<s>
The	O
cavity	O
magnetron	O
is	O
a	O
high-power	O
vacuum	B-General_Concept
tube	O
used	O
in	O
early	O
radar	B-Application
systems	I-Application
and	O
currently	O
in	O
microwave	O
ovens	O
and	O
linear	O
particle	O
accelerators	O
.	O
</s>
<s>
Unlike	O
other	O
vacuum	B-General_Concept
tubes	O
,	O
such	O
as	O
a	O
klystron	O
or	O
a	O
traveling-wave	O
tube	O
(	O
TWT	O
)	O
,	O
the	O
magnetron	O
cannot	O
function	O
as	O
an	O
amplifier	O
for	O
increasing	O
the	O
intensity	O
of	O
an	O
applied	O
microwave	O
signal	O
;	O
the	O
magnetron	O
serves	O
solely	O
as	O
an	O
oscillator	O
,	O
generating	O
a	O
microwave	O
signal	O
from	O
direct	O
current	O
electricity	O
supplied	O
to	O
the	O
vacuum	B-General_Concept
tube	O
.	O
</s>
<s>
The	O
use	O
of	O
magnetic	O
fields	O
as	O
a	O
means	O
to	O
control	O
the	O
flow	O
of	O
an	O
electrical	O
current	O
was	O
spurred	O
by	O
the	O
invention	O
of	O
the	O
Audion	B-Application
by	O
Lee	O
de	O
Forest	O
in	O
1906	O
.	O
</s>
<s>
The	O
high	O
power	O
pulses	O
were	O
generated	O
from	O
a	O
device	O
the	O
size	O
of	O
a	O
small	O
book	O
and	O
transmitted	O
from	O
an	O
antenna	O
only	O
centimeters	O
long	O
,	O
reducing	O
the	O
size	O
of	O
practical	O
radar	B-Application
systems	I-Application
by	O
orders	O
of	O
magnitude	O
.	O
</s>
<s>
New	O
radars	B-Application
appeared	O
for	O
night-fighters	O
,	O
anti-submarine	O
aircraft	O
and	O
even	O
the	O
smallest	O
escort	O
ships	O
,	O
and	O
from	O
that	O
point	O
on	O
the	O
Allies	O
of	O
World	O
War	O
II	O
held	O
a	O
lead	O
in	O
radar	B-Application
that	O
their	O
counterparts	O
in	O
Germany	O
and	O
Japan	O
were	O
never	O
able	O
to	O
close	O
.	O
</s>
<s>
By	O
the	O
end	O
of	O
the	O
war	O
,	O
practically	O
every	O
Allied	O
radar	B-Application
was	O
based	O
on	O
a	O
magnetron	O
.	O
</s>
<s>
The	O
magnetron	O
continued	O
to	O
be	O
used	O
in	O
radar	B-Application
in	O
the	O
post-war	O
period	O
but	O
fell	O
from	O
favour	O
in	O
the	O
1960s	O
as	O
high-power	O
klystrons	O
and	O
traveling-wave	O
tubes	O
emerged	O
.	O
</s>
<s>
This	O
renders	O
it	O
less	O
suitable	O
for	O
pulse-to-pulse	O
comparisons	O
for	O
performing	O
moving	O
target	O
indication	O
and	O
removing	O
"	O
clutter	O
"	O
from	O
the	O
radar	B-Application
display	O
.	O
</s>
<s>
The	O
magnetron	O
remains	O
in	O
use	O
in	O
some	O
radar	B-Application
systems	I-Application
,	O
but	O
has	O
become	O
much	O
more	O
common	O
as	O
a	O
low-cost	O
source	O
for	O
microwave	O
ovens	O
.	O
</s>
<s>
In	O
a	O
conventional	O
electron	O
tube	O
(	O
vacuum	B-General_Concept
tube	O
)	O
,	O
electrons	O
are	O
emitted	O
from	O
a	O
negatively	O
charged	O
