Module 11 − Microwave Principles
wide range of RF frequencies if it is terminated in the characteristic impedance
of the line. The electromagnetic waves traveling down the line produce electric
fields that interact with the electrons of the beam.
Figure 2-14 - Simplified TWT.
If the electrons of the beam were accelerated to travel faster than the waves
traveling on the wire, bunching would occur through the effect of velocity modulation.
Velocity modulation would be caused by the interaction between the traveling-wave
fields and the electron beam. Bunching would cause the electrons to give up energy
to the traveling wave if the fields were of the correct polarity to slow down the
bunches. The energy from the bunches would increase the amplitude of the traveling
wave in a progressive action that would take place all along the length of the TWT,
as shown in figure 2-14.
However, because the waves travel along the wire at the speed of light, the simple
TWT shown in figure 2-14 will not work. At present no way is known to accelerate
an electron beam to the speed of light. Since the electron beam cannot travel faster
than the wave on the wire, bunching will not take place and the tube will not work.
The TWT is therefore designed with a delay structure to slow the traveling wave
down to or below the speed of the electrons in the beam. a common TWT delay structure
is a wire, wound in the form of a long coil or helix, as shown in figure 2-15, view
(A). The shape of the helix slows the effective velocity of the wave along the common
axis of the helix and the tube to about one-tenth the speed of light. The wave still
travels down the helix wire at the speed of light, but the coiled shape causes the
wave to travel a much greater total distance than the electron beam. The speed at
which the wave travels down the tube can be varied by changing the number of turns
or the diameter of the turns in the helix wire. The helical delay structure works
well because it has the added advantage of causing a large proportion of electric
fields that are parallel to the electron beam. The parallel fields provide maximum
interaction between the fields and the electron beam.
Figure 2-15 - Functional diagram of a TWT.
In a typical TWT, the electron beam is directed down the center of the helix
while, at the same time, an RF signal is coupled onto the helix. The electrons of
the beam are velocity-modulated by the electric fields produced by the RF signal.
Amplification begins as the electron bunches form and release energy to the signal
on the helix. The slightly amplified signal causes a denser electron bunch which,
in turn, amplifies the signal even more. The amplification process is continuous
as the RF wave and the electron beam travel down the length of the tube.
Any portion of the TWT output signal that reflects back to the input will cause
oscillations within the tube which results in a decrease in amplification. Attenuators
are placed along the length of the helix to prevent reflections from reaching the
input. The attenuator causes a loss in amplitude, as can be seen in figure 2-15,
view (B), but it can be placed so as to minimize losses while still isolating the
input from the output.
The relatively low efficiency of the TWT partially offsets the advantages of
high gain and wide bandwidth. The internal attenuator reduces the gain of the tube,
and the power required to energize the focusing magnet is an operational loss that
cannot be recovered. The TWT also produces heat which must be dissipated by either
air-conditioning or liquid-cooling systems. All of these factors reduce the overall
efficiency of the TWT, but the advantages of high gain and wide bandwidth are usually
enough to overcome the disadvantages.
The Backward-Wave Oscillator
The BACKWARD-WAVE Oscillator (BWO) is a microwave-frequency, velocity-modulated
tube that operates on the same principle as the TWT. However, a traveling wave that
moves from the
electron gun end of the tube toward the collector is not used in the BWO. Instead,
the BWO extracts energy from the electron. beam by using a backward wave that travels
from the collector toward the electron gun (cathode). Otherwise, the electron bunching
action and energy extraction from the electron beam is very similar to the actions
in a TWT.
Figure 2-16 - Typical BWO.
The typical BWO is constructed from a folded transmission line or waveguide that
winds back and forth across the path of the electron beam, as shown in figure 2-16.
The folded waveguide in the illustration serves the same purpose as the helix in
a TWT. The fixed spacing of the folded waveguide limits the bandwidth of the BWO.
Since the frequency of a given waveguide is constant, the frequency of the BWO is
controlled by the transit time of the electron beam. The transit time is controlled
by the collector potential. Thus, the output frequency can be changed by varying
the collector voltage, which is a definite advantage. As in the TWT, the electron
beam in the BWO is focused by a magnet placed around the body of the tube.
Q-26. What is the primary use of the TWT?
Q-27. The magnet surrounding the body of a TWT serves what purpose?
Q-28. How are the input and output directional couplers in a TWT connected to
Q-29. What relationship must exist between the electron beam and the traveling
wave for bunching to occur in the electron beam of a TWT?
