14190_78

2-20 advances in the field of solid-state microwave amplifiers have produced solid-state amplifiers with enough output power to be used as the first stage in some systems. Transmitters with more than three stages usually use crossed-field amplifiers in the third and any additional stages. Both traveling-wave tubes and crossed-field amplifiers have a very flat amplification response over a relatively wide frequency range. Crossed-field amplifiers have another advantage when used as the final stages of a transmitter; that is, the design of the crossed-field amplifier allows rf energy to pass through the tube virtually unaffected when the tube is not pulsed. When no pulse is present, the tube acts as a section of waveguide. Therefore, if less than maximum output power is desired, the final and preceding cross-field amplifier stages can be shut off as needed. This feature also allows a transmitter to operate at reduced power, even when the final crossed-field amplifier is defective. Q25. What transmitter component allows the radiation of a large number of discrete frequencies over a wide band? Q26. What is the result of pulsing a pulsed rf amplifier when no rf is present? DUPLEXERS Whenever a single antenna is used for both transmitting and receiving, as in a radar system, problems arise. Switching the antenna between the transmit and receive modes presents one problem; ensuring that maximum use is made of the available energy is another. The simplest solution is to use a switch to transfer the antenna connection from the receiver to the transmitter during the transmitted pulse and back to the receiver during the return (echo) pulse. No practical mechanical switches are available that can open and close in a few microseconds. Therefore, ELECTRONIC SWITCHES must be used. Switching systems of this type are called DUPLEXERS. BASIC DUPLEXER OPERATION In selecting a switch for this task, you must remember that protection of the receiver input circuit is as important as are output power considerations. At frequencies where amplifiers may be used, amplifier tubes can be chosen to withstand large input powers without damage. However, the input circuit of the receiver is easily damaged by large applied signals and must be carefully protected. An effective radar duplexing system must meet the following four requirements: 1. During the period of transmission, the switch must connect the antenna to the transmitter and disconnect it from the receiver. 2. The receiver must be thoroughly isolated from the transmitter during the transmission of the high- power pulse to avoid damage to sensitive receiver components. 3. After transmission, the switch must rapidly disconnect the transmitter and connect the receiver to the antenna. For targets close to the radar to be seen, the action of the switch must be extremely rapid. 4. The switch should absorb an absolute minimum of power both during transmission and reception. Therefore, a radar duplexer is the microwave equivalent of a fast, low-loss, single-pole, double- throw switch. The devices developed for this purpose are similar to spark gaps in which high-current microwave discharges furnish low-impedance paths. A duplexer usually contains two switching tubes