The RFI Hunter: Looking For Noise In All The Wrong Places

Next time you get a new device and excitedly unwrap its little poly-wrapped power supply, remember this: for every switch-mode power supply you plug in, an amateur radio operator sheds a tear. A noisy, broadband, harmonic-laden tear.

The degree to which this fact disturbs you very much depends upon which side of the mic you’re on, but radio-frequency interference, or RFI, is something we should all at least be aware of. [Josh (KI6NAZ)] is keenly aware of RFI in his ham shack, but rather than curse the ever-rising noise floor he’s come up with some helpful tips for hunting down and eliminating it – or at least reducing its impact.

Attacking the problem begins with locating the sources of RFI, for which [Josh] used the classic “one-circuit-at-a-time” approach – kill every breaker in the panel and monitor the noise floor while flipping each breaker back on. This should at least give you a rough idea of where the offending devices are in your house. From there, [Josh] used a small shortwave receiver to locate problem areas, like the refrigerator, the clothes dryer, and his shack PC. The family flat-screen TV proved to be quite noisy too. Remediation techniques include wrapping every power cord and cable around toroids or clamping ferrite cores around them, both on the offending devices and in the shack. He even went so far as to add a line filter to the dryer to clamp down on its unwanted interference.

Judging by his waterfall displays, [Josh]’s efforts paid off, bringing his noise floor down from S5 to S1 or so. It’s too bad he had to take matters into his own hands – it’s not like the FCC and other spectrum watchdogs don’t know there’s a problem, after all.

20 thoughts on “The RFI Hunter: Looking For Noise In All The Wrong Places

  1. And then you live in a busy city with other houses around you on all sides… Its like high end audio freaks living in the flight path of an airport… You are in the wrong place

    1. I’ve found high pitched hums that I thought were me (and found some on me and not just sympathetically resonating on other aluminum objects too when checking) just using just a sound card and a Mini-3 Bat detector, that I found weren’t me.

      Really simply… though ideally a higher quality lower noise floor USB sound card with a 192kHz sample rate and some mods to reduce noise helps open of the view with say like Spectrum Lab or another free spectrum analyzer software: http://www.vlf.it/pernter1/x-fi_modifications.html

      The Behringer umc202hd is another sound card I invested in and eventually want to make a kind of porta pack I’m thinking with an SBC or stripped down laptop.

      Lately, I’ve been looking into investing in a 384kHz SB G6 so I can better review up to 192kHz without aliasing.

      Finally, I again for the winter season started to read into where I left off with the sensor system to get above 100kHz (was thining using Panasonic WM-61A’s for below 100kHz) and using something wideband transducer I guess instead of microphone unless I can modify and more like this design last thinking:
      https://www.youtube.com/watch?v=quuHV9pTyoo

      Of course… since all of this isn’t thick hardware and thin or no firmware… there are latency issues and is interesting comparing with the superheterodyne scanners or receivers.

      Great article topic. We definitely need more RFI hunters radio direction finding for sure since the future isn’t going to be less wireless most likely for a few.

    1. yeah the quickest way to check it though, is just cut your mains and run your rig from battery. I did it. noise was the same. Game Over. I can choke my radio all I want but I can’t choke the neigbor’s appliances.

      1. Don’t be so quick to give up. You can go to a different type of antenna. Magnetic loops tend to pickup less RFI. You may even have a problem that’s internal to your radio that’s causing the noise floor to be excessively high. Take your radio to another location and see how it behaves. Check to make sure you have a good ground.

  2. I spent several years as a “Transmission Technical Support Engineer” for New York Telephone Company, and chased noise, both RF and VF for a living. The nice thing was having all of the super test equipment at my fingertips. And then I got promoted to Bellcore and wound up teaching Noise Mitigation and Grounding, even better resources.

    This is all good information, but breaking the ground is not a good idea.

    A lot of modern refrigerators have “inverter” compressors which run continuously, not like the old ones which only ran when called for. If it were me, and I have done it, I would be on the phone and email with the manufacturer of any appliance that generates RFI, reminding them of their obligations under FCC rules and regulations.

    Is your dryer a gas dryer?

    De K5NRA, Jim 73.

  3. In a previous reincarnation I spent about 17 years working as a corporate domain expert in the area of electromagnetic compatibility, not just FCC and EN compliance but every sort of electromagnetic problem you can think of – crosstalk, power system noise, ESD, magnetic field susceptibility in tape and disk drives and even an occasional magneto-acoustic problem – pretty much every EM problem known to mankind.
    It always seemed to me that the FCC could find a reason to turn a blind eye to the spectral noise impact of any new technology that promised to make some company or other a profit, and HAM radio and radio astronomy seemed very low on their list of services to protect – hence powerline communications and other new noise sources.
    A good general rule of thumb I found was that if the problem was below 200MHz the radiation mechanism more likely involved radiation from an external cable, above 200MHz look for slot radiators and cavity resonance effects.
    I finally got got out of that end of the business and specialized in signal integrity and high speed circuit design for the following 20 years of my career – a lot fewer late nights in the lab trying to analyze the woes of a new product that had to ship PDQ to meet revenue projections – and a lot less stress :-).

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