[throw0101b]

100W can potentially blanket the entire world:

* https://www.ncdxf.org/beacon/

* https://en.wikipedia.org/wiki/International_Beacon_Project

[wglb]

Those are shortwave, that has vastly wider propagation than the 833 khz that the Bridge radio company was using.

For the broadcast station, one key to very long distance is the 250 foot towers.

[Johnythree]

The key would be the lack of interference, both from electrical noise and from other stations.

Radio Amateurs are even today communicating world wide on the 630m MF band using much lower power and very modest antennas.

[wglb]

Yes, however, they are likely using new techniques such as WSJT and related modes, that have the capability to receive many tens of DBs lower signals.

[eszed]

Around 1920 my great-grandfather, I am told, built a 1w radio transmitter in his basement in San Francisco, with an aerial running up the outside of the chimney. I've no idea what frequencies he used, but he regularly spoke with fellow enthusiasts in Europe and New Zealand - several of them came to visit, when my grandfather was a child. By 1930 there was enough interference that was no longer possible with such a low-power set.

[wildzzz]

Probably something like 200 or 100m. The higher frequencies don't start opening up until the late 20s.