Fresnel Zones
If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. But if there are obstacles near the path, the radio waves reflecting off those objects may arrive out of phase with the signals that travel directly and reduce the power of the received signal. On the other hand, the reflection can enhance the power of the received signal if the reflection and the direct signals arrive in phase. Sometimes this results in the counterintuitive finding that reducing the height of an antenna increases the signal-to-noise ratio.
Fresnel provided a means to calculate where the zones are, where a given obstacle will cause mostly in phase or mostly out of phase reflections between the transmitter and the receiver. Obstacles in the first Fresnel zone will create signals that will be 0 to 90 degrees out of phase, in the second zone they will be 90 to 270 degrees out of phase, in third zone, they will be 270 to 450 degrees out of phase and so on. Even numbered zones have the maximum phase cancelling effect and odd numbered zones may actually add to the signal power.
Read more about this topic: Fresnel Zone
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