a. Radio sets using the VHF, UHF, and SHF bands require a LOS path between the
transmitting and receiving antennas. However, radio sets using troposcatter propagation and
SATCOM multichannel systems do not require a LOS path.
b. VHF, UHF, and SHF radio transmissions behave like light waves. As with light, a
LOS condition exists when the radios from both sites can direct their transmissions on an
uninterrupted line of propagation.
c. Two manual procedures are used to determine LOS for VHF/UHF radio systems.
One procedure uses nonlinear graph paper and the other uses linear graph paper. When
available, nonlinear graph paper should be used to determine LOS. Nonlinear graph paper is
designed specifically to consider both the curvature of the Earth and the effects of refraction on
radio transmissions.
d. The factors of VHF/UHF LOS radio planning also apply to SHF microwave radios,
with some exceptions. Due to the higher transmitting frequencies of SHF, obstacles such as
hilltops, buildings, and trees must be avoided. These obstacles can either block or significantly
decrease the signal level at the receiving antenna.
e. The Fresnel zone is defined as an area of propagation (ellipsoid in shape) that exists
to establish reliable communications. Additional Fresnel zones (second, third, and fourth) exist
beyond the boundaries of the first Fresnel zone. Both LOS and first Fresnel zone clearance must
be achieved for microwave radio systems to ensure satisfactory communications.
SS0121
2-16
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