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Early microwave magnetrons
The trajectory of electrons, moving under the effect of an electrostatic field, is affected by
superimposed magnetic fields. The combined effect of electric and magnetic field on electrons had
been exploited in a variety of unconventional vacuum tubes, commonly referred to as ‘magnetrons’ by
their designers.
Tyne gives several examples of early experiments on tubes where magnetic fields generated by
solenoids control electron flow. De Forest and Von Lieben patented some kinds of such electron
devices and Moorhead sold the A-P solenoid tube for a while.
The first description of the static characteristics of a device called magnetron was described by A. W.
Hull in 1921. The tube had a linear filament in a coaxial cylindrical plate. The plate cylinder was
surrounded by a winding, used to generate a magnetic field. The radial electric field and the
superimposed magnetic field forced electrons to follow circular orbits before reaching anode. If the
magnetic field was raised, the radius of the orbits became smaller and smaller until electrons could not
reach the plate, so causing a cut-off condition in the anode current. Hull also noted that just on the
border between conduction and cut-off, adding a resonant tank circuit, the tube could sustain self-
oscillations.
Split anode magnetron designs had been approached since 1924, in order to reach more stable
oscillations at higher frequencies. In this case, the LC resonant circuit was placed between the two
anodes. From 1934 to 1935 K. Posthumus at Philips developed a four segment magnetron; he also left a
theoretic treatment of the rotating electron clouds, which gave the relations between the tube geometry,
as the number of anodes, and the intensities of electrical and magnetic fields. In 1936, Cleeton and
Williams reached the upper frequency of 47 GHz with a split anode structure. Multi-cavity devices,
forerunner of the high power magnetrons in use since WWII, had been proposed by Samuel of Bell
Telephone in 1934. Multi-cavity magnetrons were subsequently developed in Russia and in England.
The first high power microwave multicavity magnetron, suitable for radar applications, was assembled
by J.T. Randall and H.A.H. Boot at Birmingham University. The prototype, sealed by wax, operated
while continuously connected to the vacuum pump on 21st February 1940. RF bursts of about 500W
were generated at about 3 GHz. GEC, which had been asked for its industrialization, assembled the
E1188 prototype, with the typical glass spacers for the filament and for the RF output and the thin
copper end seals. Shortly later the design was modified, using an oxide coated, indirectly heated
cathode, according to the draft E1189. On 17th July a prototype of E1189 gave 12KW peak output
pulses at 9.5 cm wavelength. The early prototypes all had six cavities, the anode block being machined
using a Colt revolver drum as drilling template. In August 1940 the design of E1189 was modified with
8 resonating cavities all around a larger cathode cylinder, giving the same output power with a small 6-
lbs. permanent magnet.
Mechanical drafts and a picture of the E1189 are given in the following figure.
Fig. 1
- Left, original drafts of the E1189, the first S-band power magnetron produced by GEC. Right, a picture of the
E1189 brought to North America by the Tizard mission.
One of the eight cavities E1189, the sample No. 12, was brought to the North America by the Tizard
mission. Its detailed design was transferred to Western Electric, to Raytheon and to the Radiation Lab
at MIT, in U.S., and to R.E.L. in Canada. REL was the radar-manufacturing arm of the Canadian
National Research Council. The sample was X-rayed and eventually left in Canada. In U.S. the
Radiation Laboratory at MIT controlled the development of new radar equipment and components,
including magnetrons. The development also continued in England, pushed by military. At the very
beginning, magnetrons operated well beyond the limits of conventional electron tubes, as microwave
high-power pulse generators, but evidenced many troubles. Their operation was quite critical and
frequency spectrum generated was influenced by many factors, resulting in unwanted modes of
operation. The glass spacers and the thin copper sealing plates were delicate: many magnetrons were
damaged by improper handling when attempting to replace them in the field.
