[RFI] What Is It vs Where Is It?

[Don Kirk]

Hi Ed,

You said "what you really want to do is to figure out WHERE the noise
source is, something you will need to do no matter what it is."

All I can say regarding your above statement is "Amen to that".

I believe the problem is that folks don't have adequate DF gear so they are
looking for an easy way out that does not exist.  I prefer not to guess
what the source might be as you really need to keep any open mind or you
can make some bad decisions/guesses.

The only thing I want to know upfront regarding "what it is" is if the RFI
might be power line noise, because if it might be power line noise I will
also bring along my VHF AM direction finding gear.  I do want to know what
it's characteristics are (time of day, drift, bands, etc.) as that will
dictate my approach, but I certainly don't care knowing what the device
might be.

Great post, and thanks.
Don (wd8dsb)

On Thu, Sep 9, 2021 at 8:23 AM Hare, Ed, W1RFI <w1rfi at arrl.org> wrote:

> WHAT IS IT? By Ed Hare, W1RFI
>
> When someone has an unusual RFI problem, the first question he or she
> almost always asks is: What is it? That's an interesting question, and you
> may need to ask it during the troubleshooting process, but it is not the
> first question you should ask.
>
> First, even if someone were to say that it's a Model XYZ Panashibi
> swtiching power supply, what would that tell you? You would still have to
> go into the world and find it. The number of products sold that could make
> radio noise in in the many tens of thousands, and even it had the signature
> of a Model XYZ Panashibi supply, a Model ABC Shootzu LED bulb could have a
> very similar signature, so if someone tells you to look for the Panashibi,
> you could be off on a wild goose chase. "Knowing" what it is could actually
> make it harder to find.
>
> It could be useful, though, to know whether you are searching for a
> switching power supply, DSL or cable leakage, or a plasma TV. But be
> general, not specific, because you don't want to be misled.
>
> Other than power-line noise, most interference reported by amateurs
> involves switching regulators and power supplies. First, switching supplies
> are very common; they are used in absolutely everything. Modern LED bulbs
> and fluorescent ballasts are all probably also switching power supplies.
>
> Switching supplies do have some characteristics that make them easy to
> identify. First, they make noise every N kHz, with N typically being
> somewhere between 10 kHz to 200 kHz. In other words, a switcher will make
> usually broadband noise spaced some number of kHz apart. Put your receiver
> in AM mode and listen to the noise. A switcher will almost always have some
> 120-Hz AC hum on the noise, detectable by ear.
>
> Other signals can also cover a wide frequency range. DSL signals will just
> sound like broadband noise, or will have a distinct "digital" sound to
> them. DSL signals will have a fairly sharp frequency cutoff, appearing and
> disappearing over about 50 kHa of frequency range.
>
> Plasma TVs, if the noise is coming from the plasma screen itself, have
> noise that varies with every scene change on the screen, and you may even
> be able to correlate it with a particular TV channel by switching through
> all the channels on your own TV and see which one has scene changes
> corresponding to the change in noise.
>
> Most switching supplies make more noise at lower frequencies, tapering off
> as you switch to higher and higher bands. Most digital devices occupy
> specific frequencies, so you may not hear it at all on 3.5 MHz, but as you
> tune through HF, the noise may get strong starting at 6 MHz, and continue
> pretty steadily all the way to 20 MHz, just as examples, then disappear
> over space of a few tens of kHz.
>
> So, frequency occupancy over the entire HF range, tapering off as one goes
> higher indicates a probable switching supply, while specific spectral
> occupancy indicates a digital device carrying modulation. Switching
> supplies really are free-running oscillators, but their frequencies do not
> need to be exactly controlled. So, when the noise first appears, note the
> frequency of one of the peaks of the noise, then see if it drifts up or
> down the band. Switchers almost always drift. Stay parked on the frequency
> for a while, and keep listening. Switchers almost always exhibit little
> changes in frequency as voltage dips and surges occur, although the change
> could be less than a kHz, which is hard to tell considering the relatively
> broad nature of the noise. . Drifting and the occasional change in
> frequency are characteristic of switchers.
>
> So, I can't say this strongly enough: Start in your own home. Use a
> battery operated receiver, and turn off every circuit breaker in your home.
> If the noise goes away, it's yours. Do NOT assume that you know it's not
> your own equipment causing the noise, because I can't tell you the number
> of times a ham has wasted his or her time and ours trying to track down a
> neighborhood noise source, only to ultimately discover it was something
> like a battery charger forgotten plugged into an outlet in the garage.
>
> In any event, what you really want to do is to figure out WHERE the noise
> source is, something you will need to do no matter what it is. That battery
> operated receiver will be a valuable tool. Ideally, it will have an S