,	O
heated	O
component	O
called	O
the	O
cathode	O
and	O
are	O
attracted	O
to	O
a	O
positively	O
charged	O
component	O
called	O
the	O
anode	O
.	O
</s>
<s>
The	O
components	O
are	O
normally	O
arranged	O
concentrically	O
,	O
placed	O
within	O
a	O
tubular-shaped	O
container	O
from	O
which	O
all	O
air	O
has	O
been	O
evacuated	B-General_Concept
,	O
so	O
that	O
the	O
electrons	O
can	O
move	O
freely	O
(	O
hence	O
the	O
name	O
"	O
vacuum	B-General_Concept
"	O
tubes	O
,	O
called	O
"	O
valves	O
"	O
in	O
British	O
English	O
)	O
.	O
</s>
<s>
Early	O
conventional	O
tube	O
systems	O
were	O
limited	O
to	O
the	O
high	B-Algorithm
frequency	I-Algorithm
bands	O
,	O
and	O
although	O
very	B-Algorithm
high	I-Algorithm
frequency	I-Algorithm
systems	O
became	O
widely	O
available	O
in	O
the	O
late	O
1930s	O
,	O
the	O
ultra	O
high	B-Algorithm
frequency	I-Algorithm
and	O
microwave	O
bands	O
were	O
well	O
beyond	O
the	O
ability	O
of	O
conventional	O
circuits	O
.	O
</s>
<s>
thumb|upright	O
=	O
1.3	O
|	O
Split-anode	B-Algorithm
magnetron	I-Algorithm
(	O
c	O
.	O
1935	O
)	O
.	O
</s>
<s>
The	O
first	O
major	O
improvement	O
was	O
the	O
split-anode	B-Algorithm
magnetron	I-Algorithm
,	O
also	O
known	O
as	O
a	O
negative-resistance	O
magnetron	O
.	O
</s>
<s>
Frequency	O
also	O
drifts	O
from	O
pulse	O
to	O
pulse	O
,	O
a	O
more	O
difficult	O
problem	O
for	O
a	O
wider	O
array	O
of	O
radar	B-Application
systems	I-Application
.	O
</s>
<s>
Neither	O
of	O
these	O
present	O
a	O
problem	O
for	O
continuous-wave	B-Algorithm
radars	I-Algorithm
,	O
nor	O
for	O
microwave	O
ovens	O
.	O
</s>
<s>
The	O
cathode	O
is	O
placed	O
in	O
the	O
center	O
of	O
an	O
evacuated	B-General_Concept
,	O
lobed	O
,	O
circular	O
metal	O
chamber	O
.	O
</s>
<s>
As	O
electrons	O
sweep	O
past	O
these	O
slots	O
,	O
they	O
induce	O
a	O
high-frequency	B-Algorithm
radio	O
field	O
in	O
each	O
resonant	O
cavity	O
,	O
which	O
in	O
turn	O
causes	O
the	O
electrons	O
to	O
bunch	O
into	O
groups	O
.	O
</s>
<s>
The	O
waveguide	O
directs	O
the	O
extracted	O
RF	O
energy	O
to	O
the	O
load	O
,	O
which	O
may	O
be	O
a	O
cooking	O
chamber	O
in	O
a	O
microwave	O
oven	O
or	O
a	O
high-gain	O
antenna	O
in	O
the	O
case	O
of	O
radar	B-Application
.	O
</s>
<s>
This	O
is	O
not	O
a	O
problem	O
in	O
uses	O
such	O
as	O
heating	O
,	O
or	O
in	O
some	O
forms	O
of	O
radar	B-Application
where	O
the	O
receiver	O
can	O
be	O
synchronized	O
with	O
an	O
imprecise	O
magnetron	O
frequency	O
.	O
</s>
<s>
Large	O
S	B-Algorithm
band	I-Algorithm
magnetrons	O
can	O
produce	O
up	O
to	O
2.5	O
megawatts	O
peak	O
power	O
with	O
an	O
average	O
power	O
of	O
