340
THE
MAGNETRON
AND
THE
PULSER
diagram,
which corresponds
to a matched load, represents
compromise
between efficiency and frequency
stability.
It is possible to adjust the loading
on the magnetron
value by the suitable use of r-f transformers
As an example,
sented
by
stability
suppose
Fig.
it is desirable
10.17
at
corresponding
ducing
a transformer
which
tance from the magnetron
to point A.
one-quarter
a
In
the
The
eflect
as its center is moved
load
above
points
variations
to employ
too
possible
frequency
of
the
began only
despite
in the
elements
of
Affecting
microwave
requirements.
in
load
diagram
is to
estimate
and
a frequency
receiver
even
when
the maximum
of mag-
to magnetron
many
troubles
attributed
the information
pro-
of
Over-all
Systems
magnetrons
is
their
circumstance
has
This
of an extremely
early
and
new
and production
which
the radar
as to produce
large number
microwave
continuing
conventional
demands
on the
types
are largely responsible
of
at
tubes,
limited
forced
types.
this
country
types
of mag-
standardization.
exception,
magnetron
of a new type.
in
100 distinct
attempts
without
Design.
of tube
magnetrons
1940, there now exist over
made
case of
the
Rieke
any other single factor
Characteristics
different
in late
development
the
which does not produce
by
1.5
in load
general,
from the r-f circuits,
it is also true that
development
system
on
to be expected
During the past war it was true, almost
radar
variations
In
in design
a failure to use properly
and production
Although
path
A safe policy
more than
shortcomings
to
the design
netrons,
irregular
=
of the diagram.
diagram.
Magnetron
the
be
to
shift.
result from
adaptability
corresponding
of the effects of the r-f load on the performance
by a Rieke
10.5.
One
its dis-
corresponds
loads
cases.
is of such a character
As a corollary,
to magnetrons
vided
a very
simple
be accommodated
has contributed
reliability.
different
with their associated
in VSWR
in VSWR
Appreciation
netrons
to
large to
and making
from the center
especially
a loading of the magnetron
this variation
VSWR
frequency
by intro-
A and B represent the VSWR
distances
and B,
are
variation
low
away from the center of the diagram.
is unpredictable.
the maximum
change
A
correspond
whose behavior
and
can be accomplished
of
points
different
example,
efficiency
repre-
diagram,
it should
be realized that these
that reduce the size of a circle of con-
In Fig. 10.17 the circles about
the
high
This
line.
the magnetron
this transformer along the line in either direction
operation
corresponding
to
point
B
can
comparing
circle when displaced
operate
sets up a 2-to-1
various points on the Rieke
points represent transformations
stant VSWR
to any reasonable
such that the phase of this VSWR
By moving
wavelength,
obtained.
of
A.
a reasonable
(Sec. 11.1) in the output
to
point
to point
[SEC. 10.5
that each new
and
required
the
This has not been necessary
since
for over-all
the
associated
performance
circuit
lie exter-
MAGNETRON
10.5]
SEC.
nal to the tube and are accessible
341
CHARACTERISTICS
In microwave
to change.
the circuit elements are an integral part of the tube
porated with great care into every new design.
and must
magnetrons,
be incor-
Thus any radar system designed to meet a new set of conditions
operate on a new frequency
netron
type,
existing
or
types.
least
Since
are so closely
netron,
at
will require the development
a critical
the
related to,
a general
evaluation
over-all
and restricted
knowledge
of
the
characteristics
by,
characteristics
of
the
radar
the performance
of the important
or to
of a new magof
system
of the mag-
characteristics
of magne-
trons is essential.
Listed
below
magnetrons
teristics
of
with
a
discussion
particular
importance
usually
independent
are not
of
to
each
are
system
the
characteristics
design.
of one another
These
and
their
of
characrelation-
ships are also considered.
Wavelen@h
magnetron
S'caling.-Since
different wavelengths
magnetrons
by
wavelength
scaling
the
wavelength
from
All essential
factor
a =
the
radiation
from
range, operation
a
on
To a first approximation,
are derived
process.
scaling
of
over a limited
requires different tubes.
of different
simple over-all
altered
the
is fixed or at best variable
where
A/XO,
one another
dimensions
by
a
of the tube
A is the
are
new wavelength
desired and XOis the wavelength associated with the original dimensions.
If thk is done, the new tube at wavelength ~ will operate at the original
voltage
and current,
operating
and thus
and at a magnetic
field H
= ~ Ho,
where Ho is the
magnetic
field of the original magnetron.
The power
the power output,
increases with increasing
wavelength.
rough
rule
varies
as the square
is:
The
with wavelength
pulse
power
output
of their wavelength.
(or
input)
of
input,
A
sculed
magnetrons
The change in the size of the tube
is the basis of this rule.
The pulse power input
is often
limited by cathode emission and, since the cathode
area is proportional
to hz, pulse power input is also proportional
to ~?
Similar reasoning
shows that
if the pulse power
tvithin the
tube,
the
same
output
is limited
variation
by r-f voltage
of power
with
breakdown
wavelength
is to
be
expect ed.
This
rule is an important
At any
have
given
been
time
given
to
it may
one from
not
obtaining
high
length and a better design evolved
the
validity
of
within limits,
Pulse
wave
the
rule
Power.-The
most
is
peak
their
of system
because
special
powers
at
as a result.
is reestablished,
be used to advantage
magnetrons
the standpoint
be exact,
design.
emphasis
may
a particular
wave-
In the long run, however,
because
any
new
design
can,
at other wavelengths.
outstanding
characteristic
extremely
high
of pulsed
pulse
micro-
power
output,
made
pchsible
pulsed,
by
the
very
large
emission
yielded
by
oxide
cathodes
when
105
[SEC.
THE
AND
THE
MAGNETRON
PULSER
and the high efficiency
of magnetrons
even at very short wavelengths.
Thepulse
powers available extend over a range of 105 (0.02 kwto2000
kw)
342
Frequency in Me/see
FIG. 10,18.—Diagramshowing power and frequency distributionof representative microwave magnetronsdeveloped up to 1945.
The
demands
of microwave
radar
resulted
in a rather
extensive
development
of magnetrons whose frequencies are concentrated
more or
less into two bands.
Figure 10.18 shows on a logarithmic
chart the frequency
and
pulse
power
of
magnetrons
that
have
been
prodllred
in