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Figure 1-7.Reference designations
Medium-Frequency Communications

Neets Module 17-Radio-Frequency Communications Principles
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1-11 1-1 shows the radio-frequency spectrum broken down into nine bands used by the military. Propagation of radio waves varies widely at different frequencies. Frequencies and equipment are chosen to meet the communications application desired. We will discuss the radio-frequency spectrum in the following paragraphs. Table 1-1.—Radio-Frequency Spectrum   FREQUENCY DESCRIPTION 30 GHZ - 300 GHZ extremely high frequency 3 GHZ - 30 GHZ superhigh frequency 300 MHZ - 3 GHZ ultrahigh frequency 30 MHZ - 300 MHZ very high frequency 3 MHZ - 30 MHZ high frequency 300 KHZ - 3 MHZ medium frequency 30 KHZ - 300 KHZ low frequency 3 KHZ - 30 KHZ very low  frequency 300 HZ - 3 KHZ voice frequency Up to 300 HZ extremely low frequency Extremely Low-Frequency Communications The purpose of the EXTREMELY LOW-FREQUENCY (elf) communications system is to send short "phonetic letter spelled out" (PLSO) messages from operating authorities in the continental United States (CONUS) to submarines operating at normal mission speeds and depths. Elf has the ability to penetrate ocean depths to several hundred feet with little signal loss. This ability allows submarines to be operated well below the immediate surface and enhances submarine survivability by making detection more difficult. This is a one-way communications system from the operating authority to submarines at sea. The large size of elf transmitters and antennas makes elf transmission from submarines impractical. Very-Low-Frequency Communications The communications commitments of the Navy now cover the face of the earth. New sea frontiers to the north have opened a four-million-square-mile, ice-covered ocean of strategic importance. Our Navy must maintain control of the operating forces in an ever expanding coverage area. This additional area requires changes in communications capacity, range, and reliability. Additional needs have been particularly great in the North Atlantic and the newly opened Arctic Ocean. High-frequency circuits are too unreliable in these areas because of local atmospheric disturbances.






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