R. Fry said:
The users of these transmitters can generate ERPs so high that they can be received over relatively large areas of the globe -- day and night.
But, aside from VOA (which I believe used 250 kw transmitters at Greenville), HCJB was I believe the only operation with 500 kw transmitters on SW in the Western Hemisphere. While Costa Rica and Venezuela briefly had 1 megawatt on MW (same transmitter, a pair of Continentals) and "TWR" in Bonaire had 500 kw, also on AM, from the 60's into the 90's, there has not been any permanent high power operation with such high power elsewhere since prior to W.W. II.
The superpower AMs, such as the Riyadh one above 1500, got rather dismal coverage as they tended to be in places with poor conductivity... which is, I guess, part of the reason for using such high power to begin with.
I believe all gigawatt and above transmitters were used on AM (MW) or LW, as I don't recall any going on SW.