Lesson 1/Learning Event 4
directional
modulation,
and
low-noise
high-gain
receivers. Primarily the greatest improvement in the signal is due to the
line-of-sight transmission path.
Moreover, frequencies used are so high
that they penetrate the ionosphere.
Since the radio waves are neither
reflected nor seriously refracted, they are relatively independent of the
vagaries of the ionosphere.
b. Techniques.
At the high frequencies used for long-distance
communication, the S/N can be improved by a number of techniques, most of
which are applied to the radio receiving station.
The radio receiving
station primarily determines the action to be taken and when that action
should occur. The radio receiving operator is the first to know when the
signal quality deteriorates to dangerous levels. He receives his warning by
frequent fades and a rise in noise level. The radio receiver can do little
about the noise that rides on the signal when the signal arrives. However,
receiver design greatly reduces the amount of internally produced noise,
especially the "front-end" noise.
Specially designed radio receivers
include RF preamplifier stages called parametric amplifiers.
These high-
gain low-noise amplifiers are to be found in satellite communication ground
terminals and tropospheric scatter terminals. A list of techniques used to
improve the S/N in HF long-distance radio communication follows.
(1) Radio transmitter path.
(a) Directional
A
directional
at
the
transmitting station beams most of the radiated energy in the direction of
the receiving station.
A directional antenna at the receiving station
limits the beamwidth of the received radio signal to the direction from
which the radio signals arrive.
(b) Radio propagation predictions.
Radio propagation predictions
inform the technical controller and radio receiving operator which
frequencies will most likely provide the strongest signal for the time of
day and season of the year.
(2) Radio transmitter.
(a) Single-sideband transmission. Single-sideband (SSB) transmission
provides a stronger received signal than double-sideband transmission
because most of the available power is concentrated in the intelligence-
bearing portion of the signal.
Further, the absence of a strong carrier
prevents heterodyning with adjacent signals, as well as minimizing
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