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Magnetron Theory of Operation

A magnetron is a high power microwave oscillator in In order to sustain oscillations in a resonant circuit,
which the potential energy of an electron cloud near it is necessary to continuously input energy in the
the cathode is converted into r.f. energy in a series correct phase. Referring to Figure 2, if the instanta-
of cavity resonators similar to the one shown in neous r.f. field, due to steady state oscillations in
Figure 1. As depicted by the low frequency analog, the resonator, is in the direction shown, and, an
the rear wall of the structure may be considered the electron with velocity was to travel through the r.f.
inductive portion, and the vane tip region the capac- field such that the r.f. field retarded the electron
itor portion of the equivalent resonant circuit. The velocity by an amount, the decrease in electron
resonant frequency of a microwave cavity is there- energy will be exactly offset by an increase in the r.f.
by determined by the physical dimension of the field strength.
resonator together with the reactive effect of any
In a magnetron, the source of electrons is a heated
perturbations to the inductive or capacitive portion
cathode located on the axis of an anode structure
of the equivalent circuit. This is an important point
containing a number of microwave resonators. See
and will be recalled later.
Figure 3.

Figure 1 Figure 3
Magnetron Anode/
Resonator Cathode
Structure

Electrons leave the cathode and are accelerated

toward the anode, due to the dc field established by
the voltage source E. The presence of a strong
Figure 2 magnetic field B in the region between cathode and
Energy anode produces a force on each electron which is
Transfer mutually perpendicular to the dc field and the elec-
Mechanism tron velocity vectors, thereby causing the electrons
to spiral away from the cathode in paths of varying
curvature, depending upon the initial electron veloc-
ity a the time it leaves the cathode.

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Magnetron Theory of Operation pg. 2

As this cloud of electrons approaches the anode, it change (radians) between every adjacent pair of
falls under the influence of the r.f. fields at the vane resonator vanes and is therefore called the mode.
tips, and electrons will either be retarded in velocity, Other oscillation patterns (modes) could be
if they happen to face an opposing r.f. field, or supported by the anode structure; however, the
accelerated if they are in the vicinity of an aiding r.f. mode pattern will product the maximum number of
field. Since the force on an electron due to the mag- electron spokes, and therefore the maximum trans-
netic field B is proportional to the electron velocity fer of energy to the r.f. field, i.e., highest efficiency
through the field, the retarded velocity electrons will mode. Assuring that the magnetron maintains mode
experience less "curling force" and will therefore oscillation, to the exclusion of all other modes, is
drift toward the anode, while the accelerated veloci- one of the prime concerns of the magnetron
ty electrons will curl back away from the anode. designer.

The result is an automatic collection of electron The mode controlling techniques in a conventional
"spokes" as the cloud nears the anode (see Figure magnetron, e.g., electrically connecting alternate
4), with each spoke located at a resonator having vane tips together to assure identical potential,
an opposing r.f. field.On the next half cycle of r.f. employing geometrical similarities between alter-
oscillation, the r.f. field pattern will have reversed nate resonators to favor mode oscillation, will ade-
polarity and the spoke pattern will rotate to maintain quately maintain mode control in conventional mag-
its presence in an opposing field. netron anodes. Due to mode separation parame-
ters, the number of resonators in conventional mag-
netron anodes is limited and rarely exceeds 20
resonator vanes. Since the physical size of each
Figure 4 resonator is fixed by the desired output frequency,
Electron the overall size of the anode is limited, thereby
spokes in a restricting cathode dimensions and heat dissipation
magnetron capacity. The result is that at higher frequencies the
conventional magnetron has reduced power output
capability, lower reliability and a shorter operating
lifetime than can be realized at the lower microwave
frequencies.
The "automatic" synchronism between the electron
spoke pattern and the r.f. field polarity in a crossed The distinguishing feature of the coaxial magnetron
field device allows a magnetron to maintain relative- is the presence of a high Q stabilizing cavity
ly stable operation over a wide range of applied between the anode and the output waveguide.
input parameters. For example, a magnetron The theory of operation presented for a convention-
designed for an output power of 200 kw peak will al magnetron applies equally to the anode-cathode
operate quite well at 100 kw peak output by simply region of the coaxial structure. However, the coaxial
reducing the modulator drive level. stabilizing cavity affords very significant improve-
ments in overall magnetron performance.
You will note that the instantaneous r.f. field pattern,
shown in Figure 4, has exactly 180º of phase

