Tracking Down Radio Frequency Noise Source, With Help From Mother Nature

Amateur radio operators and shortwave listeners have a common enemy: QRM, which is ham-speak for radio frequency interference caused by man-made sources. Indiscriminate, often broadband in nature, and annoying as hell, QRM spews forth from all kinds of sources, and can be difficult to locate and fix.

But [Emilio Ruiz], an operator from Mexico, got a little help from Mother Nature recently in his quest to lower his noise floor. Having suffered from a really annoying blast of RFI across wide swaths of the radio spectrum for months, a summer thunderstorm delivered a blessing in disguise: a power outage. Hooking his rig up to a battery — all good operators are ready to switch to battery power at a moment’s notice — he was greeted by blessed relief from all that noise. Whatever had caused the problem was obviously now offline.

Rather than waste the quiet time on searching down the culprit, [Emilio] worked the bands until the power returned, and with it the noise. He killed the main breaker in the house and found that the noise abated, leading him on a search of the premises with a portable shortwave receiver. The culprit? Unsurprisingly, it was a cheap laptop power supply. [Emilio] found that the switch-mode brick was spewing RFI over a 200-meter radius; a dissection revealed that the “ferrite beads” intended to suppress RFI emissions were in fact just molded plastic fakes, and that the cord they supposedly protected was completely unshielded.

We applaud [Emilio]’s sleuthing for the inspiration it gives to hunt down our own noise-floor raising sources. It kind of reminds us of a similar effort by [Josh (KI6NAZ)] a while back.

18 thoughts on “Tracking Down Radio Frequency Noise Source, With Help From Mother Nature

  1. A correction. QRM is interference from other ham operators. The noise that this article is about is QRN. The term RFI is also used; the top expert on RFI at the ARRL (American Radio Relay League, the national amateur radio organization for the US) has the callsign W1RFI, which he chose to represent what he does for the League.

    One challenge for those of us who live in dense urban neighborhoods is that the problem can be outside our homes. One or more neighbors are likely to be close enough to our stations to cause problems, and neighbors aren’t always interested in cooperating with attempts to track down and remedy noise sources.

    1. Interestingly, those same neighbors are often so woefully ignorant to the inner workings of their own tech, that when it starts to “act glitchy” and they see a strange looking antenna coming from a house or on a car, by some ludicrous leap in logic, the poor operator suddenly becomes the source of their problem. I have even seen accusations of “causing radiation headaches” toward a local ham in the area, while the accuser crammed 14 wi-fi devices, a plasma television, and a small (but actually pretty cool) collection of working neon signs in his tiny apartment.

      Admittedly, ham radio has experience some vilification over the years, and yes it’s true that an operator can accidentally cause issues with select pieces of neighboring equipment, but everyone seems to forget that part of licensing is learning how to identify and fix those issues. We humans have a weird tendency to blame those more intelligent than us for problems they are literally trying to solve.

  2. One of my previous jobs put me on a 76 storey rooftop with 100 transmitting antennas in a grid, typically horizontally spaced about 8 feet apart. It was my job to make sure everyone worked and played well together. Many of the culprits were rusty or corroded clamped connections of dissimilar metals which very nicely made diode mixers creating radiated intermodulation.
    One of the funniest things I saw was the reaction of a group of lawyers when I had to take a regular elevator ( not freight elevator) when they saw me carrying a yagi antenna and spectrum analyzer.
    Totally freaked them out.
    They were convinced that I was sweeping suites for bugs or doing something nefarious.

    1. Reminds me of the reaction of my old landlord when I tried explaining amateur radio to her. She was convinced that I was up to no good, and I was trying to convince her that what I was doing was completely harmless. There was no meeting of the minds that day, though she did eventually help me secure permission from her boss to stick an antenna out the window.

    2. In fairness to them I’d probably think the same if I dealt with sensitive info and privacy concerns all day. Its part of what they are supposed to be thinking about, and quite possibly they had no idea the antenna were up there..
      And those tools do look like the right kind of thing for bug hunting.

      Made me smile to think of it though… Funny looks while hauling tech equipment around is rather common..

      1. It wasn’t just cautionary concerns it was a full bore freak out. Funny as could be.

        I one time escorted a DEA crew to the rooftop to place a tactical portable repeater. They decided to come “undercover” and instead of their personal or unmarked cars they parked in the basement garage loading zone in “incognito” vehicles with INS emblems. They showed up at about 10:30. I got a frantic call on the elevator from the building management to not continue to the roof but to take them to the office. Apparently the dozens of restaurants in the tunnel were all shut down for lunch because the rumors of INS agents had spread from workers in the loading zone receiving restaurant supply deliveries and all the servers and cooks had disappeared.
        After about 20 minutes nearly everyone in the building knew they were there via rumor mill from the restaurants of “don’t worry, it isn’t the INS, it’s just the DEA”.
        Also funny as it could be.
        A little thinking ahead could have completely prevented that.

        Yep. Good times.

    1. I am guessing that the Acer is the good one, the picture confused me as well. The bad one is a ‘Mitzu’

      If you read the article they have a proper picture of the offensive one. The cord on the right is from the bad one, and as you can see it isn’t even shielded by having the positive lead down the center, its two conductors side by side.

  3. Another fun one: trying to build shortwave crystal radios when there’s ethernet-over-powerline being used nearby. The latter is basically a 2-30MHz (shortwave) broadcast system using the unshielded house power wiring as a nice big antenna…

    1. Yep. I can even ear some sound from the ethernet-over-powerline noise/interferences on my vintage hifi system (Pioneer 1972), And that sound changes/evolves with direct relation to the traffic load.
      Note: that’s when I’m using my turntable + amplifier on “phono” position.

  4. Those ferrite toroids that power supply and USB leads have moulded on near the ends don’t make much difference except at UHF and maybe VHF frequencies. To stop a switch mode power supply radiating LF, MF and HF noise you need actual inductors and capacitors arraigned as low pass filters inside the power supply.

  5. The best way to keep cheesy knockoff power supplies from radiating conducted and spurious radiation is to make certain that you have removed all inductors and power transistors from the main circuit board.

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