Antenna Hungária has broadcast Kossuth Rádió (now also available via internet streaming) on 540 kHz for over 40 years with a two megawatt, 50% efficient transmitter on the 303 meter tower at Transmitter Solt. With the old transmitter, four megawatts of 11 kV utility power resulted in two megawatts of RF on 540 kHz. The new solid-state Nautel NX2000 transmitter commissioned in November 2017 consists of five 400 kW transmitters combined, and “only” 2.2 megawatts input power at 11 kV is needed. Six-inch and twelve-inch RF coaxial hardline pump the RF to the combiner and then the antenna tower. This is the most powerful medium wave transmitter in Europe, and is equal to other most powerful two megawatt medium wave transmitters in the world.
Nautel NX 2 MW AM transmitter combiner interior. - Spherical Image - RICOH THETA
Other two megawatt medium-wave
There are numerous more ≥ one megawatt transmitters. I am being a little mean and only listing here the most powerful in the world, even though many other transmitters are within 3 dB power (one megawatt).
A groundwave propagation model covering 10 kHz - 30 MHz is described in ITU-R P.368. A very simple example (too inaccurate for planning work) is from ITU grwave. During the 1960s when high altitude nuclear weapons testing was permissible, D-layer absorption increases were thought to be responsible for loss of MW skywave reception for several days over thousands of kilometers. Because ground conductivity is so important, a meaningful MW groundwave model should take into account dynamics of earth conductivity in the coverage area.