Q-30. What structure in the TWT delays the forward progress of the traveling
The MAGNETRON, shown in figure 2-17A, is a self-contained microwave oscillator
that operates differently from the linear-beam tubes, such as the TWT and the klystron.
Figure 2-17B is a simplified drawing of the magnetron. CROSSED-ELECTRON and Magnetic
fields are used in the magnetron to produce the high-power output required in radar
and communications equipment.
Figure 2-17A - Magnetron.
Figure 2-17B - Magnetron.
Figure 2-18 - Cutaway view of a magnetron.
The magnetron is classed as a diode because it has no grid. a magnetic field
located in the space between the plate (anode) and the cathode serves as a grid.
The plate of a magnetron does not have the same physical appearance as the plate
of an ordinary electron tube. Since conventional inductive- capacitive (LC) networks
become impractical at microwave frequencies, the plate is fabricated into a cylindrical
copper block containing resonant cavities which serve as tuned circuits. The magnetron
base differs considerably from the conventional tube base. The magnetron base is
short in length and has large diameter leads that are carefully sealed into the
tube and shielded.
The cathode and filament are at the center of the tube and are supported by the
filament leads. The filament leads are large and rigid enough to keep the cathode
and filament structure fixed in position. The
output lead is usually a probe or loop extending into one of the tuned cavities
and coupled into a waveguide or coaxial line. The plate structure, shown in figure
2-18, is a solid block of copper. The cylindrical holes around its circumference
are resonant cavities. a narrow slot runs from each cavity into the central portion
of the tube dividing the inner structure into as many segments as there are cavities.
Alternate segments are strapped together to put the cavities in parallel with regard
to the output. The cavities control the output frequency. The straps are circular,
metal bands that are placed across the top of the block at the entrance slots to
the cavities. Since the cathode must operate at high power, it must be fairly large
and must also be able to withstand high operating temperatures. It must also have
good emission characteristics, particularly under return bombardment by the electrons.
This is because most of the output power is provided by the large number of electrons
that are emitted when high-velocity electrons return to strike the cathode. The
cathode is indirectly heated and is constructed of a high- emission material. The
open space between the plate and the cathode is called the INTERACTION SPACE. In
this space the electric and magnetic fields interact to exert force upon the electrons.
The magnetic field is usually provided by a strong, permanent magnet mounted around
the magnetron so that the magnetic field is parallel with the axis of the cathode.
The cathode is mounted in the center of the interaction space.
Basic MAGNETRON Operation.
Magnetron theory of operation is based on the motion of electrons under the influence
of combined electric and magnetic fields. The following information presents the
laws governing this motion.
The direction of an electric field is from the positive electrode to the negative
electrode. The law governing the motion of an electron in an electric field (E field)
The force exerted by an electric field on an electron is proportional to the
strength of the field. Electrons tend to move from a point of negative potential
toward a positive potential.
Figure 2-19 - Electron motion in an electric field.
Figure 2-20 - Electron motion in a magnetic field.
Figure 2-21A - Basic magnetron. SIDE VIEW.
Figure 2-21B - Basic magnetron. END VIEW OMITTING MAGNETS.
Figure 2-22 - Effect of a magnetic field on a single electron.
This is shown in figure 2-19. In other words, electrons tend to move against
the E field. When an electron is being accelerated by an E field, as shown in figure
2-19, energy is taken from the field by the electron.
The law of motion of an electron in a magnetic field (H field) states:
The force exerted on an electron in a magnetic field is at right angles to both
the field and the path of the electron. The direction of the force is such that
the electron trajectories are clockwise when viewed in the direction of the magnetic
This is shown in figure 2-20.
In figure 2-20, assume that a south pole is below the figure and a north pole
is above the figure so that the magnetic field is going into the paper. When an
electron is moving through space, a magnetic field builds around the electron just
as it would around a wire when electrons are flowing through a wire. In figure 2-20
the magnetic field around the moving electron adds to the permanent magnetic field
left side of the electron's path and subtracts from the permanent magnetic field
on the right side. This action weakens the field on the right side; therefore, the
electron path bends to the right (clockwise). If the strength of the magnetic field
is increased, the path of the electron will have a sharper bend. Likewise, if the
velocity of the electron increases, the field around it increases and the path will
bend more sharply.
A schematic diagram of a basic magnetron is shown in figure 2-21A. The tube consists
of a cylindrical plate with a cathode placed along the center axis of the plate.