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Magnetron Theory of Operation pg. 3

Superior mode control: Operating the cavity in the

TE011 mode, and slot coupling alternate anode
resonators to the cavity, produces anode control of
such intensity as to permit the construction of coaxi-
al magnetrons with many times the number of reso-
nators that can be employed in a conventional type
magnetron. This means lower cathode emission
density, lower life and higher reliability.

Reduced RF fields in the anode: Whereas all

stored energy in a conventional is confined to the Figure 5: Coaxial Magnetron
vane resonators, in a coaxial magnetron approxi-
mately 85% of the total stored energy is contained
in the stabilizing cavity. This means reduced r.f. field
intensity at the vane tips, and less tendency to
arcing.

Improved frequency stability: The redistribution

of stored energy in the coaxial magnetron makes
the high Q stabilizing cavity the prime determiner of
magnetron output frequency. This means a lower
pushing figure, a lower pulling figure, improved
spectrum and reduced spurious emissions.

Improved tuning: In the conventional magnetron,

tuning is accomplished by inserting inductive pins in
Figure 6: Three types of tuning schemes
the rear portion of each resonator, or by capacitive
used in conventional magnetron
loading in the vane tip region.
resonator systems
Both techniques represent an adverse perturbation
to the natural geometry of the resonators which
Typical Magnetron Parameters
often results in power output variation with tuning,
starting instabilities, increased susceptibility to The following is a discussion and explanation of
arcing and a generally reduced operating lifetime typical magnetron specification parameters.
for the magnetron. In contrast the coaxial magne-
tron is tuned by moving a noncontacting plunger in Thermal Drift
the stabilizing cavity (see Figure 5). The result is a
tuning characteristic with no discontinuities, broad At the time high voltage is first applied to a magnetr-
tunable bandwidth, and none of the disadvantages ton, the thermal equilibrium of the device is sudden-
resulting from perturbations in the anode-cathode ly altered. The anode vanes being to heat at the tips
region. due to electron bombardment and the entire
anode/cathode structure undergoes a transient

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Magnetron Theory of Operation pg. 4

change in thermal profile. During the time required Temperature Coefficient

for each part of the magnetron to stabilize at its
normal operating temperature, the output frequency After the thermal drift period has expired and a
of the magnetron will "drift." The curve of output stable operating frequency has been achieved,
frequency vs. time during the period following initial changes to ambient conditions which cause a
turn on is called the "Thermal Drift" curve. Generally corresponding change in the magnetron tempera-
speaking, the maximum drift occurs during the first ture will produce a change in the output frequency.
few minutes after turn on, and slowly approaches In this content ambient changes include cooling air
equilibrium over a period ranging from 10 to 30 temperature or pressure in air cooled magnetrons;
minutes depending upon the structure mass, power mounting plate temperature in heat sink cooled
output, type of cooling and basic magnetron design. magnetrons; and flow rate or temperature in liquid
Thermal drift curves across a variety of magnetron cooled magnetrons.
types operating at the same frequency and output
power may differ radically from each other. Each The change in magnetron output frequency for
type is usually designed for a particular purpose each degree change in body temperature, as mea-
and subtle differences in the internal magnetron sured at a specified point on the outside surface of
configuration can produce radical differences in the the magnetron body, is defined as the Temperature
thermal drift curve. Coefficient for the magnetron and is usually
expressed in MHz/oC. For most magnetrons the
It should be noted that a thermal drift effect will temperature coefficient is a negative (frequency
occur not only at initial turn-on, but whenever the decreases as temperature increases) and is essen-
peak or average input power to the magnetron is tially constant over the operating range of the mag-
changed, e.g., a change of pulse duration, PRF or netron.
duty. Figure 7 shows typical thermal drift curves for
a particular magnetron plotted as a function of duty. When estimating magnetron frequency change due
The dotted line indicates the effect of a change in to temperature coefficient, keep in mind that the
duty from .001 to .0005 after thermal equilibrium temperature coefficient relates magnetron frequen-
has been initially achieved. cy to body temperature and there is not necessarily
a 1:1 relation between body temperature and, for
example, ambient air temperature. In addition, for
airborne systems, the cooling effect of lower air
temperature at altitude may offset by a correspond-
ing reduction in air density.

