HIGH-FREQUENCY SYSTEMS

The high-frequency (HF) band (3-30 MHz) is primarily used by mobile and maritime units. The military uses this band for long-range voice and teleprinter communications. This band is also used as a backup system for the satellite communications system. We will discuss satellite communications later in this chapter.

Figure 1-2 shows a typical high-frequency transmit system. In transmitting teleprinter information, the equipments shown in figure 1-2 perform the same functions as the equipments shown in figure 1-1, except the equipments in the high-frequency system do the functions in reverse order.

In the HF transmit system, the AN/UCC-1 Telegraph Terminal converts dc signals into audio tone signals. The output of the AN/UCC-1 is connected to the transmitter transfer switchboard. The C-1004 Transmit Keying and Receive Control/Teleprinter is used to key the transmitter during teleprinter operations.

Voice communications from some remote locations are also connected to the transmitter transfer switchboard. Voice communications are initiated at a handset (remote phone unit) and connected to the C-1138 Radio Set Control. The output of the radio set control is connected to the transmitter transfer switchboard. The transmitter transfer switchboard permits the operator to select the proper transmitter for the frequency to be transmitted.

Figure 1-2.-High-frequency transmit system.

Figure 1-3.-High-frequency receive system.

Figure l-3 shows atypical high-frequency receive system. Refer to the figure as we follow the signal path through the system.

1. A transmitted high-frequency signal is received by the antenna, which converts electromagnetic energy to electrical energy.

^2. The signal travels through a transmission line to an antenna patch panel, where it can be distributed to any of a number of receivers.

3. The receiver converts the RF signal into a teleprinter or voice signal, depending upon what is desired.

^4. The output of the receiver is then sent to the receiver transfer switchboard.

5. If a teleprinter signal was selected, the teleprinter signal from the switchboard goes to the AN/UCC-1 and then follows the same path as we described in the low-frequency receive section. Identical pieces of equipment are used, and they perform the same functions.

6. If a voice signal was selected, the voice signal from the receiver transfer switchboard is sent to the radio set control. The output is then sent to a handset. The voice signal can also be sent from the switchboard to a remote speaker amplifier. There, it can be placed on a speaker so that the user can listen to the received signal without holding onto the handset.