Every year, the Knights QRSS Group release their annual compendium. You can view the Winter 2020 (3rd edition) HERE
The Knights QRSS Group promotes the use of very slow mode code beacons to carry out propagation experiments on the HF bands. Often signals that are 15 to 20 dB below the noise can be seen on a computer screen as opposed to being heard by ear.
While newer digital modes can now be used for detecting very weak signals, they don't really show propagation effects. Either the digital signal was decoded or it wasn't. QRSS signals like the one shown below shows propagation over a 15-minute period.
In that image, you can see how signals fade with the Sporadic-E footprint moving and the polarisation changing. It also shows up slight doppler effects.
You can find out more about QRSS signals by visiting the QRSS Knights Groups.io page... https://groups.io/g/qrssknights/topics