Pushing Figure
The pushing figure of a magnetron is defined as the
change in magnetron frequency due to a change in
the peak cathode current. Referring back to the
Figure 7: Typical thermal drift curves earlier theory discussion, we noted that the

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Magnetron Theory of Operation pg. 5

the resonant frequency of a vane resonator is deter- The primary importance of a low pushing figure
mined by its mechanical dimensions plus the reac- near the magnetron operating point is that the push-
tive effect of any perturbation. The presence of ing figure will determine intrapulse FM, and thereby
electrons in the vicinity of the vane tips affects the will affect the spectral quality of the transmitting
equivalent capacitance of the resonator by an pulse.
amount proportional to the density of the electrons
The Pulling Figure is defined as the maximum
and, since electron density is similarly related to
change in output frequency that results when an
peak pulse current, changes in pulse current level
external, fixed amplitude mismatch, located in the
will produce changes in output frequency. The
output waveguide, is moved through a distance of
pushing figure expressed in MHz/Amp is represent-
one half wavelength relative to the magnetron.
ed by the slope of a frequency vs. peak current
Stated somewhat less formally, the pulling figure is
curve plotted for a particular magnetron type.
a measure of a magnetron's ability to maintain a
constant output frequency against changes in load
mismatch.
During the design of a magnetron, the degree to
which the output waveguide is electrically coupled
to the internal resonator structure is selected to
optimize certain performance parameters. Strong
coupling increases output power and efficiency but
also increases time jitter and sensitivity to changes
to load mismatch. Generally, the coupling is chosen
to obtain the best compromise between efficiency
and stability.
Depending upon the phase relation between
incident and reflected power at the output port of a
Figure 8: Typical thermal pushing curve
magnetron, reflected power will appear as a reac-
tance across the coupling transformer and effec-
From the curve of Figure 8, it can be seen that the
tively change the degree of coupling. Therefore,
slope is not a constant over the full range of operat-
using a fixed mismatch and varying its distance
ing current. It is therefore meaningless to talk about
from the magnetron output port will cause the mag-
a specific value for the pushing figure unless one
netron frequency to shift and the output power to
also specifies the range of peak current over which
vary concurrently.
it applies.
To standardize the measurement values, pulling
It should be noted that since power output is propor-
figure is normally measured using a fixed 1.5:1
tional to peak current in a magnetron, the pushing
VSWR; however, in very high power magnetrons a
figure at peak current levels well below the normal
1.3:1 VSWR is often used. When referring to the
operating point of the magnetron are usually unim-
pulling figure of a magnetron one should always
portant because the power output at these current
indicate the VSWR value used in the measurement.
levels is low.

