Wit's end with unknown source of RFI

Discussion in 'General Technical Questions and Answers' started by K4VBB, Dec 17, 2019.

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  1. K4VBB

    K4VBB XML Subscriber QRZ Page

    I need help to identify, and hopefully eliminate, an unknown source of RFI. I'm sure some of you have dealt with this before, so I hope to take advantage of any knowledge you may be able to provide. Please help if you can.

    First, let me describe the problem:

    On 40m I get a recurring 'whispy' noise with narrow spacing in the band at about 20KHz apart. The sound could be likened to someone holding a microphone out of the window of a moving car, or a whip antenna in a high wind.

    On the SDR they appear like a signal, but have a whispy appearance as the frequency shifts slightly every second or so. I've attached a picture. In it you'll notice the little whispy "signals" at a 20Khz spacing.

    What I've done so far:

    My system is set up well from a grounding perspective. Furthermore, I have "clean" power and ground going to all of my rigs, and also everything that is connected to them. By clean, I mean that the AC power supplied to everything runs through a series of loops of Romex running through 4 large toroids.

    Antenna is grounded at the feedpoint, and the coax is grounded at the shack. The RG-213 runs through a choke balun.

    The two rigs I have set up are run through separate power supplies. One of the power supplies is an Alinko DM-30T, and the other is a quasi-homebrew PS made from an old Dell server power supply (makes very clean power). Both rigs will pick up the errant signals no matter which PS they are connected to. They even pick it up on battery power.

    I've shut down the entire power grid in the house looking for a local source by flipping the main breaker, to no avail. The errant signals remain.

    In hopes to track it down, I connected a resonant loop antenna to my FT-891. With the FT-891 strapped around my neck, and on battery power, I have attempted to walk the yard to locate the source, but it appears that the loop antenna will not pick up the noise.

    With the exception of the 'noise' guy down the street, my station is relatively quiet. But I know what that guy's noise sounds like, and it's not him. These signals are making it difficult to operate, and I'm looking for some help to mitigate the issue. Any information you can offer will be greatly appreciated.
     

    Attached Files:

  2. K4SAV

    K4SAV Ham Member QRZ Page

    The 20 kHz spikes are your first clue. That means it won't be a power pole. But it could be a signal that is using the power lines as an antenna. It going to be something that runs at 20 kHz, something like a battery charger, wall wart, switching power supply, etc. I don't know what frequencies are used in the solar switching controllers or in some HVAC systems.

    Sounds like you have eliminated things inside your house and that makes it more difficult since the source is apparently somewhere outside your house. To track this you will need something that can hear the RFI. Try different frequencies. RFI with lots of harmonics is usually stronger at lower frequencies. Once you can hear it on a portable receiver, then you can track it.

    Are you using that 20 meter dipole on 40 meters? If so, I have some other suggestions.

    Jerry, K4SAV
     
  3. KA2IRQ

    KA2IRQ Ham Member QRZ Page

    Some things just radiate a signal! Sometimes that signal is just coming in through your antenna.

    Can you make a recording of it with your cellphone and post it? Sometimes hearing it is a bigger clue.
     
  4. K4VBB

    K4VBB XML Subscriber QRZ Page

    Actually, I'm using the G5RV for 40. I'll likely get something better for 40 after I put up my beam.
     
  5. K4VBB

    K4VBB XML Subscriber QRZ Page

    I most certainly will.
     
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    Is this a SDRPlay radio by chance?

    If it is I think I know what is going on. Regardless who made it, if you do not already know how to adjust RF Gain, break out the book and learn. Many of the SDR radios especially the really wideband models from 10 Khz to light go into oscillation from two strong stations inter-modulation distortion. aka intermod. Offender come from Commercial AM band.

    So once you know how to adjust RF Gain, try turning it down, and turn the audio volume up. Use the RF gain to control volume. Turning the gain down can stop the osculations. Some SDR have AM/FM filters. On the spectrum analyzer it looks a lot like a commercial AM signal across the band at a fixed spacing like every 17 Khz. Tune to it and it is buzzing whispy sound. Anyway try the RF Gain.
     
    Last edited: Dec 17, 2019
    N2EY likes this.
  7. K4VBB

    K4VBB XML Subscriber QRZ Page

    It is not, but it is an RTL-SDR dongle. I'll try your suggestion in hopes that it'll resolve this, but my hope that this problem is the SDR dongle is limited. I only have one rig that has an IF output for SDR (IC-7410). My other rigs are an FT-450D and an FT-891. I'm receiving the noise with all of these (except when the loop antenna is connected), and regardless whether the SDR dongle is connected.

    I'm not going to rule that out until I've tested it, and I hope you're right. It'd be a simple fix and given the Chinese construction of the SDR dongle it would not surprise me. But given that I get the noise regardless of the state of the dongle, it's unlikely. I'll test it when I get home, and also include a recording of the noise.
     
  8. K4SAV

    K4SAV Ham Member QRZ Page

    Intermod is a possibility, and a good guess if it's a cheap SDR. Since you also have an FT-891, FT-450D and an FT-891 connect one of those to your G5RV and see if you hear it. That won't be conclusive if you still hear it, but if you don't hear it, then it's likely being created inside the SDR.

    Edit: I should have read closer. Seems you have already tried that.

    Intermode is a constant problem with guys operating 160 meters. AM stations can create intermod products every 10 kHz sometimes. Beside cheap radios, there are lots of mixers around, (like rusty fences and other intermittent connections) that can do the same thing. That's a lower probability considering what you are seeing, but it's possible.

    Jerry, K4SAV
     
    Last edited: Dec 17, 2019
  9. K4VBB

    K4VBB XML Subscriber QRZ Page

    Looks like I'm not the only one with this problem. I found this while looking around on the web for examples.

    https://www.sigidwiki.com/wiki/Weird_Signal_on_Multiple_Frequencies

    Scroll to the bottom and click "play" to listen.

    This is the EXACT noise I'm getting with an almost identical signature I'm seeing in the SDR. Unfortunately, it has yet to be identified. Since the guy that uploaded the information lives in Essex, UK, I'm no closer to identifying what it might be.
     
    WD9U likes this.
  10. KH6AQ

    KH6AQ Ham Member QRZ Page

    You say "In hopes to track it down, I connected a resonant loop antenna to my FT-891. With the FT-891 strapped around my neck, and on battery power, I have attempted to walk the yard to locate the source, but it appears that the loop antenna will not pick up the noise."

    A better antenna may work for the FT-891, such as:

    1) 40 meter HamTenna (7.5' whip) or a telescoping whip such as the 72" MFJ-1963 ($15 at DX Engineering)
    2) SO-239 to 3/8-24 threaded stud antenna mount adapter ($12 at Amazon), or an MFJ-7748 adapter and a UHF male-to-male adapter (DX Engineering stocks both)
    3) A trailing ground wire connected to the FT-891 ground screw can be used if increased signal strength is needed (6' to 33')
     

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