c. Satellite communications (SATCOM) multichannel systems use the 7.25 to 8.4 GHz
portion of the SHF band. These systems do not require a LOS path between the ground terminal
transmitting and receiving antennas (Figure 2-1C). SATCOM systems, including tactical
satellite (TACSAT) systems, normally have an operating range of 10,000 miles for a single link.
2.
System Planning.
a. The planning of a tactical multichannel radio system involves identifying the location
of signal sites. Key factors for successful communications are the terrain over which the radio
links will operate and the height of the antenna at both the transmit and the receive sites.
b. A path is the route over which the transmissions travel. Path lengths for links differ
with the terrain and antenna heights. Distances as great as 100 miles may be spanned in a single
link under favorable LOS conditions. Radio paths over flat terrain, using standard antenna masts
25 to 50 miles apart, can usually be spanned in a single link. In computing LOS radio paths, 30
miles is the planned yardstick for average propagation distances. This average is based on the
curvature of the Earth. However, it does not take into account obstacles or terrain features within
the 30-mile path.
c. VHF, UHF, and SHF radio transmissions in LOS communications behave like light
waves, as in a flashlight beam. If pointed to a spot in the distance, the light beam will travel
essentially in a straight line, being bent slightly due to the effects of the Earth's atmosphere
(called refraction). A LOS path does not exist when an obstacle is placed between the transmit
and receive sites, so the receiver can no longer see the light from the sender's flashlight. As with
light, a LOS condition exists when the radios from both sites can direct their transmissions on a
uninterrupted line of propagation.
d. There are procedures to assist signal officers to determine whether a LOS path exists
between two points. The first procedure pertains to the LOS path for VHF/UHF radio systems.
The second procedure pertains to the LOS path for SHF microwave radio systems. Different
methods are used to compute radio transmission pats from VHF/UHF and SHF.
3.
LOS Procedures or VHF/UHF Radio Systems.
a. There are two manual procedures which can be used to determine LOS for VHF/UHF
radio systems. One procedure involves the use of nonlinear graph paper. The other procedure
involves the use of linear graph paper.
2-3
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