I feel that one of the charms of amateur radio is its unpredictability. It feels like throwing a message in a bottle into the vast ocean, not knowing where the currents would take it, or who would read the message. Radio waves often take unpredictable paths when traveling. To a large extent, how these waves propagate is determined by the state of our planet’s ionosphere. The ionosphere consists of layers of charged particles that affect how RF signals travel. These layers move and shift and undergo cycles of strengthening and weakening, all under the influence of the Sun. Radio waves can bounce off the ionosphere, and essentially “skip” around the Earth.

To understand how waves propagate in different bands a protocol known as WSPR (pronounced “whisper”) was developed in 2008 by Joe Taylor (K1JT). It is an acronym for “Weak Signal Propagation Reporter”. It can tell us what is possible with low-power transmissions and see which radio bands have a path to which points on the globe.

A WSPR transmission conveys the sender’s call sign, station location, and power level using a compressed data format with strong forward error correction (FEC). The message is modulated using frequency-shift keying (FSK) at a very low bit rate. Sending a single WSPR message takes almost two full minutes! The WSPR protocol is effective at signal-to-noise ratios (SNR) as low as -28 dB in a 2500 Hz bandwidth, some 10 to 15 dB below the threshold of audibility.

My QCX transceiver contains an inbuilt WSPR mode. I set it up and sent a single WSPR message on the 40m band with about 4-5 watts of power. It took almost 2 minutes to send, and I was worried about heating the BS170 transistors in the power amplifier since I had never really stress-tested them in this manner. To my relief, no magic smoke was released. I monitored the transmission on a local WebSDR to make sure it was sending a decipherable message. After it was done sending, I checked the WSPRNet website to see if any stations received my feeble signal. I wasn’t expecting anything spectacular, but to my surprise, the signal had traveled much farther than I imagined!

I had reached Antarctica, Hawaii, New Zealand, Australia, the Canary Islands, and Norway (to name a few)! Well, that proves that the antenna I built is working. There are lots of tools to analyze the WSPR data. I liked the analysis tools available on WSPR Rocks. For example, you can view a SNR vs distance chart. It was interesting to see that some distant stations copied my signal better than stations which were nearby. On the website you can see the names of the stations and other details.

I also pulled all the data into a Google Sheet for analysis.

I reached 26 locations with a single WSPR transmission. Incredible! The results have encouraged me to try building a dedicated WSPR beacon using the Raspberry Pi for use on other bands. Stay tuned!