3.75	O
kW	O
.	O
</s>
<s>
thumb|upright	O
=	O
1.4	O
|	O
9.375	O
GHz	O
20	O
kW	O
(	O
peak	O
)	O
magnetron	O
assembly	O
for	O
an	O
early	O
commercial	O
airport	B-Application
radar	I-Application
in	O
1947	O
.	O
</s>
<s>
In	O
a	O
radar	B-Application
set	O
,	O
the	O
magnetron	O
's	O
waveguide	O
is	O
connected	O
to	O
an	O
antenna	O
.	O
</s>
<s>
As	O
in	O
all	O
primary	O
radar	B-Application
systems	I-Application
,	O
the	O
radiation	O
reflected	O
from	O
a	O
target	O
is	O
analyzed	O
to	O
produce	O
a	O
radar	B-Application
map	O
on	O
a	O
screen	O
.	O
</s>
<s>
Several	O
characteristics	O
of	O
the	O
magnetron	O
's	O
output	O
make	O
radar	B-Application
use	O
of	O
the	O
device	O
somewhat	O
problematic	O
.	O
</s>
<s>
This	O
wide	O
bandwidth	O
allows	O
ambient	O
electrical	O
noise	O
to	O
be	O
accepted	O
into	O
the	O
receiver	O
,	O
thus	O
obscuring	O
somewhat	O
the	O
weak	O
radar	B-Application
echoes	O
,	O
thereby	O
reducing	O
overall	O
receiver	O
signal-to-noise	O
ratio	O
and	O
thus	O
performance	O
.	O
</s>
<s>
In	O
some	O
applications	O
,	O
for	O
example	O
,	O
a	O
marine	B-Algorithm
radar	I-Algorithm
mounted	O
on	O
a	O
recreational	O
vessel	O
,	O
a	O
radar	B-Application
with	O
a	O
magnetron	O
output	O
of	O
2	O
to	O
4	O
kilowatts	O
is	O
often	O
found	O
mounted	O
very	O
near	O
an	O
area	O
occupied	O
by	O
crew	O
or	O
passengers	O
.	O
</s>
<s>
In	O
practical	O
use	O
these	O
factors	O
have	O
been	O
overcome	O
,	O
or	O
merely	O
accepted	O
,	O
and	O
there	O
are	O
today	O
thousands	O
of	O
magnetron	O
aviation	O
and	O
marine	B-Algorithm
radar	I-Algorithm
units	O
in	O
service	O
.	O
</s>
<s>
Recent	O
advances	O
in	O
aviation	O
weather-avoidance	O
radar	B-Application
and	O
in	O
marine	B-Algorithm
radar	I-Algorithm
have	O
successfully	O
replaced	O
the	O
magnetron	O
with	O
microwave	B-Algorithm
semiconductor	I-Algorithm
oscillators	I-Algorithm
,	O
which	O
have	O
a	O
narrower	O
output	O
frequency	O
range	O
.	O
</s>
<s>
More	O
modern	O
variants	O
use	O
HEMTs	B-Algorithm
or	O
GaN-on-SiC	O
power	O
semiconductor	O
devices	O
to	O
generate	O
the	O
microwaves	O
,	O
which	O
are	O
substantially	O
less	O
complex	O
and	O
can	O
be	O
adjusted	O
to	O
maximize	O
light	O
output	O
using	O
a	O
PID	O
controller	O
.	O
</s>
<s>
The	O
attempt	O
to	O
measure	O
the	O
electron	O
mass	O
failed	O
because	O
he	O
was	O
unable	O
to	O
achieve	O
a	O
good	O
vacuum	B-General_Concept
in	O
the	O
tube	O
.	O
</s>
<s>
Hull	O
's	O
magnetron	O
was	O
not	O
originally	O
intended	O
to	O
generate	O
VHF	B-Algorithm
(	O
very-high-frequency	B-Algorithm
)	O
electromagnetic	O
waves	O
.	O
</s>
<s>
Most	O
of	O
these	O
