Figure 1-13.—UHF/HF voice relay circuit.

When a ship requests simplex operation on duplex circuits, the shore station may be required to shift transmitters prior to acknowledging call-up. If no reply is received within 45 seconds, the ship should repeat the call-up procedures. If a third attempt is required, the ship should check equipment to ensure proper operation. Semiduplex Semiduplex communications circuits, used primarily on task force/task group/ORESTES, are a combination of the simplex and duplex modes. All stations except the net control station (NECOS) transmit and receive on the same frequency. The NECOS transmits and is received on a second frequency. The NECOS may transmit continuously, whereas all other stations must transmit in accordance with simplex procedures. UHF/HF RELAY The UHF/HF relay method permits long-range, uninterrupted communications during periods of hazardous electromagnetic radiation (HERO). Figure 1-13 shows a block diagram of a UHF/HF voice relay circuit. Modern radio and radar transmitting equipments produce high-intensity RF fields. It is possible for RF energy to enter an ordnance item through a hole or crack in its skin or to be conducted into it by firing leads, wires, and the like. Here is an example of HERO. An aircraft carrier is arming aircraft on board. During arming operations, all HF transmitters must be secured to prevent possible detonation of the ordnance. To maintain its ship-shore communications, the carrier transmits to a relay ship via a UHF circuit. The relaying ship then retransmits the signal on a HF circuit to a terminated NAVCOMTELSTA. On-line Figure 1-13.—UHF/HF voice relay circuit. 1-12