Ground reflections

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Often the signal waves are angled downward slightly, usually the result of diffraction over an obstacle in the distance. If there is mostly-flat ground in front of the antenna, the ground reflection can be efficient.
[img][647:500]http://www.hdtvprimer.com/ANTENNAS/GroundReflect.jpg[/img]
These two waves pass through each other without affecting each other. But the antenna responds to the instantaneous sum of the two overlapping waves. Where the two waves subtract, there will be places where reception is very weak.

The result is a striped region of alternating strong and weak layers parallel to the ground. Thus there are cases where lowering the antenna might put it in a stronger signal. Unfortunately a strong spot for one channel can be a weak spot for a different channel, so compromise might be necessary.

The ground doesn