early	O
magnetrons	O
were	O
glass	O
vacuum	B-General_Concept
tubes	O
with	O
multiple	O
anodes	O
.	O
</s>
<s>
However	O
,	O
the	O
two-pole	O
magnetron	O
,	O
also	O
known	O
as	O
a	O
split-anode	B-Algorithm
magnetron	I-Algorithm
,	O
had	O
relatively	O
low	O
efficiency	O
.	O
</s>
<s>
While	O
radar	B-Application
was	O
being	O
developed	O
during	O
World	O
War	O
II	O
,	O
there	O
arose	O
an	O
urgent	O
need	O
for	O
a	O
high-power	O
microwave	O
generator	O
that	O
worked	O
at	O
shorter	O
wavelengths	O
,	O
around	O
10cm	O
(	O
3GHz	O
)	O
,	O
rather	O
than	O
the	O
50	O
to	O
150cm	O
(	O
200MHz	O
)	O
that	O
was	O
available	O
from	O
tube-based	O
generators	O
of	O
the	O
time	O
.	O
</s>
<s>
However	O
,	O
the	O
German	O
military	O
considered	O
the	O
frequency	O
drift	O
of	O
Hollman	O
's	O
device	O
to	O
be	O
undesirable	O
,	O
and	O
based	O
their	O
radar	B-Application
systems	I-Application
on	O
the	O
klystron	O
instead	O
.	O
</s>
<s>
This	O
was	O
one	O
reason	O
that	O
German	O
night	O
fighter	O
radars	B-Application
,	O
which	O
never	O
strayed	O
beyond	O
the	O
low-UHF	O
band	O
to	O
start	O
with	O
for	O
front-line	O
aircraft	O
,	O
were	O
not	O
a	O
match	O
for	O
their	O
British	O
counterparts	O
.	O
</s>
<s>
Likewise	O
,	O
in	O
the	O
UK	O
,	O
Albert	O
Beaumont	O
Wood	O
proposed	O
in	O
1937	O
a	O
system	O
with	O
"	O
six	O
or	O
eight	O
small	O
holes	O
"	O
drilled	O
in	O
a	O
metal	O
block	O
,	O
differing	O
from	O
the	O
later	O
production	O
designs	O
only	O
in	O
the	O
aspects	O
of	O
vacuum	B-General_Concept
sealing	O
.	O
</s>
<s>
According	O
to	O
Andy	O
Manning	O
from	O
the	O
RAF	O
Air	O
Defence	O
Radar	B-Application
Museum	O
,	O
Randall	O
and	O
Boot	O
's	O
discovery	O
was	O
"	O
a	O
massive	O
,	O
massive	O
breakthrough	O
"	O
and	O
"	O
deemed	O
by	O
many	O
,	O
even	O
now	O
,	O
to	O
be	O
the	O
most	O
important	O
invention	O
that	O
came	O
out	O
of	O
the	O
Second	O
World	O
War	O
"	O
,	O
while	O
professor	O
of	O
military	O
history	O
at	O
the	O
University	O
of	O
Victoria	O
in	O
British	O
Columbia	O
,	O
David	O
Zimmerman	O
,	O
states	O
:	O
</s>
<s>
As	O
the	O
discussion	O
turned	O
to	O
radar	B-Application
,	O
the	O
US	O
Navy	O
representatives	O
began	O
to	O
detail	O
the	O
problems	O
with	O
their	O
short-wavelength	O
systems	O
,	O
complaining	O
that	O
their	O
klystrons	O
could	O
only	O
produce	O
10W	O
.	O
</s>
<s>
Bell	O
Telephone	O
Laboratories	O
took	O
the	O
example	O
and	O
quickly	O
began	O
making	O
copies	O
,	O
and	O
before	O
the	O
end	O
of	O
1940	O
,	O
the	O
Radiation	O
Laboratory	O
had	O
been	O
set	O
up	O
on	O
the	O
campus	O
of	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