The strapping technique, connecting the cavities according to some schemes, prevented modes of
oscillation different from the pi fundamental one. Strapped magnetrons were soon available, capable of
generating clean pulses of several tens to hundreds kilowatts. The close co-operation among
manufacturers, research laboratories and users of the three countries, Great Britain, U.S. and Canada,
resulted in a very fast improvement of the early magnetron types, with everyday introduction of high
performance devices for every application and, virtually, for every new radar equipment.
In the next pages some of British and U.S. early S-band pulsed magnetrons are shown. Their look
closely recalls the E1189 prototype. U.S. magnetrons evolved with the addition of mounting brackets
and of a rugged glass boot over the filament connections, to prevent damages caused by in-field
improper handling.
Early British S-band magnetron tubes
Fig. 2 –
CV64 was the first echelon strapped magnetron, 40KW-pulse power at 3300MHz. CV186 was very similar, 35KW
at 3320MHz, for use in Lancaster bombers turret gun laying radar system. CV1479 delivers 450KW pulses at 3045MHz.
US early S-band magnetrons
Fig. 3 -
Some of the early U.S. S-band magnetrons. Western Electric 706A to 706C were fixed frequency cavity magnetrons
inspired to the E1189 brought by the Tizard mission; shortly later were replaced by the  706AY to 706GY types, capable of
200KW output pulses. WE 714AY gave 125KW pulses. Bottom, the Raytheon 2J27, capable of 265KW output pulses;
although retaining the same basic structure of other magnetrons, its design included several improvements, such as the glass
boot to protect the filament seals, the thick mounting flange and the reinforced glass spacer in the coaxial out conductor.
The X-band evolution
Once the radar sets based upon S-band magnetrons had become operative, British and U.S. researches
moved to the next border: the development of multicavity magnetron capable of operation in the X-
band region. A better resolution was expected, the radar sets being compact enough to be easily
installed even in small combat airplanes, as night fighters.
The British approach approximately retained the overall dimensions of S-band magnetrons, while
increasing the number of cavities to operate with acceptable magnetic flux densities. A large number of
cavities however resulted in a random operation, due to the increasing difficulty to avoid unwanted
modes of oscillation. At the end, a 12 slot anode was selected as best compromise between
performances and production easiness, being possible to accurately milling the slots with available gear
cutting machines. The magnetron was approved as CV108. It looked very similar to the S-band CV64,
with the exception of the output probe: a glass surrounded antenna, which had to be inserted into a
waveguide piece.
In U.S., Western Electric followed a different design approach, scaling-down the dimensions of S-band
magnetrons. U.S. advanced manufacturing processes made possible to form precise anode blocks in a
single operation, starting from a cylinder of oxygen-free copper. Western Electric developed its 725A,
a double ring strapped X-.band magnetron, which soon became the most popular magnetron ever made
and the reference for many new design. Capable of delivering 40 to 60KW peak power at 9375MHz,
725A had a glass boot on the filament seals and a rugged flange to be easily handled even in field
service operations. Two figures give an idea of its success: some 89.480 units of 725A were delivered
during WWII to British Empire under the Lend-Lease Law and in the mid 950s, the same magnetron
could be bought on the surplus market for as little as 4,50 USD, versus 25,00 USD asked for a 2K25
klystron.
The structure of the 725A was copied in several designs, both in U.S. and in Great Britain. 730A was a
725A with a cathode bi-pin base and the waveguide flange moved to the top. 2J21 was interchangeable
with 725A and 2J48, 2J49, 2J50 from Raytheon just differed from it for their tuning frequency. British
BTH designed the CV208, which was a 725A repackaged to be interchangeable with CV108.
Some pictures of X-band early magnetrons are given in the following page.
X-band Magnetrons
Fig. 4 –
Some pictures of the early X-band pulsed magnetrons, all derived from the 725A. Top, the British CV208, with its
glass surrounded probe to be inserted in the waveguide adapting section. CV208 had the same shape of CV108. Center and
right, two pictures of the 725A. Bottom, a 730A and a 2J49, both variants of the 725A. The last magnetron is a 2J42, with
axial cathode and integral magnet.
