Last Tuesday at 1744 UTC (1:44 PM EDT) UR3RM, a ham radio station in Ukraine blindly sent out a message on 7040.138 kHz. It was automated. It was text. Maybe someone would hear it. Maybe not.

The “maybe not” part is easy to understand because UR3RM’s transmitter was putting out one milliwatt, .01 watts. To put that in perspective, a Class 2 Bluetooth transmitter, the ones good for around 30 feet, run 2.5 milliwatts.

UR3RM was using a mode called WSPR for Weak Signal Propagation Reporting. Unlike most of ham radio, this is a one-way mode. Not only is there little expectation anyone will be listening, but there’s even less that the signal would make it back. Radio propagation isn’t always a two-way path.

WSPR’s biggest selling point is you can do it on the cheap. It’s easy to set yourself up for not much more than $100 and often a whole lot less. And, though a ham radio license is needed to transmit, anyone can put up a receiver. And the US ham license test is multiple-choice, all published and online.

Most WSPR transmitters run very low power, many well under a watt like UR3RM. And sometimes, like UR3RM that peanut whistle goes far. Tuesday’s 1744 UTC transmission was heard on the Australian island of Tasmania, a distance of 15,140 km. Stated more impressively, the transmission/reception worked out to 9,235,000 miles per watt! This isn’t being done with fancy gear and immense antennas. This particular transmission took place on what we quaintly still call “short wave” radio. WSPR’s greatest accomplishment is it lets this be done on noisy, unreliable, staticky radio bands. And, it lets the receiver know what it’s gotten is good without any confirmation from the sender.

There is a price to pay for making all this reliable: bandwidth. A WSPR signal is 6 Hz wide. A typical voice channel would be around 2,500 Hz. This allows the tiny WSPR of power to be more concentrated and much more effective.

Low bandwidth also limits the signaling rate. In today’s gigaworld, you’ll be shocked to know WSPR runs at 1.4648 baud. No typo. The structured WSPR transmission sends 50 characters in 110.6 seconds, beginning one second after each even minute.

Each message contains the station’s callsign, a grid locator, and transmitter power expressed in dBm. So, when the station in Tasmania picked up the Ukrainian transmission he immediately knew where it was from and how much power got it there.

Because of their very narrow bandwidth, WSPR signals can often be decoded when human ears can’t detect the signal is even there. It’s claimed a signal 28 dB below the noise in a 2500 Hz bandwidth receiver can be decoded with WSPR. I’ve had the volume turned up and watched stations decoded that were totally indistinguishable from the background noise by my ears.

The narrow bandwidth actually allows a receiver to hear and decode multiple stations at once, often handfuls at a time when the bands are open. Since the receiver has no way to tell the originating stations “job well done,” the reception is reported to a central hub on the Internet. Want to know what ham bands are good for contacting what parts of the world at this moment? Head over to wsprnet.org, where these are plotted out and otherwise quantified.

WSPR was produced by Joe Taylor, a ham operator (K1JT) and Nobel Physics prize winner. In the past, he’s developed other transmission/reception methods to help with moonbounce and meteor scatter radio work.

Like so many other radio advances, this one is really helped by the advent of inexpensive SDR receivers. Though the $20-ish variety sold on Amazon, eBay and others doesn’t do well on these long-distance low frequencies, more sophisticated models are now selling for under $100. The software to decode (and the transmit software too) is free and open source. Prebuilt or mostly built transmitters are also widely available for under $100. Some folks have even figured out how to make a Raspberry Pi act like a 10 milliwatt WSPR transmitter (pictured at top), though some outboard filtering to make sure it only transmits where it’s supposed to is necessary.

Every ham radio band is different, and with the current solar sunspot cycle down near the minimum, conditions are definitely lousy. But WSPR is so vigorous and resilient that even now worldwide communication is possible with flea power. If you’re interested, you can actually dip your toe in the water for free. The dozens of receivers aggregated through http://sdr.hu all have WSPR as an available mode. If you’re like me, you’ll end up spending hours listening to radio signals you actually don’t hear and wouldn’t understand if you did!

Top image credit: Gerolf Ziegenhain/CC BY-SA 3.0

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