Long distance reception of a Spanish station on Medium Wave during daylight hours

Since I did a previous scan of the Medium Wave band in March, I have found several other very weak signals buried in the noise. One of these however is just about audible and can be heard in the middle of the day. I built a very basic loop aerial with some wire would around a cardboard box (30 x 45 cms) and tuned it with a variable capacitor. The result was that I can now ID two of the signals.

The signal on 855 kHz was Radio Nacional 50kw transmitter near Santander on the north coast of Spain, a distance of 992 kms or 616 miles. As can be seen from the map above, the path is almost completely over the ocean.

The aerial mast in Spain is just 300m or so from the water while I am 10 kms from the south coast of Ireland.

Radio mast near Santander

The other signal heard was from the test DRM transmitter of France Blue (8kw) on 1071 kHz near Brest, a distance of 500 kms or 310 miles.

I was wondering if the signals were possibly sky wave but I don't think so. They are there in the middle of the day every day and they are really steady like you might expect from a ground wave signal.

The radio used was a very basic Sony radio which was indoors. The loop aerial was just sitting on top of it.

992 kms seems like a remarkable distance for a ground wave signal on the medium band. There are plenty of much closer transmitters in the UK that I can't hear but the sea path to Spain seems to be making all the difference.