Thanks for the report on what you are hearing.
I've set up one of the VLF/LF/HF Active antennas at the workshop feeding one of the prototype Sentinel 4 receivers through a 150 kHz low pass filter to remove the LW and MW broadcast stations.
Best so far was hearing WSPR transmissions on 137.5 kHz from stations in Germany and Spain. The only problem was that the 4G modem we use in the workshop for internet access fell over that night so the spots from G4HYG/RX2 didn't get to the WSPRnet database.
I'm rather surprised how far 1W effective radiated power at 137.5 kHz can get.