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-
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