2-40
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.