The tuned circuit is made up of cavities in which oscillations take place and are
physically located in the plate.
When no magnetic field exists, heating the cathode results in a uniform and direct
movement of the field from the cathode to the plate, as illustrated in figure 2-21B.
However, as the magnetic field surrounding the tube is increased, a single electron
is affected, as shown in figure 2-22. In figure 2-22, view (A), the magnetic field
has been increased to a point where the electron proceeds to the plate in a curve
rather than a direct path.
In view (B) of figure 2-22, the magnetic field has reached a value great enough
to cause the electron to just miss the plate and return to the filament in a circular
orbit. This value is the CRITICAL Value of field strength. In view (C), the value
of the field strength has been increased to a point beyond the critical value; the
electron is made to travel to the cathode in a circular path of smaller diameter.
View (D) of figure 2-22. shows how the magnetron plate current varies under the
influence of the varying magnetic field. In view (A), the electron flow reaches
the plate, so a large amount of plate current is flowing. However, when the critical
field value is reached, as shown in view (B), the electrons are deflected away from
the plate and the plate current then drops quickly to a very small value. When the
field strength is made still greater, as shown in view (C), the plate current drops
When the magnetron is adjusted to the cutoff, or critical value of the plate
current, and the electrons just fail to reach the plate in their circular motion,
it can produce oscillations at microwave frequencies. These oscillations are caused
by the currents induced electrostatically by the moving electrons. The frequency
is determined by the time it takes the electrons to travel from the cathode toward
the plate and back again. a transfer of microwave energy to a load is made possible
by connecting an external circuit between the cathode and the plate of the magnetron.
Magnetron oscillators are divided into two classes: Negative-Resistance and ELECTRON-RESONANCE
A negative-resistance magnetron oscillator is operated by a static negative resistance
between its electrodes. This oscillator has a frequency equal to the frequency of
the tuned circuit connected to the tube.
An electron-resonance magnetron oscillator is operated by the electron transit
time required for electrons to travel from cathode to plate. This oscillator is
capable of generating very large peak power outputs at frequencies in the thousands
of megahertz. Although its average power output over a period of time is low, it
can provide very high-powered oscillations in short bursts of pulses.
Q-31. The folded waveguide in a BWO serves the same purpose as what component
in a TWT?
Q-32. What serves as a grid in a magnetron?
Q-33. a cylindrical copper block with resonant cavities around the circumference
is used as what component of a magnetron?
Q-34. What controls the output frequency of a magnetron?
Q-35. What element in the magnetron causes the curved path of electron flow?
Q-36. What is the term used to identify the amount of field strength required
to cause the electrons to just miss the plate and return to the filament in a circular
Q-37. a magnetron will produce oscillations when the electrons follow what type
Figure 2-23A - Split-anode magnetron.
Figure 2-23B - Split-anode magnetron.
Figure 2-24 - Movement of an electron in a split-anode magnetron.
The split-anode, negative-resistance magnetron is a variation of the basic magnetron
which operates at a higher frequency. The negative-resistance magnetron is capable
of greater power output than the basic magnetron. Its general construction is similar
to the basic magnetron except that it has a split plate, as shown in figure 2-23A
and B. These half plates are operated at different potentials to provide an electron
motion, as shown in figure 2-24. The electron leaving the cathode and progressing
toward the high-potential plate is deflected by the magnetic field and follows the
path shown in figure 2-24. After passing the split between the two plates, the electron
enters the electrostatic field set up by the lower-potential plate.
Here the magnetic field has more effect on the electron and deflects it into
a tighter curve. The electron then continues to make a series of loops through the
magnetic field and the electric field until it finally arrives at the low-potential
Oscillations are started by applying the proper magnetic field to the tube. The
field value required is slightly higher than the critical value. In the split-anode
tube, the critical value is the field value required to cause all the electrons
to miss the plate when its halves are operating at the same potential. The alternating
voltages impressed on the plates by the oscillations generated in the tank circuit
will cause electron motion, such as that shown in figure 2-24, and current will
flow. Since a very concentrated magnetic field is required for the negative-resistance
magnetron oscillator, the length of the tube plate is limited to a few centimeters
to keep the magnet at reasonable dimensions. In addition, a small diameter tube
is required to make the magnetron operate efficiently at microwave frequencies.
A heavy-walled plate is used to increase the radiating properties of the tube. Artificial
cooling methods, such as forced-air or water-cooled systems, are used to obtain
still greater dissipation in these high-output tubes.
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