COUPLING METHODS. - Energy (rf) can be removed from a magnetron by means of a COUPLING LOOP. At frequencies lower than 10,000 megahertz, the coupling loop is made by bending the inner conductor of a coaxial cable into a loop. The loop is then soldered to the end of the outer conductor so that it projects into the cavity, as shown in figure 2-35, view (A). Locating the loop at the end of the cavity, as shown in view (B), causes the magnetron to obtain sufficient pickup at higher frequencies.

Figure 2-35A. - Magnetron coupling methods.

Figure 2-35B. - Magnetron coupling methods.

The SEGMENT-FED LOOP METHOD is shown in view (C) of figure 2-35. The loop intercepts the magnetic lines passing between cavities. The STRAP-FED LOOP METHOD (view (D), intercepts the energy between the strap and the segment. On the output side, the coaxial line feeds another coaxial line directly or feeds a waveguide through a choke joint. The vacuum seal at the inner conductor helps to support the line. APERTURE, OR SLOT, COUPLING is illustrated in view (E). Energy is coupled directly to a waveguide through an iris.

Figure 2-35C. - Magnetron coupling methods.

Figure 2-35D. - Magnetron coupling methods.

Figure 2-35E. - Magnetron coupling methods.

MAGNETRON TUNING. - A tunable magnetron permits the system to be operated at a precise frequency anywhere within a band of frequencies, as determined by magnetron characteristics.

The resonant frequency of a magnetron may be changed by varying the inductance or capacitance of the resonant cavities. In figure 2-36, an inductive tuning element is inserted into the hole portion of the hole-and-slot cavities. It changes the inductance of the resonant circuits by altering the ratio of surface area to cavity volume in a high-current region. The type of tuner illustrated in figure 2-36 is called a SPROCKET TUNER or CROWN-OF-THORNS TUNER. All of its tuning elements are attached to a frame which is positioned by a flexible bellows arrangement. The insertion of the tuning elements into each anode hole decreases the inductance of the cavity and therefore increases the resonant frequency. One of the limitations of inductive tuning is that it lowers the unloaded Q of the cavities and therefore reduces the efficiency of the tube.

Figure 2-36. - Inductive magnetron tuning.

The insertion of an element (ring) into the cavity slot, as shown in figure 2-37, increases the slot capacitance and decreases the resonant frequency. Because the gap is narrowed in width, the breakdown voltage is lowered. Therefore, capacitively tuned magnetrons must be operated with low voltages and at low-power outputs. The type of capacitive tuner illustrated in figure 2-37 is called a COOKIE-CUTTER TUNER. It consists of a metal ring inserted between the two rings of a double-strapped magnetron, which serves to increase the strap capacitance. Because of the mechanical and voltage breakdown problems associated with the cookie-cutter tuner, it is more suitable for use at longer wavelengths. Both the capacitance and inductance tuners described are symmetrical; that is, each cavity is affected in the same manner, and the pi mode is preserved.

Figure 2-37. - Capacitive magnetron tuning.

A 10-percent frequency range may be obtained with either of the two tuning methods described above. Also, the two tuning methods may be used in combination to cover a larger tuning range than is possible with either one alone.

ARCING IN MAGNETRONS. - During initial operation a high-powered magnetron arcs from cathode to plate and must be properly BROKEN IN or BAKED IN. Actually, arcing in magnetrons is very common. It occurs with a new tube or following long periods of idleness.

One of the prime causes of arcing is the release of gas from tube elements during idle periods. Arcing may also be caused by the presence of sharp surfaces within the tube, mode shifting, and by drawing excessive current. While 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. Therefore, 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 voltage is raised further until arcing again occurs and is left at that value until the arcing again ceases. Whenever 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 normal rated voltage is reached and the magnetron remains stable at the rated current, the baking-in is complete. A good maintenance practice is to bake-in magnetrons left idle in the equipment or those used as spares when long periods of nonoperating time have accumulated.

The preceding information is general in nature. The recommended times and procedures in the technical manuals for the equipment should be followed when baking-in a specific type magnetron.

The Crossed-Field Amplifier (Amplitron)

The CROSSED-FIELD AMPLIFIER (cfa), commonly known as an AMPLITRON and sometimes referred to as a PLATINOTRON, is a broadband microwave amplifier that can also be used as an oscillator. The cfa is similar in operation to the magnetron and is capable of providing relatively large amounts of power with high efficiency. The bandwidth of the cfa, at any given instant, is approximately plus or minus 5 percent of the rated center frequency. Any incoming signals within this bandwidth are amplified. Peak power levels of many megawatts and average power levels of tens of kilowatts average are, with efficiency ratings in excess of 70 percent, possible with crossed-field amplifiers.

Because of the desirable characteristics of wide bandwidth, high efficiency, and the ability to handle large amounts of power, the cfa is used in many applications in microwave electronic systems. This high efficiency has made the cfa useful for space-telemetry applications, and the high power and stability have made it useful in high-energy, linear atomic accelerators. When used as the intermediate or final stage in high-power radar systems, all of the advantages of the cfa are used.

Since the cfa operates in a manner so similar to the magnetron, the detailed theory is not presented in this module. Detailed information of cfa operation is available in NAVSHIPS 0967-443-2230, Handling, Installation and Operation of Crossed-Field Amplifiers. As mentioned earlier, crossed-field amplifiers are commonly called Amplitrons. You should note, however, that Amplitron is a trademark of the Raytheon Manufacturing Company for the Raytheon line of crossed-field amplifiers. An illustration of a crossed-field amplifier is shown in figure 2-38.

Figure 2-38. - Crossed-field amplifier (Amplitron).

Q.44 Why is the pi mode the most commonly used magnetron mode of operation?
Q.45 What two methods are used to couple energy into and out of magnetrons?
Q.46 Magnetron tuning by altering the surface-to-volume ratio of the hole portion of a hole-and-slot cavity is what type of tuning?
Q.47 Capacitive tuning by inserting a ring into the cavity slot of a magnetron is accomplished by what type of tuning mechanism?