Marshall Amplification Field Marshall 100 Owner's Manual page 32

Field Marshall Tracking Receiver
Diffraction
Fortunately, radio waves can bend around objects. Diffraction
works best around metal edges in buildings, but also happens
with hills, trees and mountains. Diffraction increases the range
of your transmitter beyond line-of-sight, because the signal
bends over the crest of the hills.
This means your transmitter's range is not determined strictly
by the above line-of-sight chart. Diffraction allows some of the
signal to "hug the earth" and go further than the strict horizon,
tapering off gradually as you go beyond it. A transmitter beyond
a hill or in a deep ravine would be undetectable if it weren't for
diffraction around the edges. A powerful transmitter can use
diffraction to punch a signal beyond the line-of-site limit.
Diffraction around trees combined with reflections can create
complex patterns within the forest. You should hold your
antenna horizontally in a forest because trees produce mostly
vertical interference.
It is important to remember that the frequency is very important.
Higher frequency means less diffraction. Higher frequency
transmitters may perform equally well at close range on flat
ground, but will not do well at a distance or in hilly terrain. This
is unfortunate, because higher frequencies allow smaller
antennas. The frequency of your Marshall receiver, between
173 and 220 MHz gives a good compromise between antenna
size and ground-hugging (diffraction) ability.
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