Broadband noise on 40M @ KJ6EO > HELP!

Discussion in 'Ham Radio Discussions' started by KJ6EO, Oct 18, 2020.

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  1. KJ6EO

    KJ6EO Ham Member QRZ Page

    Hello and thank you for reading my post. For some time now, I've had this nasty broadband noise on 40m (see the attached file of a screenshot from my WebSDR receiver). Interestingly enough, the noise switches off exactly at 12:00 noon, and switches back on again at 6:00pm (Pacific Time). When this noise is being generated, it's 40db over S9, and 40 meters is totally useless, I can't hear a thing. I have HF in my mobile so I drove around the neighborhood while listening to my IC-706mkIIg on 40m. The noise is strongest about 4 houses down the street. But I have no way of knowing which side of the street it's coming from. Oddly enough, where the noise is the strongest, there is a good sized ground mounted power transformer (maybe 3'x3'x3'). I wouldn't expect this to be a power transformer though, since the interference switches on and off on a regular schedule. If I had to take a guess, I would imagine that this is being caused by a Pool Pump with a VFD. As I said, the noise is very strong. It's radiating approximately a quarter mile in all directions from the epicenter.

    If anyone could identify this noise, I would certainly appreciate your help. If you want to see it live, and hear it live, just visit my WebSDR ( at any time besides 12 noon and 6pm Pacific Time. Thank you in advance for any guidance or suggestions you might have.

    Attached Files:

  2. WT4W

    WT4W Ham Member QRZ Page

    Do 160 and 80 get covered up also? If it was a VFD I'd expect the lower bands to be even worse than 40.
    KJ6EO likes this.
  3. N3PM

    N3PM Ham Member QRZ Page

    If you can see/hear the noise down into the upper part of the BCB, triangulation with a portable radio may be worth a shot.
    Mike N3PM
    KJ6EO and N0TZU like this.
  4. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    I presume you already verified that the noise isn’t from your own house.

    Get or borrow a portable SW receiver, preferably with a BFO or SSB mode. Tune it to a strong noise frequency at your house, then walk around the neighborhood to get a better idea where it’s coming from.

    Ideally it should also have an external antenna jack, to which you could connect a simple home brew loop antenna for better directionality if needed.

    These days no ham shack should be without one to track down the inevitable new RFI source.
    KJ6EO likes this.
  5. KJ6EO

    KJ6EO Ham Member QRZ Page

    No interference on 80 or 160
  6. KJ6EO

    KJ6EO Ham Member QRZ Page

    Thanks for your suggestion. I have HF in my truck. By driving around the neighborhood, I have located the general area where the interference is coming from. However, I don't know which side of the street it's on, or weather it's the house nearest my truck, or in
    adjacent houses. The general area is just 4 houses down the street. I do have a portable SW Radio, but I think I'll discover the same
    thing I did with my mobile HF, just the general area. I might have to just go from house to house asking questions. I'd prefer not to
    do that. I'd rather know exactly which house it is.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That could be for a few obvious reasons, such as:
    • Your home station antenna works well on 40m but not well (not big enough) on 80m or 160m
    • Or the "emitter" of the noise is power-line connected with a length of cable that resonates around 7 MHz and radiates most efficiently there, and not so much on other bands
    If it "runs all night and through the morning" and is only "off" for 6 hours a day, that would be a weird way to run pool equipment but anything's possible.

    Possibly "grow lights" for indoor cultivation of plants (often, the kind that are illegal to sell)?

    I vote for taking a handheld portable receiver and if possible using a directional antenna on it (a very small loop is inefficient but can be quite directional) to try to triangulate the noise source and ultimately zeroing in on the location of the source.
    KJ6EO likes this.
  8. KJ6EO

    KJ6EO Ham Member QRZ Page

    Yes, I have verified that the noise isn't coming from my house. I do have a wellbrook 1m active loop. I hooked it up to my mobile HF Rig and drove down the street to the general location of the noise. The noise is so strong that the loop didn't provide any directionality. I tried
    using my body as shielding, but that didn't work either. Whatever is generating the noise, it's capable of wiping out an entire quarter mile
    radius on 40 meters. The noise seems to be centered around 6.600 mhz, but it's very broad banded extending maybe 900 khz above and below 6.600.
  9. W1VT

    W1VT Ham Member QRZ Page

    It may not be practical, but the noise is often strongest at the meter if you can get there safely.

    The ARRL has never been able to put together a comprehensive noise ID program, despite having a full time staffer at the RFI desk for the past few decades.
    There are tools for identifying power line noise, but there is no massive database of noise signatures available. Though, in theory, if every member were able contribute a few signatures a year ...

    Something that loud may have VHF or UHF harmonic that can be picked up with a small Yagi at close range.
    Ideally, one would pair the Yagi with a HT that can be set to the AM mode, not FM.
    FM is designed to suppress, not receive, noise.
    Last edited: Oct 18, 2020
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The 1m active loop is probably too big and sounds like it's preamplified, which is worse.

    You can use a smaller loop that's passive, and if necessary insert a 20 dB shielded pad (BNC types sold into the instrumentation industry are very inexpensive, surplus, and sold all over the place) in that line to reduce its sensitivity further.

    Sometimes although the noise obviously peaks around 40m, it's "still there" but just more attenuated on other frequencies, possibly even in the AM broadcast band. Old-fashioned portable AM radios (which today aren't easy to find, but many of us have old ones from the 60s) with an internal "loopstick" antenna can sometimes be good noise detectors and the loopsticks are quite directional.
    KJ6EO likes this.

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