to	O
develop	O
various	O
types	O
of	O
radar	B-Application
using	O
the	O
magnetron	O
.	O
</s>
<s>
By	O
early	O
1941	O
,	O
portable	O
centimetric	O
airborne	O
radars	B-Application
were	O
being	O
tested	O
in	O
American	O
and	O
British	O
aircraft	O
.	O
</s>
<s>
In	O
late	O
1941	O
,	O
the	O
Telecommunications	O
Research	O
Establishment	O
in	O
the	O
United	O
Kingdom	O
used	O
the	O
magnetron	O
to	O
develop	O
a	O
revolutionary	O
airborne	O
,	O
ground-mapping	O
radar	B-Application
codenamed	O
H2S	O
.	O
</s>
<s>
The	O
H2S	O
radar	B-Application
was	O
in	O
part	O
developed	O
by	O
Alan	O
Blumlein	O
and	O
Bernard	O
Lovell	O
.	O
</s>
<s>
The	O
cavity	O
magnetron	O
was	O
widely	O
used	O
during	O
World	O
War	O
II	O
in	O
microwave	B-Application
radar	I-Application
equipment	O
and	O
is	O
often	O
credited	O
with	O
giving	O
Allied	O
radar	B-Application
a	O
considerable	O
performance	O
advantage	O
over	O
German	O
and	O
Japanese	O
radars	B-Application
,	O
thus	O
directly	O
influencing	O
the	O
outcome	O
of	O
the	O
war	O
.	O
</s>
<s>
Centimetric	B-Application
radar	I-Application
,	O
made	O
possible	O
by	O
the	O
cavity	O
magnetron	O
,	O
allowed	O
for	O
the	O
detection	O
of	O
much	O
smaller	O
objects	O
and	O
the	O
use	O
of	O
much	O
smaller	O
antennas	O
.	O
</s>
<s>
The	O
combination	O
of	O
small-cavity	O
magnetrons	O
,	O
small	O
antennas	O
,	O
and	O
high	O
resolution	O
allowed	O
small	O
,	O
high	O
quality	O
radars	B-Application
to	O
be	O
installed	O
in	O
aircraft	O
.	O
</s>
<s>
Centimetric	O
contour	O
mapping	O
radars	B-Application
like	O
H2S	O
improved	O
the	O
accuracy	O
of	O
Allied	O
bombers	O
used	O
in	O
the	O
strategic	O
bombing	O
campaign	O
,	O
despite	O
the	O
existence	O
of	O
the	O
German	O
FuG	O
350	O
Naxos	O
device	O
to	O
specifically	O
detect	O
it	O
.	O
</s>
<s>
Centimetric	O
gun-laying	O
radars	B-Application
were	O
likewise	O
far	O
more	O
accurate	O
than	O
the	O
older	O
technology	O
.	O
</s>
<s>
Since	O
then	O
,	O
many	O
millions	O
of	O
cavity	O
magnetrons	O
have	O
been	O
manufactured	O
;	O
while	O
some	O
have	O
been	O
for	O
radar	B-Application
the	O
vast	O
majority	O
have	O
been	O
for	O
microwave	O
ovens	O
.	O
</s>
<s>
The	O
use	O
in	O
radar	B-Application
itself	O
has	O
dwindled	O
to	O
some	O
extent	O
,	O
as	O
more	O
accurate	O
signals	O
have	O
generally	O
been	O
needed	O
and	O
developers	O
have	O
moved	O
to	O
klystron	O
and	O
traveling-wave	O
tube	O
systems	O
for	O
these	O
needs	O
.	O
</s>
<s>
All	O
magnetrons	O
contain	O
a	O
small	O
amount	O
of	O
thorium	O
mixed	O
with	O
tungsten	B-Application
in	O
their	O
filament	O
.	O
</s>