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Magnetron Theory of Operation:

Frequency Agile Magnetrons

Frequency agility (FA) in regard to radar operations, For this reason, CPI produces a broad range of FA
is defined as the capability to tune the output tuning mechanisms for coaxial magnetrons; each
frequency of the radar with sufficient speed to mechanism offering the optimum combination of
produce a pulse-to-pulse frequency change greater parameters for a particular application.
than the amount required to effectively obtain
decorrelation of adjacent radar echoes. Frequency Agile Magnetron Classes

It has been firmly established that FA, together with Frequency agile magnetrons fall into four classes:
appropriate receiver integration circuits, affords
reduced target scintillation/glint, improved ability to • Dither Magnetrons (D) -- Output rf frequency
detect targets in a clutter environment, elimination varies periodically with a constant excursion,
of 2nd time around echoes, and improved resis- constant rate and a fixed-center frequency.
tance to electronic countermeasures, over that
possible with a fixed frequency or tunable radar • Tunable/Dither Magnetrons (T/D) -- Output rf
system. It is important to note that, with the excep- frequency varies periodically with a constant
tion of ECM resistance, increasing the excursion and constant rate.The center
pulse-to-pulse frequency spacing will increase the frequency may be slowly tuned by hand or by
amount of system performance improvement that external servomotor drive to any point within the
can be realized to a maximum occurring at the point tunable band.
where full pulse echo decorrelation is obtained
(nominally 1/tp). Pulse-to-pulse frequency spacings • Accutune(tm) Magnetrons (A) -- Output rf
greater than this critical value produce no further frequency variations are determined by the
increase in system performance, and, in fact, may waveshape of an externally generated, low
result in a performance decrease due to the large level, voltage signal. With appropriate selection
"IF" inaccuracies arising from the need for the AFC of a tuning waveshape, the Accutune magne-
to correct larger pulse to pulse frequency errors. tron combines the features of dither and
tunable/dither magnetrons.
On the other hand, as regards resistance to elec-
tronic jamming (ECCM), the greater the • Accusweep(tm) Magnetrons (As) -- Our best
pulse-to-pulse frequency spacing, the more difficult and most versatile tuning system. The output rf
it will be to center a jamming transmitter on the tuning rate and waveshape are infinitely
radarfrequency to effectively interfere with system variable within the design limits of each device.
operation. Customer inputs are typically any waveform
from random to square wave and a + 5 volt com-
Each radar system application must be considered mand.
separately to determine which FA parameters will
best satisfy the particular need. Just as the FA All CPI frequency-agile magnetrons provide a refer-
requirements of each radar differ, so also do the ence voltage output which is an accurate analog of
mechanisms differ for optimally producing the the instantaneous rf output frequency. This signal
required agility parameters. No single tuning greatly simplifies automatic frequency control of the
scheme has been found which will universally satis- system local oscillator frequency. The analog
fy the requirements of every FA application. voltage is produced either by a self-generating,

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Magnetron Theory of Operation:

Frequency Agile Magnetrons

permanent magnet device requiring no external Using this relationship, one finds that a radar oper-
drive, or by a precision resolve or LVDT (Linear ating at a 0.5 s pulse duration will have efficient
Voltage Displacement Transducer) acting in decorrelation between target echoes if pulses differ
conjunction with one of CPI's solid-state frequency in frequency by at least 2 MHz. Note that the
readout modules. required frequency separation is a function only of
The Accutune and Accusweep magnetrons operate the pulse duration.
with a servo loop, feedback control, tuner drive and According to the plot of the figure, as frequency
thereby utilize CPI's solid-state servo amplifier separation increases above the value 1/T, pulse
together with the frequency readout module. decorrelation continues to improve, however, the
amount of improvement is negligibly small for large
Agile Magnetron Design Considerations
increases in pulse frequency separation. In practi-
At first glance one might conclude that the largest cal situations, the improvement in decorrelation
frequency change at the highest rate will give the obtained by increasing the frequency separation to
best radar performance. Unfortunately, this is not a values greater than 1/T is usually more than offset
true statement. by other factors. For example, as pulse-to-pulse
There have been many separate theoretical studies frequency difference increases, the receiver circuit-
and comprehensive experiments performed to ry needed to assure stable LO (Local Oscillator)
establish the relationship between radar perfor- tracking also increases, in both complexity and
mance improvement and pulse-to-pulse frequency physical size. The accuracy necessary for LO track-
difference. An understanding of the theoretical ing relates directly to the IF bandwidth needed to
basis for the conclusions reached in these efforts is pass the resultant video signal. Any increase in IF
important. In order to preserve the continuity of our bandwidth, needed to offset inaccuracies in LO
discussion, we will show only the results of these tracking, will reduce overall receiver sensitivity and
studies in this section. tend to defeat the original purpose. Experience has
shown that if one designs for pulse-to-pulse
Effective performance improvement is achieved frequency separation as near as possible to, but not
when the frequency difference between radar le ss than, 1 /T (where T is the shortest pulse dura-
pulses is large enough to eliminate any correlation tion used in the radar) optimum system perfor-
between the return echoes. A plot of correlation mance will be achieved. Experimental studies have
coefficient versus pulse to pulse frequency differ- shown that performance improvement varies as N,
ence is shown below. where N is the number of independent (decorrelat-
ed) pulses integrated within the receiver circuitry, up
to a maximum of 20 pulses.
CORRELATION

