NOOB rfi question

Discussion in 'General Technical Questions and Answers' started by KJ7UBV, Dec 2, 2021.

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  1. KJ7UBV

    KJ7UBV Platinum Subscriber Platinum Subscriber QRZ Page

    New ham here just getting started on HF with a question about a wierd "whooshing" type of signal I keep picking up. It happens at all times of the day and is most pronounced on 20 and 40 meters. Sometimes it stays on a particular frequency but usuall drifts around a bit. I've googled and enlisted fellow hams to try and figure it out with no luck. I've also listened to every RFI recording on ARRL's website as well as several others to compare with no luck either.

    Here's a short clip I recorded this morning.

    73 KJ7UBV Paul :confused:
  2. W1VT

    W1VT Ham Member QRZ Page

    A big clue is the propagation right now on 20 and 40 meters. There are skip zones associated with HF propagation that can be used to estimate radio source distance, even with an unknown antenna.
    You report suggests that is very close and not propagated via the ionosphere.

    Zak W1VT
  3. KJ7UBV

    KJ7UBV Platinum Subscriber Platinum Subscriber QRZ Page

    I'm in Las Vegas so there's an immeasurable number of RFI sources. The "Strip" is about 20 miles from my location and the nearest power substation is about a mile and a half away. I've gone through my entire home turning off all my electronic doodads and machines with no effect. I guess it could possibly be something from one of the local military bases but I have no idea how to verify if that's the source. I haven't had any black helicopters circling or landing at my home so I guess that's a good sign. :eek::D
    PU2OZT likes this.
  4. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    It’s likely a switching power supply, their switching frequency usually drifts. What is the spacing of the harmonics? I can’t tell from your video.

    Did you shut off power to your entire house to verify it’s not there? Remember to remove batteries from anything with backup.
    PU2OZT, KJ7UBV and K7TRF like this.
  5. K7GQ

    K7GQ Subscriber QRZ Page

    I agree with the post above - that's a switching power supply. Did you simply turn off things, or complete circuits at the breaker as is always suggested?
    KJ7UBV likes this.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    With those kind of signal strengths it's likely in your own home or in a close neighbor's home (especially if you're in an apartment building). As posted above it's almost certainly some kind of device like a battery charger or LED light strip or something else that uses a switching power supply. The regular repetition of the signal on your band scope is a telltale clue that it's some kind of device that has a regular switching frequency but one that isn't crystal controlled as it sometimes drifts up or down in frequency. That's classic switching power supply noise.

    The standard way to chase these down is by first eliminating potential culprits in your own home. If possible run your receiver off a battery and turn off all the circuit breakers in your home to see if the noise goes away. If it does then turn breakers back on one at a time to find the branch circuit (room) with the problem then try unplugging devices until you find the problem unit. Wall wart style power supplies are often the problem as are battery chargers. If you can't run your rig off a battery then at least turn off all breakers except what supplies your rig and unplug every other device in your ham shack.

    Some things like laptop computers and alarm systems have internal batteries that can stay energized and active even when the breaker is off so beware that those could be problems as well.

    If it's not in your own home then start looking around the neighborhood, a hand held AM radio or battery operated shortwave receiver or other similar devices can help you track down neighborhood RFI sources or at least figure out what house it's coming from.
    KJ7UBV likes this.
  7. KJ7UBV

    KJ7UBV Platinum Subscriber Platinum Subscriber QRZ Page

    I had not even considered that possibility. I have a Powerwerx SS-30DV literally sitting next to the radio and the power cable near the antenna cable. I'm going to try moving some things around and see if that's the issue. I haven't tried the CB isolation route yet but that'll be next.:)
  8. KJ7UBV

    KJ7UBV Platinum Subscriber Platinum Subscriber QRZ Page

    I think the power supply is the culprit. I've moved it a bit further from the radio and moved the power cables further from the point where the antenna feedline enters the house. It hasn't eliminated it yet, but I did notice quite a drop so far so I think I'm on the right trail here. :)
    PU2OZT likes this.

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