Further improvements
The in-field replacement of early magnetrons was very cumbersome. The need of a precise and uniform
magnetic field in the interelectrode region asked for complex procedures in checking the magnetic field
intensity and the proper mechanical alignment of the magnetron. The solution was found integrating
the magnetron with its magnet. The magnetron became a plug-in replacement, factory assembled and
already adjusted for the maximum efficiency, just requiring the insertion of the cathode and of the RF
output connectors. The latest improvement step came when ceramic materials replaced glass parts of
the body.
Fig. 5
Top, a typical integral magnet X-band magnetron. Bottom, a typical ceramic insulated magnetron.
Other magnetron devices
Multicavity pulsed magnetrons became by far the most used devices in radar transmitters. But this kind
of velocity modulated tubes soon was made available in different variety of devices for other
applications.
CW multicavity magnetrons
These tubes were introduced for RF heating purpose. The most important applications were in
diathermy electro-medical equipment and in microwave ovens for food heating/cooking. The principle
of CW magnetrons is not different from their pulsed equivalent, but for operating parameters. Anode
voltage must be limited to a safe value, within the anode dissipation capability, and the magnetic field
should be accordingly low.
Fig. 6 -
Telefunken MG8-200 delivers 200W at 3.3GHz. It is intended for diathermy electromedical equipment.
Split-anode magnetrons, 5J29
There are some very odd devices, designed for VHF/UHF military jammers, based upon the well-
known split-anode architecture. Two fluid cooled heavy copper anodes were connected to a tuned
transmission line, to generate about 150 W at frequencies from some 150 to about 900MHz.
Fig. 7 -
5J29 was the first of three split-anode magnetrons designed by General Electric for high-frequency jammers. The
two anode copper blocks are internally connected by a short copper tubing which acts as shorting termination of the external
tuning line, also granting inside the circulation of the cooling fluid.
5J30, Split-anode Magnetron
Fig. 8 -
General Electric 5J30, split anode magnetron. Each anode block is fluid cooled through coaxial copper tubing. The
two blocks are insulated from each other, the resonant line being shorted by a tuning stub on the side opposite to the
magnetron. Thoriated tungsten filamentary cathode with shielding rings at each end and small shielding vanes along the
filament axis.
5J32 Split-anode Magnetron
Fig. 9 -
The GE variant 5J32, which differs from the 5J30 for having double-ended connections to the anode blocks.
Raytheon 4J60 to 4J65 (QK59 to QK64)
Fig. 10Three pictures of a QK60, manually tunable from 2450 to 2720MHz
This family of low-power pulsed or CW magnetrons was intended for radar jammers. The six models cover from
2230 to 4030MHz, each manually tuned over the following ranges:
¨
4J60 = QK59
2230 to 2470MHz
¨
4J61 = QK60
2450 to 2720MHz
¨
4J62 = QK61
2700 to 3010MHz
¨
4J63 = QK62
2990 to 3330MHz
¨
4J64 = QK63
3310 to 3670MHz
¨
4J65 = QK64
3650 to 4030MHz
Integral magnet, coaxial output. 6.3V at 3.8A heater. 50W out at 1.2KV CW mode operation. 1.5KV pulsed
operation. Pinout: 1,2 heater cold – 7,8 heater-cathode.
This family was probably derived from the experimental CM16B described in Microwave Magnetrons, by G.B.
Collins. In ‘Radar System Engineering’, Ridenour lists the family as suitable for pulsed operation, retaining
approximately the same voltage and power levels given for CW operation: in this way it was possible to generate
pulses of any length.
X-band minimagnetron, CV2380
Fig. 11 –
This unusual mini-magnetron can deliver about 100mW pulses at 9400MHz. It should be mounted in a special
waveguide resonator, with 2450 Oersted magnetic field.