Download 346 THE MAGNETRON AND THE PULSER [SEC. 105 Pulse

346
THE
MAGNETRON
AND
THE
PULSER
105
[SEC.
Pulse
Voltage.—The input impedance'
of most pulsed magnetrons lies
bet ween 700 and 1200 ohms.
For example, magnetrons
with an input
of 260 kw (100-kw
output)
of 2500 kw (1000-kw
for 6000-kw
Serious
require a pulse voltage
output)
input
(2500-kw
attempts
have
of about
15 kv; inputs
require 30 kv, and one magnetron
output)
been
requires a pulse voltage
made
to
design
designed
of 50 kv.
magnetrons
that
operate at lower input impedances because such tubes would simplify
design of line-type pulsers.
These attempts have been unsuccessful,
350 ohms
was the lower
limit
in mid-1946.
The
with high impedances
is, however,
impedances
as high
as
10,000
ohms
have
shows
input
pulse
power,
pulse
voltage,
existing
6000
the
10-cm
kw.
magnetrons
Power
of 40 per
easily achieved,
with
outputs
input
can
be
with input
produced.
and
Table
input
powers
estimated
of magnetrons
and tubes
been
pulse
design
10.2
impedance
ranging
by
would
the
and
from
assuming
of
2 to
an
efficiency
cent.
TARJ.E10,2.—INFuT
CUAEACTERISITCSOF MICMW.*VE
Tube
I
No.
MAGNETROM
I
I
Input pulse
power, kw
I
I
1
Pulse voltage,
kv
Input
I
I
15
2
4J60
2J38
25
2J32
4J31
HP1OV
impedance,
ohms
1
1125
1000
250
5
15
2500
6000
30
50
360
900
415
By varying the magnetic field in which the magnetron operates, any
given design can be made to operate satisfactorily
over a range of pulse
voltage
of roughly
50 per cent.
For satisfactory
tube will exhibit about the same input impedance
can be obtained from performance
charts.
Pulse-1ength
early
Limitations.—The
operation,
however,
values.
experiments
the
Exact
of the British
figures
with
high-power
pulse techniques
revealed a characteristic
of oxide cathodes
which is responsible in large measure for the high pulse power of magnetrons.
It
was
emit as much
d-c emission.
More
found
that
under
as 20 amp/cm2
recently,
pulsed
conditions
as compared
to
currents as high as 100 amp/cm'
oxide
about
cathodes
0.2
can
amp/cm2
have been obtained.
This current is carried partly by primary electrons and partly by secondary electrons liberated from the "oxide cathode by back bombardment.
1The word impedance,
as used in this section, means the voltage-current
magnetron at the operating point.
The
dynamic impedance,
current curve near the operating point, is very
See Sec. 108.
for
the
ratio of a
slope of the voltage-
much lower, usually around 100ohms.
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-
THE
348
vantage
of
arrangement
An
power
MAGNETRON
this method
is that
at high pulse powers.
unsymmetrical
operation
tuning
cavity
cavity
is tightly
type
is shown
is changed
coupled
sparking
of
in
AND
tuning
Fig.
may
P ULSER
occur
which
10"23.
by distorting
THE
has
The
within
the
advantages
frequency
the diaphragm.
to one of the resonant
[SEC.
of
tuning
for
the
highsingle
Since the tuning
cavities,
and since all the
The changein frequency is accomplished
FtQ. 10.21.—Magnetronwith sprocket tuning.
by changingthe inductanceof the resonators.
resonant
cavities
the oscillating
metrical
type
magnetron
are very
frequency
of
Advantages
ability
to
power
distorts
limits
very
high
the
schemes
experience
electric
tuning
field
tuning
the straps,
This unsympatterns
range to
within
about
simplicity
Figure
powers.
10.24
the
6 per
and
its
shows
adjustment.
have been tried,
above
and still others will be
represent the basic methods.
with radar demonstrated
extreme
by
is altered.
are its mechanical
pulse
of tuning
other tuninz
but that
to one another
its effective
but the three t~pes described
Wartime
desirable
coupled
of the method
handle
as a function
Various
devised,
tuning
and therefore
cent.
closely
of the entire magnetron
range was not.
that tunability
was very
output
10.5