Origins of the Magnetron
 

Hi All,

Just a bit of RF history obtained from "Forty Years of Radio
Research," by George C. Southworth while I was doing some research for
a correspondent:

"As early as 1921, Dr. A.W. Hull of the General Electric Company
described the characteristics of a device which he called a
magnetron. It consisted of an axial filament surrounded by a
cylindrical plate between which a constant radial electric field
could be impressed. Superimposed on this arrangement and at right
angles to the electric field was an axial magnetic field.

... "Later the inductive load was reduced to a tiny Lecher frame.
A magnetron with this configuration was placed inside a small
waveguide by Dr. Harold S. Howe of the University of Michigan to
obtain a top frequency of nearly 50,000 mc. (1938).

"A few years earlier, 1934, Dr. Cleeton and Professor Williams
also of the University of Michigan had ... discovered a pronounced
absorption band for ammonia at about 27,300 mc. This ultimately
led to a new time-keeping device of high accuracy.

... "Finally the plate was made from a single block of copper with
appropriate segments milled inside its hollow periphery, each
representing an individual oscillator. This important step was
taken by physicists at the University of Birmingham in the late
thirties. They also applied plate power in short pulses and at
much higher levels than had been used previously. This was not
only more appropriate for good magnetron operation but it
provided an almost ideal signal for radar use. ... Details of this
device were brought to America first by Sir Robert Watson-Watt and
Drs. Cockcroft and Bowen in September, 1940."

73's
Richard Clark, KB7QHC

Origins of the Magnetron
 

I think most of this, and a whole lot more concerning where the
development of the cavity magnetron lead, is in Robert Buderi's book,
"The Invention That Changed the World." Potentially very interesting
stuff for any techie.

In the last paragraph you quoted, there is what I consider a mistake
that can lead to misunderstanding of how things work. It suggests that
each cavity is an oscillator. The cavities are no more oscillators
than a bottle. Each is a resonator, whose resonance is excited by the
stream of electrons flying past, in much the same way as the bottle is
a Helmholz resonator which is excited by the stream of air flowing
past. In each case, what's going on in the resonator affects the
stream flowing by in a way that lets it further excite the resonator
synchronously with the resonant energy that's already there.

Cheers,
Tom

Richard Clark wrote:
Hi All,

Just a bit of RF history obtained from "Forty Years of Radio
Research," by George C. Southworth while I was doing some research for
a correspondent:

"As early as 1921, Dr. A.W. Hull of the General Electric Company
described the characteristics of a device which he called a
magnetron. It consisted of an axial filament surrounded by a
cylindrical plate between which a constant radial electric field
could be impressed. Superimposed on this arrangement and at right
angles to the electric field was an axial magnetic field.

... "Later the inductive load was reduced to a tiny Lecher frame.
A magnetron with this configuration was placed inside a small
waveguide by Dr. Harold S. Howe of the University of Michigan to
obtain a top frequency of nearly 50,000 mc. (1938).

"A few years earlier, 1934, Dr. Cleeton and Professor Williams
also of the University of Michigan had ... discovered a pronounced
absorption band for ammonia at about 27,300 mc. This ultimately
led to a new time-keeping device of high accuracy.

... "Finally the plate was made from a single block of copper with
appropriate segments milled inside its hollow periphery, each
representing an individual oscillator. This important step was
taken by physicists at the University of Birmingham in the late
thirties. They also applied plate power in short pulses and at
much higher levels than had been used previously. This was not
only more appropriate for good magnetron operation but it
provided an almost ideal signal for radar use. ... Details of this
device were brought to America first by Sir Robert Watson-Watt and
Drs. Cockcroft and Bowen in September, 1940."

73's
Richard Clark, KB7QHC

Origins of the Magnetron
 

On 31 May 2006 15:31:11 -0700, "K7ITM" wrote:

In the last paragraph you quoted, there is what I consider a mistake
that can lead to misunderstanding of how things work. It suggests that
each cavity is an oscillator. The cavities are no more oscillators
than a bottle.

