Radar System Engineering

364 THE MAGNETRON AND THE PULSER [SEC. 10.8
pulser, and the latter determines in some cases the variation in power
output with small incremental changes in applied voltage. For example,
a magnetron operating at 20 kv and 13.5 amp absorbs power at the rate
of 270 kw, yet a decrease in applied voltage of only 3 kv will drop the
current by 7 amp and virtually stop the oscillation. Since AV/A~ is
small, a satisfactory pulser must produce a fairly flat-topped pulse free
from voltage changes larger than 5 per cent of the peak voltage.
The V-1 characteristics of most magnetrons show irregularities which
are associated with various modes of oscillation whose number, magnitude,
and position vary with magnetron type, r-f load applied to the
magnetron, magnetic field, and
Operatingpoint
20 — --------------------
\
cathode condition.
Short-circuit and open-circuit
conditions must be allowed for if
- Mode change
the pulser is not to be damaged. A
magnetron, or any other load, may
15 -
develop an infinite impedance because
of mechanical breakage of
some part of the circuit, or through
3
~
%
g
9 10 -
~
E
%
~
5 -
Peak plate current in amperes
FIG. 1O.29.—V-Icharacteristicsof 4J77magnetron.
Field, 2700gauss;A = 11.140cm.
failure of the cathode to emit. This
is not serious in the case of the hardtube
pulser, but the pulse-forming
n e t w o r k variety generally discharges
its unconsumed energy in
some abnormal and undesirable
way. No pulsing system should be
considered which can be destroyed
by a single discharge of a high-power
pulse in some unexpected part of the
circuit. An average-current device
intended to turn off the primary
power supply if the load fails to draw power could hardly do so in less than
a few pulse cycles, and thus cannot be relied upon to protect against this.
A short-circuited load can prove disastrous to a hard-tube pulser in a
few pulse cycles unless provision is made to turn off the primary power
when the load draws too much current, since most of the energy which
the hard-tube pulser normally delivers to the load will then appear as
anode heating of the switch tube. Since it is hardly practicable to design
switching tubes heavy enough to withstand this sort of abuse for long,
overload protection must be provided. A relatively fast-acting averagecurrent
device is suitable for this purpose.
The electrostatic capacity of the load should nearly always be kept
to a minimum, Not only is the energy spent in charging this capacity