> meter. First, connect it to your antenna and hear the noise. Now, take it
> outside with a small antenna and if you still hear the noise, your job will
> be pretty easy. Note the S meter reading, and take a walk. You should be
> able to find the peak pretty easily, isolating it to a few houses on HF.
>
> As you tune higher in frequency and still hear it, you need to be closer
> to the source, so when you are near it, think 10 meters, if the noise can
> be heard there. It IS possible to get tricked, because on your neighborhood
> walk, you will hear devices from each house, so at least be sure that the
> noises you hear do in some way resemble the noise you hear at your house.
>
> Now, here comes the tricky part, because in some neighborhoods, you will
> not have much access to the private properties surrounding you. But you can
> use that S meter to get a real good idea what house the noise is coming
> from. To do this, you need to place the antenna of the battery-operated
> receiver a specific distance away from the electrical wiring in each house,
> and take an S meter reading of the noise. I suggest that you can use one or
> two feet as that distance, and judging the S meter by eye is sufficient.
> The noise will be significantly stronger radiating from the offending house.
>
> You can sometimes use the outside electrical meter in a building, or an
> outside light on the porch or driveway, making sure is it NOT 12 volt
> lighting that can have a different switching supply than what you are
> looking for. (Of course, it could be the very source you are looking for.)
>
> If you can't go near each building around you, if you have overhead
> electrical you can use the S meter to sniff the ground wire that is
> connected to each pole, at least isolating the noise to a few houses. If
> the wiring is underground, you can use the pad mounted transformers that
> are scattered around the neighborhood, although it is sometimes hard to
> know what houses are connected to each one.
>
> Now, once you have found the house, you have a difficult decision to make.
> Do you approach a stranger, or just live with the noise? People don't
> always react well to accusations that the device they just bought at
> BigMart is being operated in violation of some federal law that they don't
> understand, but how to diplomatically approach a neighbor is another
> subject. There is one key, though, and that is under FCC rules, it is the
> responsibility of the operator of noisy devices to not cause harmful
> interference, so if this is a neighbor's equipment, it's his or her
> obligation to fix it.
>
> Asking Where Before What?
> By Paul Cianciolo, W1VLF
>
>
> A message from ARRL Laboratory RFI Engineer, Paul Cianciolo, W1VLF:
>
> In the course of investigating interference (RFI) issues, two questions
> are frequently asked.
>
> The first: “What does this RFI sound like to you?” And sometimes a video
> or screen capture is included when an amateur reaches out about their
> interference problem. The question then becomes, “What does this look like
> to you?”
>
> Aside from a few very distinctive types of RFI, it is difficult to
> identify what specific device is causing interference based on the sound it
> produces, or the visual signature in a waterfall.
>
> The most useful question to ask first regarding interference isn’t “What
> is producing this RFI?” but “Where is this interference coming from?” Once
> the source of RFI is determined to be coming from a specific location, the
> process of identifying the actual device causing issues is much easier.
>
> For example, at my own station, there are no fewer than 20 individual
> devices that can potentially cause RFI (and some do!). This is just one
> room in one house, in one of many houses in the neighborhood.
>
> In my case, figuring out where interference is coming from is easy: it’s
> my own home. It wasn’t difficult for me to locate individual sources of RFI
> and deal with them on case by case basis as I worked on reducing
> interference around my station.
>
> Another example: Let’s say I use a very popular imported LED bulb in my
> home and it’s producing RFI. Let’s call it part of the DimBulb brand, model
> no. 123A. I purchased the bulb online. There is no question that this bulb
> is the source of my noise, and I can deal with it.
>
> Although this is an imaginary brand and model number, the potential for
> one of those bulbs to be in service in your neighborhood, with many homes
> nearby, is highly likely. Even knowing without a doubt the source of RFI is
> that very same bulb, would you ask all your neighbors if they have one of
> those bulbs? Would they even know offhand if they did? Or would you locate
> where the interference is coming from and then determine what is causing it?
>
> Rather than trying to figure out what you think is causing interference
> and chasing after that particular device, you’re better off tracking down
> where RFI is coming from and going from there. Try to ask yourself “where”
> before asking “what.”
>
> Almost half the RFI cases of unknown origin I deal with are found to be in
> the amateur’s own home. Never underestimate the power of simply taking a
> trip to your main breaker and listening for noise on batteries. It’s the
> best first step when determining “Where is this RFI coming from?” before
> figuring out “What is causing this RFI?”
>
>
>
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