And by the virtue of shortwave propagation, it could be heard across the world. For the past month and a half (from when the news of its impending shutdown was revealed) I was regularly picking it up in Australia right up until the bitter end.
HF propagates through skywave (most reliably from 5-30MHz), which is where the signal bounces off the ionosphere.
In the MF (AM broadcast) band, you can observe this at night - in Australia I can pick up the 50kW Melbourne ABC station (public broadcaster) at 774kHz with a good radio, just about across the entire country.
In the LF (longwave) band, the earth’s surface and the ionosphere start to behave more like a waveguide than skywave. This is actually more reliable/consistent than even HF, but you need massive transmitting antennas due to the large wavelengths involved.
HF also generally wins for distance covered per watt - despite the massive power of Radio 4 longwave, I’d have no chance of hearing it reach Australia.
It’s still quite a lot of power for AM (amplitude modulation) broadcast on any band.
If Radio 4 was on shortwave with 500kW transmitters, I probably would have been able to listen in on my radio at home in Australia, no UK-located KiwiSDR required.
Japan used to have a 500kW transmitter on 774kHz (mediumwave/AM broadcast band) for NHK Radio 2, and when the local broadcaster on 774 kHz went off air at midnight for scheduled maintenance, I could pick it up on my car’s stereo quite easily.
And by the virtue of shortwave propagation, it could be heard across the world. For the past month and a half (from when the news of its impending shutdown was revealed) I was regularly picking it up in Australia right up until the bitter end.