(3) Cross polarization is a configuration in which the transmit
and receive antennas at a multichannel station are oppositely
polarized. Depending on your equipment's capabilities, you may mount
the transmit and receive antennas on a common mast, or on separate
masts.
Cross polarization reduces the required transmitter-to-
receiver frequency separation.
Part B of this lesson discusses
frequency separation.
4.
LOS Radio Transmission Paths.
A multichannel system, linked by radio, consists of multiplex
equipment, two radio terminals, and the relay stations needed to span
the distance between them.
There are two terms you need to be
familiar with for the discussion of field radio circuits. These two
terms are path and link.
A radio circuit path is the route over
which the radio transmission travels.
A link (or hop) is the
distance between a radio transmitter and the receiver that receives
the transmission.
Path lengths for links differ with the terrain
over which the signals travel and the heights of the antennas at
opposite terminals.
Figure 2-3 shows several typical LOS path
profiles.
Under favorable LOS conditions, a single link may span distances of
up to 100 miles. Conversely, over terrains with moderate obstacles,
hops may be limited to spans no greater than 10 miles.
Over flat
terrain, using standard antenna masts, distances of approximately 30
miles is standard.
This is the average (or yardstick) signal
propagation distance used for planning. This average is based solely
on the Earth's curvature, and does not take into account obstructions
or favorable opportunities such as terminals located with unusually
high terrain features.
a. Electromagnetic
Propagation.
In
LOS
electromagnetic waves in the very high frequency (VHF), UHF, and
super high frequency (SHF) ranges behave much like light waves (such
as a flashlight beam).
They travel essentially in straight lines,
being bent only slightly by the Earth's atmosphere. We refer to the
travel of electromagnetic waves through free space (such as the
Earth's atmosphere) as electromagnetic propagation.
Applied to LOS
communications, this means the transmission of radio signals through
the atmosphere.
LOS propagation is the ability of the transmitter to "see" the
receiver electromagnetically.
This is much like signaling another
person with a flashlight. As long as the receiver can see the light
of the sender, an LOS path exists. If an obstacle
2-8
SS0016
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