FACTORS
INFLUENCING RADIO COMMUNICATIONS--Line-of-Sight
Ideal Radio Transmission occurs under conditions whereby a
visually clear path exists between Receiver and Transmitter antennas. A visually
clear path or line of sight ensures the strongest possible signal with minimal
attenuation (reduction) due to environmental factors. To have a clear line
of sight there must be no obstacles between the two locations. Line of sight
can be verified using binoculars to ensure a clear path between Receiver and
Transmitter antennas. However, even under line of sight conditions, signal
attenuation occurs depending on the distance between the two antennas as well
as proximity of obstacles to the line of sight path. When RF radiates from
one antenna to another it spreads out such that an elliptical pattern is created
between the two antenna; this elliptical area is known as the Fresnel Zone.
The cross-section or height of the Fresnel Zone depends on the distance between
antennas and the frequency of the signal. Obstacles protruding into the Fresnel
Zone affect signal strength and integrity. Hard obstacles protruding into
the Fresnel Zone can deflect signals resulting in short reception delays or
in some cases prevent the signal from reaching its intended destination. Soft
obstacles such as trees typically attenuate the signal resulting in shorter,
less reliable transmissions. The Fresnel Zone can be calculated based on signal
path distance and signal frequency. The height of the Fresnel Zone can be
used to estimate the required height of antennas to provide sufficient clearance
from obstacles. As a general rule, antennas should be raised/located to ensure
60% of the First Fresnel Zone is clear of obstructions. The radio link will
then behave essentially the same as it would with a clear path.
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