MAGNETRON OPERATION

MAGNETRON OPERATION.— The electric field in the electron resonant oscillator consists of the alternating current (at) and direct current (de) fields. The action of electronics taking energy away from the ac fields is an undesirable effect. The dc field extends radially between adjacent anode segments by the RF oscillations induced in the cavity tank circuits of the anode block. For more information on magnetron operation, refer to your appropriate operating proce- dures for your fire-control system. MAGNETRON COUPLING METHODS.— The RF energy can be removed from a magnetron by a coupling loop. At frequencies lower than 10,000 MHz, the coupling loop is made by bending the inner conductor of a coaxial cable into a loop and soldering the end to the outer conductor so that the loop projects into the cavity. To obtain sufficient pickup at higher frequencies, the loop is located at the end of the cav- ity. MAGNETRON TUNING.— A tunable magne- tron permits the system to be operated at a precise frequency anywhere within a band of frequencies, as determined by the magnetron’s characteristics. The resonant frequency of a magnetron can be varied by varying the inductance or capacitance of the resonant cavities. MAGNETRON SEASONING.— During initial operation, a high-power magnetron arcs from the cathode to the plate and must be properly broken-in or baked-in. Actually, arcing in magnetrons is very com- mon. It may occur with a new tube or following a long period of idleness. One of the prime causes of arcing is the liberation of gas from tube elements during idle periods. Arcing may also be caused by sharp surfaces within the tube, mode shifting, and excessive current. Although the cathode can withstand considerable arcing for short periods of time, continued arcing will shorten the life of the magnetron and may destroy it entirely. Hence, each time excessive arcing occurs, the tube must be baked-in again until the arcing ceases and the tube is stabilized. The baking-in procedure is relatively simple. Magnetron voltage is raised from a low value until arcing occurs several times a second. The voltage is left at that value until arcing dies out. Then the volt- age is raised further until arcing again occurs, and it is left at that value until the arcing again dies out. When- ever the arcing becomes very violent and resembles a continuous arc, the applied voltage is excessive and should be reduced to permit the magnetron to recover. When the normal rated voltage is reached and the magnetron remains stable at the rated current, the baking-in is complete. It is a good maintenance prac- tice to bake-in magnetrons left idle in the equipment, or those used as spares, when long periods of non- operating time have accumulated. Crossed-Field Amplifier The crossed-field amplifier (CFA) is constructed very similar to a magnetron. The major difference is that the CFA requires an RF input in addition to the electrical input and the magnetic field. The CFA anode is very similar to that of a regular trapezoid block magnetron, with the exception of the drift tube section. The drift tube section serves two purposes: (1) to dampen out electron bunches, and thus oscillations, as the spokes move past the output port, and (2) to provide a path for RF energy from the input to the output. The CFA is normally used in a chain of two or more CFAs in series. When the CFA is not pulsed with high voltage, the tube presents a low-impedance- to-input RF and passes it through to the output port. When used in a chain, one or more CFAs can be energized, depending on the output power required. For example, for close-in targets, the final stage or stages may not be needed, but all CFAs are required for detecting small targets at maximum range. The cathode of a CFA is cylindrical, just as in magnetrons; however, most are cold cathodes (no fila- 2-11