The range of a NDB beacon is controlled primarily by its power:

The formulas (for approximate answers) to determine range are:

Max range = 3 x Square root of Power in Watts (water)
Max range = 2 x Square root of Power in Watts (land)

Note 1: The formula does not include the effect of ground conductivity or frequency.
Note 2: To double the range requires four times the power.

NDBs are a medium range nav aid using frequencies from 190 KHz to 1750 KHz, in Europe usually 200 KHz to 455 KHz. These frequencies straddle the MF and LF bands but the system could also be described as using hectometric and kilometric wavelengths or it could be described loosely as an MF or hectometric aid because most of the frequencies are in one band. Surface waves are the primary method of signal propagation giving theoretical ranges of 300 NM over land and over 600 NM over the sea.

But what appen to these waves when they pass from land to water or vice versa? These electrical differences in conductivity between land and sea affect radio waves. As low frequency waves cross the shoreline, they move abruptly from an area of low conductivity (land) to an area of high conductivity (water). That rapid change in conductivity changes the radio wave's "phase velocity."As the phase velocity changes, the wave's frequency (what you dialed into the radio receiver) stays the same. The laws of conservation of energy and momentum come into play, and the wave bends. This is called refraction.