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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