Why it changes the path
Frequency, polarization, ground conductivity, terrain, antenna height, and loss determine useful distance. Lower frequencies and vertical polarization often favor surface-wave range.
What to observe
Separate local direct coverage from a true surface-following path and compare results over different terrain and ground conditions.
The common mistake
Calling every contact made below the ionosphere “ground wave.”
Propagation models and field regions depend on frequency, geometry, terrain, atmosphere, polarization, antenna pattern, and measurement distance. Use them to plan and compare—not to guarantee a contact or installed range.