Radar System Engineering

354 THE MAGNETRON AND THE PULSER [SEC. 10.6
changing difficulties arise a systematic attack is possible.
conclusions of this study are given here.
The important
Mode changes are of two types. One type is caused by anode currents
so high that they exceed the conditions for oscillation and cause transition
into a state of nonoscillation or into another mode of oscillation. The
~r/-
second type arises from a failure of the oscillations in the desired mode to
build up rapidly enough with respect to the voltage rise at the start of the
pulse. The first type is encountered usually in lower-power c-w magnetrons,
rarely in pulsed tubes. The
Pulser load curve
second type, called “mode-skipping,”
is common to nearly all high-
,\
\ ‘\
power pulsed magnetrons and thus
I:,)(*’’”’”
is of importance here.
Mode-skipping is dependent not
only on the characteristics of the
V-1 curve for
II oscillations
magnetron but also on the characteristics
of the pulser. In practice,
I
,1 ‘ - (desired mode) 1
it has been necessary to consider the
magnetron and pulser as a unit.
‘bPulse current _
F1~. 10.25,—V-I plot of a magnetronillustrating
the problem of mode skipping.
V, = magnetronstarting voltage
V. = critical voltage above which oscillations
will not start
VO= pulservoltage with no load.
For given magnetic field and r-f
loading, there is only a limited range
of anode voltage over which the
magnetron will build up oscillations
in the, desired mode. On the other
hand, if high-power oscillations are
to be maintained, a plate voltage at
least as high as this starting voltage
This calls either for a very low-
(V.) must be maintained at large currents.
impedance pulser or for a high pulser voltage VO at zero load current.
Consider the case illustrated in’ Fig. 10.25. To achieve the final pulse
current 1, the pulser must be adjusted to a value of VO considerably higher
than the starting voltage Vs. Under proper conditions, when the pulse
is applied to the magnetron, the V-1 curve is that shown dotted in Fig.
10.25. The pulse voltage never rises above the critical value Vc, because,
as it reaches Vs, the magnetron draws current and loads the pulser. If
oscillation fails to start promptly after the pulse voltage has reached Vs,
the voltage continues to rise and may exceed Vc before current is drawn.
Then, since the conditions for oscillation no longer exist, the voltage rises
to Vo and oscillations in the desired mode cannot take place. This is
what occurs when a magnetron skips modes. The V-1 trace for such an
event is also shown in Fig. 10.25. The condition for oscillation in the
desired mode occurs when the magnetron starts to oscillate and to draw
current in the time interval taken for the pulse voltage to pass from Vs