I had built a QRP Labs U3S and run it on 40m and 20m. My main interest is in NVIS communications (Near Vertical Incindence Skywave). Many years back I visited a facility in Hermanus, South Africa where they test for various HF propagation conditions. The term Ionosonde comes to mind.
Well I made my own system that seems to work very well using WSPR as the source on the U3S and WSJT-X together with a SDR-IQ as the receiver. Allow me to present some further detail.
In KG34ac (Lichtenburg) I run the U3S using a 40m / 20m trap inverted vee. I built the simple traps myself on tiny pieces of PCB using a 30 turn winding on a red toroid with a parallel 47 pF capacitor. On 40m the U3S puts out 200 mW and on 20m, 50 mW approximately. The WSPR sequence is repeated every six minutes.
In KG34ac (Molopo), 29 km away as the crow flies I run WSJT-X in WSPR mode talking indirectly via virtual audio (Software) and Spectravue (RF Space’s SDR software) to the SDR-IQ SDR HF receiver usually used as a CW monitor for the RBN (Reverse Beacon Network) when appropriate..
These two options allow some pretty interesting observations. I use WSPRD at http://wsprd.vk7jj.com/ to accumulate the data and create an appropriate graph from that data using LIBRE Calc (Freeware office software).
The results coincide with real world propagation opportunities for NVIS communications. I drew this graph from recent data.
Now that’s using WSPR for a purpose! The SNR values are usually well into negative values for example -27 or nothing at all. Anything above the zero line is a very good indicator that suitable conditions exist.
73 de Eddie ZS6BNE