It should be noted that the number of pulses, which

COEFFICIENT

can be effectively integrated, cannot be greater

than the number of pulses placed on the target
during one scan of the antenna and, therefore, the
antenna beamwidth and scan rate become factors
PULSE-TO-PULSE
FREQUENCY SEPARATION which must also be considered in determining the
integration period of the radar.

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Magnetron Theory of Operation:

Frequency Agile Magnetrons

Using the above, a design value for Agile Excursion Example:

can now be expressed in terms of radar operating
parameters. Assume one desires to add agility to a radar having
the following operating parameters:

Agile Excursion = N/T Pulse duration - 0.25, 0.5 & 1.0 µSec.
Duty Ratio - 0.001
Pulses on target - 16 per scan
ONE CYCLE OF
AGILE RATE Using the formulas derived above one obtains:
Agile excursion = N/T = 16/0.25 = 64 MHz
Pulse to pulse frequency separation =
FREQUENCY

AGILE
EXCURSION
1/T = 1/ 0.25 = 4 MHz
PRR = Duty / T = 0.001/ (0.25x10-6)
TIME = 4000 Hz
Time for 16 pulses = 16/ 4000 = 0.004 Sec
Agile Rate* = 1/(2x0.004) = 125 Hz

* The 2 in the denominator accounts for the fact that

two excursions through the agile frequency range
Agile Excursion: The total frequency variation
of the transmitter during agile operation. occur during each cycle of agile rate.
Agile Rate: The number of times per second The agile parameters used above were derived
that the traansmitter frequency traverses the
agile excursion and returns to its starting using clutter reduction as the prime objective. Elimi-
frequency. nation of target scintillation requires the satisfaction
NOTE: There are two excursions of the frequen- of one additional constraint, namely that the agile
cy band for each cycle of agile rate. excursion in Mhz should be at least equal to 150/D,
where D is the characteristic distance, in meters,
between major reflecting points on the target cross
section. For most practical situations, an excursion
Where N is the number of pulses placed on the
which satisfied the requirements of clutter reduction
target during one radar scan, or 20 whichever is
will usually be sufficient to satisfy the requirements
smaller, and T is the shortest pulse duration used in
of target scintillation also.
the system.
Determination of the required agile rate is now Further information and data on agile coaxial mag-
required. The object is to traverse the full agile netrons can be obtained by requesting the
excursion range in the time needed to transmit the Frequency Agile Magnetron Story booklet.
number of pulses on the target during one antenna
scan.

