HIGH-FREQUENCY COMMUNICATIONS

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. Lf Receive 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. HIGH-FREQUENCY COMMUNICATIONS The high-frequency (hf) band is shared by many domestic and foreign users. Portions scattered throughout the band are assigned to the military. The Navy’s 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 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. 2 - 5