Foundations of Amateur Radio #214
July 13, 2019
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Your Software Defined Radio around the
What would you do if you found that at random
times your garage door opener didn't work, or
the Wi-Fi network dropped out, or you
couldn't switch off a light with an RF
That's the position I found myself in and the
times at which this was happening were madly
unpredictable. One moment everything would
work fine and the next all things radio would
As a radio amateur you're likely nodding your
head and thinking, radio interference,
there's some direction finding in your
future. Sure enough, that's the case, but
before that, I needed to know if the
interference was random, if it had a
particular pattern and how widespread it was,
since it seemed to impact multiple different
devices using different parts of the radio
Initially I focussed on getting a recording
of it. I turned on my radio, tuned it to a 2m
frequency and recorded the noise. Only one
problem. There was no noise. All I could see
was an extreme signal strength, but it wasn't
showing up as noise.
I enrolled the help of my RTL dongle and
recorded some raw data, essentially capturing
a 3 MHz slice of noise centred around 147
MHz. All that revealed was that there was
noise. I already knew that.
At that point I decided that a bigger hammer
was needed. Something you can do if you have
a $5 RTL-SDR dongle and some free software,
in my case I used a tool called rtl_power and
a visualisation tool called gnuplot.
rtl_power is a nifty piece of software. It
takes measurements and averages out the power
level across the measurement range. To make
it work, you specify a starting frequency, a
stopping frequency, how big a step to use to
average, how often you want to measure and
for how long.
For my little investigation I started with
measuring between 0 and 1.7 GHz, at 1 MHz
intervals, every 2 minutes for 10 days. That
creates a big CSV file that you can process
with gnuplot into a picture that tells a
Seriously, it showed me that the interference
was very wide, 0 to 300 MHz, it occurred
every 20 or so hours, lasted up to six hour
at a time. There were other things happening
as well, similar patterns, but across an even
larger frequency range, from 0 to 600 MHz,
but in shorter duration and of lesser
Based on the times alone, I can immediately,
almost certainly, eliminate any source under
Based on the timings I can also determine
that the noise is likely not created by an
automatic process, given that they vary in
duration and the way they're clustered around
The variation of the interference allows me
to determine that there are at least three
separate types of noise, each with specific
characteristics and times, sometimes
It's too early to tell if this pattern will
continue. One possible next step is to set up
the same measurement tool and powering it
from a battery. Once I've got that working, I
expect to turn off the house power during an
interference session and determine if the
noise is coming from my house, or if it's an
external source, which seems likely.
Once I've determined if it's in house or not,
I can start either eliminating gadgets by
switching off specific power circuits, or I
can start direction finding and locating a
nearby source of pain.
At that point I can decide what to do next.
That said, at the moment it looks like
several televisions around me are creating an
RF noise storm of epic proportions.
I've documented all of how I did this and you
can find it and the scripts I created on the
web at vk6flab.com.
One thing that has happened since I started
documenting my efforts is the idea that we
could collectively as a community make
measurements like this and document the state
of our RF space and how it changes over time.
I plan to update my code to incorporate this
idea, perhaps log in 24 hour blocks and
generate a chart over that time, perhaps make
it into a video.
One challenge ahead of us would be to come up
with a universal way to calibrate our various
dongles, so we all report the same signal
level in the same way. One thought is to use
the sun as a global calibration, but I'm not
yet sure how that might be implemented.
One thing's for sure. If you've ever wondered
what use can a $5 RTL dongle possibly be,
this is one thing that you just cannot do
with a traditional radio. That's not to say
there's a place for both in the world, just
different tools for different problems.
I'm Onno VK6FLAB
This article is the transcript of the weekly
'Foundations of Amateur Radio' podcast,
produced by Onno (VK6FLAB) Benschop who was
licensed as radio amateur in Perth, Western
Australia in 2010. For other episodes, visit
http://vk6flab.com/. Feel free to get in
touch directly via email: email@example.com
If you'd like to join a weekly radio net for
new and returning amateurs, check out the
details at http://ftroop.vk6flab.com/, the
net runs every week on Saturday, from 00:00
to 01:00 UTC on Echolink, IRLP, AllStar Link,
Brandmeister and 2m FM via various repeaters,
all are welcome.
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