Hi Tom,

I defer to the author's explanation, and the nature of writing for a
wide audience. This group would be such an example.

The author mentions, in portions not quoted, that the electron
beam/current/what-have-you streams at a grazing angle along the arc of
the inside of the plate, crossing these openings. The cavities are,
then, parallel plate loads.

It stands to reason, on the other hand, that there is only one output
from ostensibly one cavity whose fields are sharing the passing stream
of electrons that is also resonating. In fact, this operation (also
described by the author) led to understanding and development of the
Klystron tube and other traveling wave amplifiers.

73's
Richard Clark, KB7QHC

Origins of the Magnetron
 

Richard,

Randall & Boot's original magnetron used to be displayed in the
London, England, Science Museum. It was all by itself in a very
large, securely locked glass case. No magnet. I don't know whether it
is still there.

It lay there, all forlorn, hardly noticed, about the same
insignificant size as a small, half-size, rusty can of baked beans.

My sentiments lie with R and B, slaving away in the laboratory at
Birmingham University while the Luftwaffer rained down bombs and
incendiaries on the city. At the time, the top-secret goings-on were
unknown to me, and I spent my time in a corrugated-iron air raid
shelter in the back garden just a few miles down the road.

A few years later, having joined the RAF as a Radar technician, I had
the pleasure of holding a production model in one hand and the magnet
in the other. At the other end of the workshop bench a parabolic dish
rotated once every two seconds. It is not true that a 50 kW peak pulse
power at 3000 Mhz sterilises one's reproductive organs. I have
fathered 5 children.

It was left to the Japanese to populate the World's kitchens with
microwave ovens. Beyond the first, no magnetron has ever been made in
the industrial city of Birmingham, England. But they don't make many
motor cars there any more either.

Origins of the Magnetron
 

Reg, G4FGQ wrote:
"It was left to the Japanese to populate the World`s kitchens with
microwave ovens."

The Japanese proved adept at improving and producing high quality
technical products. Japanese didn`t market the first microwave oven.
Raytheon introduced its "Radar Range" soon after WW-2 ended. Japanese
copies were innovative, reliable, and cheap, so they won instant
acceptance worldwide.

The magnetron has been called the best import ever from Britain and I
think that comparison even included Bob Hope and Liz Taylor. At the time
of the magnetron gift to the U.S., British war production was already
bursting at the seams and the U.S. was well advanced in radar and had a
few tricks up its sleeve to improve the British gear.
I`ve noticed early British airborne radar using yagi antennas. That
seemed quaint to me.

Best regards, Richard Harrison, KB5WZI

Origins of the Magnetron
 

Richard Harrison wrote:

At the time
of the magnetron gift to the U.S., British war production was already
bursting at the seams and the U.S. was well advanced in radar and had a
few tricks up its sleeve to improve the British gear.

All the major powers were well advanced in radar at the time. All the
ideas were already in place, and engineers everywhere were starting to
put them together. However, as Reg points out, freedom from bombing
raids is a wonderful aid to creativity.

The USA developed ways to mass-produce the magnetron, notably a method
of building up the cavity from laminations rather than needing to have a
skilled machinist mill it out from solid (and before Roy gets a word in,
they fixed the oil leaks too).

I`ve noticed early British airborne radar using yagi antennas. That
seemed quaint to me.

Those were the phased arrays for the earlier VHF radar, and consisted of
two or four two-element yagis clustered around the nose (of a
two-engined aircraft, obviously). This gave a fairly good
forward-looking capability. Both sides did much the same, and given the
relatively long wavelength, it's hard to think how better to do it.

The huge benefit of the magnetron was that it operated at much shorter
wavelengths, which frees up the antenna design and provides much better
spatial resolution - witness the downward-looking "H2S" radar which was
the magnetron's first major deployment.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Origins of the Magnetron
 

"Ian White GM3SEK" wrote in message
...


