitors into separate AC outlets on different circuits and did not fix issue.
10. Ran a ground up to each monitor and video card on the PC (PC is already grounded) and nothing.
11.Repositioned screen to see if that made any difference I thought it did but turned out not to.
 So that was how my Saturday was spent and from doing all the above I was able to find out a few things that I hope may ring a bell on how to solved the RFI isssue
1. As I move my hand closer to the monitor the RFI increases.
2. When the rig is on the dummy load the RFI is gone.....but so is everything else regarding signals.
Now it's Sunday and I have come to the understanding that I am going to just have to live with the problem. I know where it is coming from that sometimes that is half the battle! I am going to do some reading and thinking about the issue. I came into the shack to just roll the desk back up against the wall and tidy up the room. I then turned on the K3 along with the PC and for some reason the RFI is still there but not as much at all!!! Seems doing nothing is doing something!!


  1. I would say you have a common mode current problem induced in the coax cable by the monitor's internal circuitry, wich is radiating EMI. A coax cable is theoretically neutral to common mode currents because the shield is much thicker than the center wire and forces the induced signal to be differential rather than common mode, but as nothing is perfect you get a little bit of both. I'm not sure what your shack connection diagram looks like and if you have a proper RF ground plane, but as a general rule try to use a better coax in the shack, route it as far as possible from the monitors and use a proper choke right at the reciever input. Or, replace the monitor :).

    1. Good afternoon Razvan, The coax that comes from the attic is LMR400 and then the inter connections from rig to LP 100A is RG8X. I do have clamp on chokes from a local ham store, but what do you recommend for a choke at the receiver end of things?

  2. Mike,
    Good evening, and YO9IRF is 100 percent correct, one of club statesmen gave a presentation on Baluns and chokes. He suggested placing Mix 43 ferrite split beads on cabling inside the shack. As mentioned, the problem is common mode currents and differential, I'm in the process of choking 'all' my cables. Likewise Google, Palomar Engineers for additional information to consider, our club is getting ready to place a significant order.
    73, KA3DRR

    1. Hello Scot, thanks very much of the reference to Palomar I was at their site and it's very informative. I sure do have lots of cables in the shack and what you and YO9IRF mention seems to make sense. Have a good week.

  3. Mike,

    Common mode may be only a part of your problem. Here are a couple of links for some good information (if you haven't already stumbled upon them:

    There is plenty to be found through searches of the internet, some very good, and some of which conflicts with others.

    You may need to put a common mode choke at the feedpoint of your antenna and at your radio. The common "ugly balun" coil of coax is not likely to make any difference although for some it does appear too which points out the fact that when dealing with common mode noise that what works for one does not always work exactly the same for someone else.

    What I alluded to in my first sentence that common may not be the only problem is that your antennas being relatively close to the source of the RFI/EMI may simply be picking up RFI/EMI emitted by the monitors on those frequencies. In which case, no matter what choking you do, some of it will always get through. Hence the "touching the monitor made it stronger" type of observation.

    Good grounding and common mode chokes always are a good idea as these not only prevent ingress into your system RFI/EMI from outside but also helps protect outside systems from RFI/EMI from your system.

    I am constantly improving and updating in this regard, sometimes these changes make a very noticeable difference, sometimes very suitable to the point that any difference is nearly unidentifiable.

    cheers, Graham ve3gtc

    1. Good morning Graham, thanks for taking the time add the links to your post. I have seen these links in the past but have put it in the back of my mind to read my age my mind seems to fail me now and then and I don't get around to it. Yes my antenna is close to the setup and I imagine this also has some effect as well. Thanks for taking the time to comment and the suggestions are great as well. This time over the weekend I am going to sit down and read through the materiel in the links you sent.

  4. Hello Mike,

    Things like that can run you "nuts". My biggest problem is "dimmers" and "motion detectors" on the security system. Then there's the "day-night" light on the front porch. I have to just turn it completely "off" when I'm listening for weak signals.

    I find one of the great advantages of running QRP is getting "out of town" and away from all the electrical noise. Hope you're able to solve your dilemma there.

    1. Good evening John, you are so right this type of thing can take you to the dark side and back!! At one point I put 1:1 baluns on the feed line. It seem to had taken care of things.....YAH on one band but on another band things were 10 fold what they had been!!