A New Toy For A Radio Nerd
If you've been a long time reader of the blog, you know that I've always been a bit interested in all things radio. While I do hold an amateur radio license, I've found myself drawn more towards picking up the strange and interesting things that are found on the air, be it my more recent experiments in ACARS, or hunting for weather balloons and gliders, There was one thing that was a common thread in all of this - I was merely listening and reporting on the signals that I was hearing.
While I definitely enjoy listening in on what's going on, as a Ham radio operator, I am also keen on being "heard" out there. While I still manage to get on the air (though not nearly as much as I would like) I need to be physically at the controls of my radio while transmitting. While being in physical control of the radio is a mandatory requirement of being a ham radio operator, I started to get a bit intrigued by an area of the hobby where I don't necessarily need to be at the controls during transmitting. That part of the hobby revolves around the concept of running low powered beacons, which are primarily used as a service to fellow amateur radio operators for them to map out current radio propagation.
Amateur radio operators have long chased distant contacts and rare signal paths but for the longest time that was always a bit of a challenge since radio propagation is always something that is continuously changing.
In recent years there has been a huge explosion in the use of new digital modes, with FT8 being the most popular mode, and is on track (if not already) to surpass the more traditional amateur radio communication modes of morse code and single side band voice communication. As part of this boom of digital modes there was introduced a new digital mode, which turned the traditional pursuit of one on one conversation into something that is more akin to scientific exploration. This new mode, called WSPR (pronounced appropriately "whisper") represented a fascinating intersection of cutting-edge digital signal processing, global collaboration, and the age-old fascination with how radio waves travel around the world.
What Is WSPR?
WSPR, which stands for Weak Signal Propagation Reporter, is the perfect name for what it does. This digital radio protocol was specifically designed to test and map radio propagation conditions across the globe using signals so faint they would be completely inaudible to the human ear.
Unlike traditional amateur radio modes where operators engage in conversations with one another, WSPR stations act as automated beacons, continuously transmitting and receiving brief messages that contain just three pieces of information: (callsign, location, and transmission power level). Though simple, these transmissions create something far more valuable than individual contacts - they generate a living map of how radio waves are travelling around the Earth at any given moment in time.
The Power of Weakness
What makes WSPR truly remarkable is its ability to decode signals that exist far below the noise floor. While a human ear might hear nothing but static, WSPR can successfully decode signals that are far weaker than the background noise. This extraordinary sensitivity means that stations can achieve global communication using transmitter powers as low as one watt - something comparable to a small LED flashlight.
This capability opens up possibilities that would have seemed like science fiction just a few decades ago. Ham radio operators, using tiny transmitters with nothing more than a simple wire antenna in their backyards can have their signals detected on the other side of the planet.
It's All About The Timing
WSPR stations alternate between transmitting and receiving on even and odd minutes, creating a global network where everyone takes turns speaking and listening. This coordinated dance allows thousands of stations to share the same frequencies without interfering with each other, while maximizing the chances of successful signal reception.
The technical specifications are equally impressive. WSPR uses an extremely narrow bandwidth of about 6 Hz, roughly the width of a single piano key's frequency range. This narrow focus is what enables the protocol to extract coherent signals from what appears to be pure noise.
When a WSPR station successfully decodes a transmission, the receiving station typically uploads this information to a central database at wsprnet.org. This crowdsourced data collection creates a real-time, global map of radio propagation. This allows amateur radio operators to see which paths are open, which frequencies are working the best, and how conditions are changing throughout the day.
The applications of this data extend beyond amateur radio. Scientists study WSPR data to understand ionospheric behavior, solar effects on radio propagation, and other atmospheric phenomena. The protocol has become a valuable tool for research that might of otherwise required expensive specialized equipment.
Thinking Up A Plan
The main thing that appealed to me is that WSPR requires minimal operator attention. Unlike the traditional modes that I mentioned earlier, which require someone be present at the radio, WSPR stations can run fully unattended. The fact that it only requires very low power to work also meant that I didn't need to tie up my main transceiver, only a fairly small transmitter was needed.
Since I had some experience with Software Defined Radio (SDR) USB dongles on my other radio projects, I wanted to do something similar for my proposed WSPR beacon, since this allow me to have everything run off an old laptop or Raspberry Pi that would look after all duties (transmitting, receiving and posting results to the central database).
The issue is that the majority of the more common SDR's are receiver only, with a lot of the SDR's that can transmit being fairly high powered (Flexradio for example), and expensive. Thankfully there are a handful of USB based SDR transceivers that are actually pretty cheap and provided the power I needed.
Three in particular that I looked at where:
- HackRF One (~$300) - 1 MHz to 6 GHz, half-duplex, 20 MHz bandwidth, 8-bit ADC
- LimeSDR Mini (~$159) - 10 MHz to 3.5 GHz, full-duplex, 30.72 MHz bandwidth, 12-bit ADC
- ADALM-PLUTO (PlutoSDR) (~$150-200) - 325 MHz to 3.8 GHz, full-duplex, 20 MHz bandwidth, 12-bit ADC
After reading some reviews and looking up what the current availability was on Amazon for a reasonable price, I settled on ordering the HackRF One, since I found one on Amazon on sale for 40% off.
So, after laying down my money, I now need to wait for it to arrive (currently it's in some warehouse in China). Since I am now playing a bit of a waiting game, I'll finish off here, but my plan is to have a working WSPR station up and running which I hope to be able to tell you about next month.
