Radar System Engineering

SEC. 105] MAGNETRON CHARACTERISTICS 347
Whatever the source of the emission, if too large a current is drawn for
too long a time, sparking and other instabilities result. The exact
relationship between peak cathode emission and pulse length depends
on the type, temperature, and age of the cathode. To a fair degree of
approximation, the maxi mum permissible peak emission varies inversely
as the square root of the pulse length: 1-= = l/~. Thus a cathode
which will emit 20 amp for a pulse duration of 2 psec will probably emit
40 amp during a 0.5-psec pulse. In consequence, greater energy per plllse
can safely be obtained for the longer pulses.
Pulse durations greater than 5 psec are rarely employed when magnetrons
are used as transmitter tubes. Frequency modulation during the
pulse becomes a serious problem for longer pulses, even if sparking
troubles are overcome. Pulse durations as short as 0.25 psec have been
used successfully, particularly with high-frequency magnetrons whose
starting times are short.
Tuning of Magnetrons.-To change the frequency of a magnetron
more than a few megacycles per second requires that a change be made
in the resonant circuits of the anode. Either the effective capacity or
effective inductance must be varied, and, since the resonant circuits are
within the evacuated portion of the tube, variation of either of them is a
troublesome problem.
For this reason early magnetrons were not tunable, and only a later
need for increased flexibility of radar systems forced the design of tunable
tubes. The practical advantage of tunable over fixed-tuned magnetrons
is obvious. If operation on a number of frequencies is contemplated, a
single tunable magnetron can replace a whole set of fixed-frequency
magnetrons, and only with a tunable magnetron is it possible in general
to obtain r-f power at a specified frequency. The performance characteristics
of tunable magnetrons m-e equivalent to those of the corresponding
fixed-frequency tubes, and there is thus no reason, except availability,
for not using them.
Tuning of the higher-frequency microwave magnetrons is accomplished
by inserting conducting cylinders into the’ inductive portion of
each resonant cavity, thus decreasing the effective inductance. This
construction, sho}vn in Pig. 1021, provides a tuning range as high as 12
per cent,
At frequencies lower than about 5000 Me/see, the magnitude of the
longitudinal displacement required in inductive tuning becomes inconvenient,
and other tuning methods are adopted. Figure 10.22 sho!vs a
“ C-ring” type of tunable magnetron, in which a conducting surface can
be moved toward or away from the straps and capacitive portion of the
resonant cavities, thus changing their effective capacity. The disad-