Without consulting text books my brain cell seems to remember NDB having a Ground Wave. If you had a very long range NDB and you were attempting to use it for navigation over the horizon(ie the curve of the earth)then the surface over which it travelled would be quite important, and some surfaces would absorb its energy more than others. A short range NDB like the ones we are used to at Airports might not suffer quite so much because we using mainly the Space wave bit of it, although having said that, it can still be bent by coastal refraction and other local terrain effects. Southends' NDB is bent if you ever try an approach...well that was my excuse anyway.