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Magnetron Theory of Operation:

Beacon Magnetrons

Beacon magnetrons are small conventional magne- Anode

trons with peak power output less than 4 kW and
The anode is the foundation of the magnetron
average power output of less than 5 watts. Typical-
circuit. It generally consists of an even number of
ly, they weigh 8 ounces.
microwave cavities arranged in radial fashion as
The technical requirements for this class of magne- shown. There are three possible anode configura-
trons demand precise frequency control of the mag- tions:
netron. The temperature stability factor is of great
• Hole and slot
importance since it allows frequency control without
• Vane tip
additional electronics in the total radar transponder.
The magnetron itself requires tunability but must • Rising sun
have the properties of a fixed frequency magnetron
after adjustment and locking. Thus, the techniques
of temperature compensation must work over a
band of frequencies. Also, frequency stability is
essential over typical temperature ranges of -65oC
to +100oC, and typical shock of 100G, and vibration
environments of 15G (generally those of missile HOLE-AND-SLOT STRAPPED RISING SUN
and aircraft electronic systems). VANE
Figure 2.
Construction of beacon magnetrons can be simpli-
fied to contain five basic building blocks. They are
the anode, tuner, cathode, output, and magnet. Advantages and disadvantages of each type
involve consideration of operating characteristics
These may be arranged in block diagram fashion as and construction techniques. The hole-and-slot and
shown in Figure 1. vane type normally have every other cavity
The following sections will discuss each of the five strapped to each other by a conducting metal strip.
parts of the magnetron. The hole-and-slot type and the rising sun type are
usually machined by hobbing methods out of solid
copper stock. The vane type is generally made up
MAGNET
of individual vanes assembled and brazed into a
support ring. This requires assembly labor and
brazing fixtures.

The anode provides the basic magnetron with its

TUNER ANODE CATHODE operating frequency. The central area provides C
(capacitance) and the outer perimeter contributes L
(inductance) to fulfill the relationship
F = 1/(2π √LC)
OUTPUT Each anode is cold checked for "Q" - value and
frequency. This involves general microwave imped-
Figure 1.
ance and resonance measurements techniques.

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Magnetron Theory of Operation:

Beacon Magnetrons
Tuner and is used where extremely wide tunability is
required. The attachment must necessarily involve
The tuner is the device which provides some mag-
a bellows or diaphragm arrangement in order to
netrons with the ability to vary from the basic
allow for mechanical movement and still contain the
frequency determined by the anode. Tuners fall into
necessary vacuum envelope.
three basic categories:
Figure 4 shows a simplified capacitive tuner-anode
• Capacitive
assembly. The magnetron tuner is generally com-
• Inductive
posed of two parts, internal and external. The inter-
• Combination of both
nal portion described above is that part which is
A fundamental description of each is shown in enclosed by the vacuum envelope. The external
Figure 3. The capacitive type is so named because portion is attached to the internal portion by some
in it a tuning member is introduced into the anode mechanical means and provides the drive mecha-
cavities affecting the E-field and hence the capaci- nism to actually move the tuner the required
tance of the anode. This type can be constructed of distance to change L and C and therefore change
either metal (copper) fingers which are inserted frequency.
between adjacent anode vanes in the central
portion of the anode or a dielectric or metallic ring
which is inserted into the anode between its central TUNER DRIVE ROD
vane straps.
BELLOWS

Top view Top view Bottom view CAPACITIVE

TUNING RING

ANODE
ASSEMBLY

Figure 4.

Capacitive Inductive Combination Cathode

Figure 3. The cathode of a magnetron is the part which

makes the magnetron an active device. This
provides the electrons through which the mecha-
The inductive type tuner is much the same as the
nism of energy transfer is accomplished. The cath-
capacitive but the tuning member enters the cavi-
ode is usually located in the center of the anode and
ties in the back wall region where the H-field and
is made up of a hollow cylinder of emissive material
Inductance are affected. The combination of the two
surrounding a heater.
is a complicated affair which affects both L and C

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Magnetron Theory of Operation:

