Stray Signals

When is a radio not a radio? When it’s ‘software defined’ – The B.C. Catholic

About a year ago, a friend, Kent Johansen, who happens to be the
designer of the sensors that power the earthquake early warning system in our
schools, gave me a device he called a software defined radio – SDR for

Although I have had a lifelong fascination with all aspects of
radio, I had ignored developments in the field in recent decades as I focused
on my work in computer technology and physics.

When he handed the device to me, it was the first time I had
heard mention of the term “software defined radio,” let alone seen one.

I was completely befuddled. A radio, to me at least, had dials,
lights, perhaps even meters or a digital display of some sort. This device had
none of that. In fact it was just a little bigger than one of my thumbs. It had
a USB plug at one end, and a very small brass terminal near the other.

Accompanying it was a small antenna that attached to the brass
terminal. “Here you go”, Kent said to me, “a complete radio that can tune from
25 MHz all the way up to nearly 2 Ghz.”

Well, I remember thinking, what exactly was I going to do with
this little device? Most of my experience with radio in my teenager years was
with radio signals below 25 MHz, namely AM radio, which centres around 1 MHz
(for example, Vancouver sports station TEAM 1040 operates at 1.040 MHz, and
shortwave radio, which typically runs from around 5 MHz to 25 MHz (part of what
is called the HF, high frequency, band.

Frequencies above 25 MHz were mostly unfamiliar to me, with the
exception of the aircraft band (108 – 137 MHz), and the FM band (88 – 108 MHz).
My friend made vague allusions to using the SDR for listening to the International
Space Station, or to signals bouncing off meteor trails, and he referred to
signals from weather satellites.

Having obtained an amateur (ham) radio licence several decades
ago, I was also vaguely familiar with something called the 2-metre band (144 –
148 MHz), part of what is called the VHF (very high frequency) spectrum that
runs from 30 to 300 MHz.

It took me a while to actually begin experimenting with the SDR.
I did a lot of background reading, learning that SDR really had military
origins as it made it possible to listen to wide swaths of frequencies very
easily. The basic concepts for SDR evolved in the 1970s, but did not become
mainstream until relatively recently when the concept was adapted to TVs.

My SDR dongle, made by NooElec, is a particularly good unit,
costing under $30. I have found it to be a solid performer that can be left
running for days, if not weeks on end, gathering for instance data from
satellites (more on this aspect in a future column).

Unfortunately, as with many electronic devices, knock-off models
with dubious performance are to be found of many brands of SDRs. I now also
have a second SDR, made by SDRplay, a model called the RSP2. It is considerably
more expensive than the NooElec model, but it covers the entire spectrum from 0
to 2 GHz.

Software defined radio tuned to Vancouver station Rock 101 illustrating the waveform and HD radio
signals on either side.

When I first plugged in the NooElec SDR it did nothing. I didn’t
realize I actually needed some software to unleash its potential – specifically
I needed SDR software, basically a utility that could use the circuitry in the
SDR itself to tune and demodulate

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