Suspect RFI from overhead mains cable

Discussion in 'Interference Problems' started by Jeff - G0WSA, Aug 26, 2017.

  1. Jeff - G0WSA

    Jeff - G0WSA New Member

    Hi There,

    I'm setting up my shack in our new home (though we've been here two years) and I have noticed there is a high level of noise across the entire HF spectrum. In general terms, all the HF bands seem to have S7-9 noise. I notice no peaks, no modulation, no notching - just old-fashioned white noise.

    What I could do with is a bit of advice on how to hunt this down. The equipment I have available to me is:

    Yaesu FT-920 connected to an horizontal OCFD or a commercial discone (vertical). I note that the noise almost disappears on the vertical antenna.

    FunCube Dongle on my laptop

    A borrowed RF-explorer hand-held spectrum analyser (from 15MHz upwards)

    I live on the edge of a village near Oxford, with a neighbour on one side and fields to the other sides. As I walk around my property with the spec analyser, I see high levels of noise on one side of my house (the farthest away from the neighbour) It is noisiest close to the pole which brings mains and telephone into the house.

    I've turned off the power at the fuse box to our house but this makes no difference.

    We have a solar farm one field away but walking around it with the analyser I see no real noise, this only returns when I go back to my house.

    The analyser has a simple whip antenna and I do notice the noise level change significantly when I rotate the antenna through 180 degrees on the horizontal and it seems to drop away when I orientate the antenna vertically.

    What I can't figure out is whether this is on the mains or the phone line (we have FTTC by BT)

    Is there anything more I can do to isolate this? Is powering off at the fuse box enough to discount anything in our property? Do I need to disconnect the BT HomeHub or telephone wiring? I'm hoping for a power cut to the village to see what difference, if any, that makes!

    What about further investigation? Is it worth trying to make a small antenna for the fun cube dongle or a different antenna for the spec analyser? If so, what type would people suggest? I'm happy to contact the mains distribution company but I've a feeling their time will be more expensive than mine so don't want to do that until I'm a bit more confident with the issues.

    All (any) help would be much appreciated.


    Jeff, G0WSA
  2. Ken G3SDW

    Ken G3SDW Moderator

    Hi Jeff, seems as you have done all the basic tests, so well done for that.

    Over head phone can carry a host of problems, but in most cases its source will be from a close by neighbor.

    If you can do a walk about again then try to reduce the antenna when the RFI is at its strongest and keep reducing it until you are on top of the location were the RFI is coming from.

    Have you spoken to you neighbor, as now you have proved that it is not coming from your house then it must be coming from someone else`s house and soon you have to speak to your neighbor next door, hopefully you are on good terms.

    Let us know how you get on.
  3. Jeff - G0WSA

    Jeff - G0WSA New Member

    Hi Ken,

    Thanks for the speedy response.

    I think I've discounted my neighbour already, as I approach her house from mine the signal drops. On my property (both inside and out) the interference appears stronger on the side nearest my overhead drop lines (mains and telephone) and that is on the opposite side of the property from my neighbour. I'm going to spend a bit of time with our dear friend Google to see what I can make to ease the DF process!

    I'll then add a photo from Google Earth with all the relevant bits marked - as they, a picture is worth a thousand words.

    Thanks again

    Jeff, G0WSA
  4. Ken G3SDW

    Ken G3SDW Moderator

    OK Jeff will look forward to hearing from you soon
  5. Jeff - G0WSA

    Jeff - G0WSA New Member

    So, I noticed that when I dropped my aerial to ground level the noise disappeared. I decided to try a different aerial in a (slightly) different position so made myself a doublet, cut for 40M and fed with home-made ladder line. I had it up in time for this weekends CQWW RTTY contest and noticed a huge drop in the noise floor for the weekend. Last night it rained heavily and this morning I notice the noise is back (S9+ on 20M today, whereas it was S1/S2 yesterday). I may be grasping at straws but can't help feeling it's coming from the overhead mains cable bringing the power onto my property (this pole also carries the overhead telephone cable btw) The noise is certainly much more pronounced beneath the cables - though I cannot tell whether it is the mains or telephone cable. I'm now keeping a close eye on things to see if it quietens down as the weather dries up ..... I'll let you know what I discover!
    73, Jeff - G0WSA
  6. Ken G3SDW

    Ken G3SDW Moderator

    Cheers Jeff look forward to your news

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