Beacon Magnetrons
A cross- section of a simple magnetron cathode is The next step in the magnetron's construction is to
shown in Figure 5. Many types of magnetron cath- attach the cathode to the tuner-anode assembly.
odes have been developed; each designed for a This procedure also requires extreme care in the
specific advantage. The fabrication of magnetron axial line-up and orientation of the cathode and
cathodes is carried out in very meticulous and anode. Any eccentricity between anode and cath-
precise environments. Each braze and weld must ode will produce variations in magnetron operation
be inspected for completeness in order not to upset and can cause serious internal arcing or malfunc-
the designed heat flow characteristics. Magnetron tion. Figure 6 shows a simplified cathode-tuner-an-
cathodes are designed to operate at particular tem- ode assembly.
peratures and owing to the phenomenon called
“back bombardment” they cannot tolerate wide
TUNER DRIVE ROD
variations in construction and assembly techniques.
As a further check on operating temperature of
cathodes used in high reliability magnetrons, the
cathode-heater assembly alone is evacuated and
operated at a predetermined heater voltages and
the cathode temperature checked with an optical ANODE ASSEMBLY
pyrometer. This technique reveals any flaw or
defect in construction prior to the time the cathode
is actually assembled in a magnetron.

CATHODE
EMISSIVE CATHODE ASSEMBLY
MATERIAL
Figure 6.

Output
CATHODE SUPPORT The output circuit in a magnetron is that portion of
the device which provides the coupling to the exter-
nal load. The RF energy produced in the cavities
may be coupled by either a coaxial or waveguide
type of output. Figure 7 shows both types. The
coaxial design involves either a probe, a loop or a
tapped vane coupling to the anode and concentric
coaxial line through the vacuum envelope to the
output connector. Suitable matching sections must
HEATER be included along the line to provide for the correct
INSULATOR impedance transformations and coupled load which
Figure 5.
appears at the anode. The center conduction of the
coaxial line is insulated and supported along its
length by either glass or ceramic beads.

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CPI Beverly Microwave Division, Telephone: +1 (978) 922-6000 • FAX: +1 (978) 922-8914 • Email: bmdmarketing@cpii.com Rev. 7/16
 Beverly Microwave Division
www.cpii.com/bmd

Magnetron Theory of Operation:

Beacon Magnetrons
Magnetic Circuit
COUPLING LOOP
The magnetic circuit associated with the magnetron
is necessary to provide the crossed field type of
operation which provides for the synchronization of
INSULATOR SUPPORT
the electron trajectories. The magnetic circuit
shown here is composed of an external permanent
VACUUM SEAL magnet and associated internal pole pieces. The
type and composition of the permanent magnet
COAXIAL vary with particular requirements of field strength
and stability. Size and weight are also important
CONNECTOR considerations. The transmission and focusing of
the magnetic field from the external permanent
magnet to the interaction gap between the anode
COUPLING SLOT and cathode is accomplished by the use of high
permeability metal pole pieces shaped to focus the
field lines as sharply as possible.
OUTPUT
WINDOW
FOCUSING POLE PIECE
TRANSFORMER

FLANGE
WAVEGUIDE

Figure 7.

The waveguide type of output is made up of a

coupling slot in the back wall of a cavity, a 1/4 trans-
former, a vacuum seal window (either glass or
ceramic) and a section of output waveguide. The
sizes of the coupling slot and 1/4 transformer are
determined by frequency, bandwidth and load FOCUSING POLE PIECE PERMANENT MAGNET
coupling considerations. The type of vacuum seal
window used is determined by the power output Figure 8.
and pressurization requirements. Placement of the
output window is extremely critical as far as position
along the line is concerned, because any high
VSWR which may be reflected back from the load
that will cause a voltage maximum at the window
will cause overheating and subsequent rupture of
the vacuum seal.

For information on CPI products visit our webpage at www.cpii.com/bmd, or contact:

Pg. 12
CPI Beverly Microwave Division, Telephone: +1 (978) 922-6000 • FAX: +1 (978) 922-8914 • Email: bmdmarketing@cpii.com Rev. 7/16