multipath amplification

Body

*Open Field Propagation*
Although the formulas in the previous section are useful in some
circumstances, the actual range of a VHF or UHF signal is affected by
reflections from the ground and surrounding objects. The path lengths of
the reflected signals differ from that of the line-of-sight signal, so
the receiver sees a combined signal with components having different
amplitudes and phases.

The reflection causes a phase reversal. A reflected signal having a path
length exceeding the line-of-sight distance by exactly the signal
wavelength or a multiple of it will almost cancel completely the desired
signal ("almost" because its amplitude will be slightly less than the
direct signal amplitude). *On the other hand, if the path length of the
reflected signal differs exactly by an odd multiple of half the
wavelength, the total signal will be strengthened by "almost" two times
the free space direct signal. *

In an open field with fl at terrain there will be no reflections except
the unavoidable one from the ground. It is instructive and useful to
examine in depth the field strength versus distance in this case. The
mathematical details are given in the Mathcad worksheet "Open Field Range."

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