The elevation profile
The key tool is the elevation profile: the height of the ground along the straight line from the transmitter to a point. Add the curvature of the Earth and the height of the antennas and you can see at a glance whether the path is clear, partly blocked or in deep shadow. The coverage map of this site computes a profile for every direction around the transmitter, from real elevation data stored on our own server.
That is why moving an antenna 100 m or raising it by a few metres can change a shadow into a good signal: it changes the profile for a whole sector.
Shadow zones and reflections
Behind a hill the signal is much weaker, but it is rarely zero: diffraction over the crest and reflections from other slopes can still deliver a usable signal, especially at low frequencies. In the map such areas appear as orange and red patches next to green areas. Narrow valleys can channel the signal and give long thin arms of coverage.
Buildings and trees are not in elevation data. The tool adds an environment loss at the receiver (open, rural, suburban or urban) that grows slowly with frequency.
Improving a weak spot
In order of effectiveness: raise the antenna, move it to higher ground, choose a lower frequency, use a higher-gain directional antenna aimed at the weak area, and increase power as the last resort because 3 dB costs a doubling of power. Compare the options in the coverage map before you climb the tower.