Those were the phased arrays for the earlier VHF radar, and consisted of
two or four two-element yagis clustered around the nose (of a
two-engined aircraft, obviously). This gave a fairly good
forward-looking capability. Both sides did much the same, and given the
relatively long wavelength, it's hard to think how better to do it.

The huge benefit of the magnetron was that it operated at much shorter
wavelengths, which frees up the antenna design and provides much better
spatial resolution - witness the downward-looking "H2S" radar which was
the magnetron's first major deployment.

The VHF radars were still around into the late 1970's, maybe beyond. The US
Navy had them on carriers for air search. I think the nomenclature was
AN/SPS-29 and/or AN/SPS-37. The one I recall was in the 218 - 220 MHz and
it was hell on TV channel 13! The antenna was referred to as a bedspring
array; the rectangular framework for the dipole radiating elements resembled
a giant bedspring.

Origins of the Magnetron
 

Sal M. Onella wrote:

"Ian White GM3SEK" wrote in message
...


Those were the phased arrays for the earlier VHF radar, and consisted of
two or four two-element yagis clustered around the nose (of a
two-engined aircraft, obviously). This gave a fairly good
forward-looking capability. Both sides did much the same, and given the
relatively long wavelength, it's hard to think how better to do it.

The huge benefit of the magnetron was that it operated at much shorter
wavelengths, which frees up the antenna design and provides much better
spatial resolution - witness the downward-looking "H2S" radar which was
the magnetron's first major deployment.

The VHF radars were still around into the late 1970's, maybe beyond. The US
Navy had them on carriers for air search. I think the nomenclature was
AN/SPS-29 and/or AN/SPS-37. The one I recall was in the 218 - 220 MHz and
it was hell on TV channel 13! The antenna was referred to as a bedspring
array; the rectangular framework for the dipole radiating elements resembled
a giant bedspring.


Sure, but none of those would fly very well. The discussion was really
about airborne radar, where there are tough limits on antenna size.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Origins of the Magnetron
 

Back in the early 70's I got a pair of magetron magnets that came out
of old airborne radio set - can't remember the nomenclature - that the
USAF converted into a weather radio. They were quite large and strong,
and 'U' shaped. Someone swiped them in a move. Anybody know where I
might find a set? Google and eBay haven't turned up anything.

Origins of the Magnetron
 

Richard Clark wrote:

Hi All,

Just a bit of RF history obtained from "Forty Years of Radio
Research," by George C. Southworth while I was doing some research for
a correspondent:

"As early as 1921, Dr. A.W. Hull of the General Electric Company
described the characteristics of a device which he called a
magnetron. It consisted of an axial filament surrounded by a
cylindrical plate between which a constant radial electric field
could be impressed. Superimposed on this arrangement and at right
angles to the electric field was an axial magnetic field.

This sounds like a Faraday disk motor where
the copper or brass disk was replaced by an
electron stream in a vacuum:
http://w1tp.com/s_motr.jpg



... "Later the inductive load was reduced to a tiny Lecher frame.
A magnetron with this configuration was placed inside a small
waveguide by Dr. Harold S. Howe of the University of Michigan to
obtain a top frequency of nearly 50,000 mc. (1938).

"A few years earlier, 1934, Dr. Cleeton and Professor Williams
also of the University of Michigan had ... discovered a pronounced
absorption band for ammonia at about 27,300 mc. This ultimately
led to a new time-keeping device of high accuracy.

... "Finally the plate was made from a single block of copper with
appropriate segments milled inside its hollow periphery, each
representing an individual oscillator. This important step was
taken by physicists at the University of Birmingham in the late
thirties. They also applied plate power in short pulses and at
much higher levels than had been used previously. This was not
only more appropriate for good magnetron operation but it
provided an almost ideal signal for radar use. ... Details of this
device were brought to America first by Sir Robert Watson-Watt and
Drs. Cockcroft and Bowen in September, 1940."

73's
Richard Clark, KB7QHC