power over long distances. It provides eight channels
of frequency-division multiplex rtty traffic on each
transmission. The AN/FRT-72 transmitter is designed
specifically for this purpose. It produces 50-kW peak-
envelope power (25-kW average power) and covers a
frequency range of 30 to 150 kHz. Low-frequency
transmitters are normally used only on shore stations.
The low-frequency receive system receives lf
broadcasts and reproduces the intelligence that was
transmitted. A typical lf receive system is shown in
figure 2-4. The antennas receive the lf signal and send
it to the multicoupler and patch panel. The
multicoupler and patch panel (AN/SRA-17 and
AN/SRA-49) allow the operator to select different
antennas and connect them to various receivers. In the
system shown in figure 2-4, the receiver can be either
the AN/SRR-19A or the R-2368A/URR. These
receivers operate in the frequency ranges of 30 to 300
kHz and 14 kHz to 30 MHz, respectively.
The receiver audio is fed to the SB-973/SRR
receiver transfer switchboard. As we explained earlier,
this allows the received audio to be connected to
numerous pieces of equipment. In figure 2-4, the audio
is connected to either an AN/URA-17 or CV-2460
convertor comparator, which converts the received
signal to dc for use by the teletype (tty) equipment.
From the convertor, the dc signal is fed to a dc patch
panel (SB-1203/UG). The signal can then be sent to
any crypto equipment attached to the patch panel. The
crypto equipment decrypts the signal and routes it to
the red patch panel (SB-1210/UGQ). The signal can
then be patched to a teletype printer for plain text
printing, or to a reperforator, where a paper tape will be
punched and stored for later printing.
The high-frequency (hf) band is shared by many
domestic and foreign users. Portions scattered
throughout the band are assigned to the military. The
Navys communications requirements have grown
rapidly, severely taxing its portion of the spectrum.
Satellite communications has relieved some of this
congestion and, for some types of service, has replaced
hf for long-distance communications, pushing hf into a
back-up role. However, even with the use of satellite
communications, hf will continue to be in high demand
for sometime. We will cover satellite communications
in chapter 3.
Naval communications within the hf band are
grouped into four general types: point-to-point, ship-
to-shore, ground-to-air, and fleet broadcast. All but the
fleet broadcast are normally operated two-way.
Point-to-point systems provide communications
over long-distance trunks or via links between fixed
terminals. A trunk is normally a message circuit
between two points using cable, fiber, or
telephone circuits. A link is a transmitter/receiver
system connecting two locations. The two locations
normally use directional, high-gain antennas that
increase the effective radiated power, reduce the
chance of interference, and boost the sensitivity of the
Figure 2-4.Lf receive.
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