MICROWAVE TUBES-Introduction

• In your previous courses, it is evident

that devices like., vacuum tube, diode,
BJT and FET etc.. can be used to
generate/amplify the signals.---
Oscillators or Amplifiers.
• But conventional devices/tubes are
not susceptible at greater ranges of
frequencies
• Hence, specialized microwave tubes
are used
• Tubes generate and amplify (as
Oscillators or Amplifiers) high levels
of microwave power more cheaper
than solid state devices.
 MICROWAVE TUBES-classification
DC Signal with
high magnitude

AC Signal with AC Signal with

low magnitude high magnitude
 MICROWAVE TUBES-classification
Information and
Control signals Single Cavity Reflex
are in parallel Klystron

Two-Cavity Klystron
O-type Tubes
Multi- Cavity Klystron Hybrid Version
Linear Beam Tubes
of tubes are also
in use
Tubes Travelling Wave Tube

Cross-field tubes
Magnetron
M-type Tubes

Information and
Control signals
are in series
Limitations and Losses of conventional tubes

A l
C  L
d A
 Limitations and Losses of conventional tubes
• It is always good to go with the limitations of conventional tubes when operated
at UW frequencies:
1. Inter-electrode capacitance effect
2. Lead inductance effect
3. Transit time effect
4. Gain-bandwidth product
5. Effects due to RF losses
6. Radiation losses

1. Inter-electrode Effect: 3. Transit time effect

2. Lead inductance effect
 MICROWAVE TUBES
4. Gain-bandwidth Product limitation: Gain bandwidth product is independent of
frequency. So for a given tube higher gain can be only obtained at the expense of
narrower bandwidth.
5. Effects due to RF losses : Exposure to very high RF intensities can result in heating
of biological tissue and an increase in body temperature. Tissue damage in humans
could occur during exposure to high RF levels because of the body's inability to cope
with or dissipate the excessive heat that could be generated.
6. a) Skin effect: This effect is introduced at higher frequencies. Due to it, the current
flows from the small sectional area to the surface of the device. Also at higher
frequencies, resistance of conductor increases due to which the there are losses.
b) Dielectric loss: Dielectric material is generally different silicon plastic
encapsulation materials used in microwave devices. At higher frequencies the losses
due to these materials are also prominent
 MICROWAVE TUBES
• Definition: Klystrons are a special type of vacuum tubes that find applications as
amplifiers and oscillators at microwave frequencies, with principle of operation is
velocity modulation.
• Thus the device used for amplifying microwave signals is known as Two-cavity
Klystron and the device used for oscillating microwave signals is known as Single-cavity
Klystron
• In the year 1937, American engineers Russell and Sigurd Varian developed klystrons.
• In Klystrons the kinetic energy of a moving electron beam is utilized for amplifying
and generating microwave signals.
• TWT i.e., Travelling Wave Tubes are also used for amplification of RF signals and have
similar application as klystrons.
• But in TWT, a continuous interaction is maintained between the field and electron
beam. While in klystrons interaction between the two is allowed to occur only at the
cavities of the structure.
Klystrons are majorly classified as: Single cavity Reflex Klystron & Two-cavity Klystron
MICROWAVE TUBES: Single Cavity Reflex Klystron
MICROWAVE TUBES: Applegate Diagram
 Performance parameters & Applications
Performance parameters:
• The operating frequency range generally offered is 1 to 20 GHz.
• It delivers output power in the range 10mW to 2.5 W.
• The tuning range of klystron lies between 5 GHz at 2W to 30 GHz at 10 mW.
• Theoretically, the efficiency is considered 22.78% while practically the achieved
efficiency is only 10 to 20%.
Applications
As reflex klystrons are oscillators thus find applications in
• local oscillators for receivers,
• radar receivers,
• radio receivers.
• signal sources in microwave generators and
• pump oscillators of parametric amplifiers.
Two cavity Klystron : Structure

Collector
or anode
 Reflex Klystrons : Structure
• As we can see that the above figure consists of 2 cavities namely Buncher-
cavity and Catcher-cavity.
• The RF signal (I/P)to be amplified is provided at the Buncher-cavity.
• The electron gun comprises cathode, heating element and anode.
• The electron beam is produced by the cathode by making use of heating element
and the high positive potential at the anode provides required acceleration to the
electron beam initially.
• The region between two cavities is known as drift space.
• To generate high-density focused electron beam inside the tube, an external
electromagnetic winding is used that generates a longitudinal magnetic field.
• This is done in order to prevent the spreading of the beam inside the tube.
• The amplified RF signal is achieved at the catcher cavity.
• Also, a collector is present near the second cavity that collects the electron bunch.
 Working of Two-cavity Klystron Amplifier
• As we can see that the above figure consists of 2 cavities namely Buncher-
cavity and Catcher-cavity.
• Initially, electrons are emitted from the electron gun and the anode present in the
structure provides the desired acceleration to the beam.
• In the absence of any RF input, the electron will tend to move with their
respective uniform velocities to reach the catcher cavity and gets collected at the
collector.
• But when external RF signal is applied at the input of the buncher cavity then
this causes the generation of a local electric field inside the tube.
• This electric field causes the bunching of electrons as the field applies acceleration
and deceleration to the moving electron, according to the polarity of the signal by
which the field is generated.
• Basically the reason for causing acceleration and deceleration is that when the
direction of movement of electron is opposite to the direction of the field, then, in
this case, the electrons experience a decrease in their moving velocity.
 Two cavity Reflex Klystron : Applegate Diagram

Buncher grid

Catcher grid
Catcher
grid
 Performance parameters & Applications
Performance parameters:
• Frequency : 250 MHz to 100 GHz (60 GHz nominal).
• Power : 10 kW – 500 kW (CW) 30 MW (pulsed).
• Power Gain : 15 dB – 70 dB (60 dB nominal).
• Bandwidth : Limited 10 – 60 MHz Generally used in fixed Frequency Applications
Applications
The applications of klystron amplifiers involve in
• Satellite,
• High-energy physics,
• Wideband high-power communication,
• Radar, medical,
• Particle accelerators, etc.
• Klystron amplifiers can produce far superior outputs of microwave power compare with
Gunn diodes which are named as solid-state microwave devices.
Single cavity Vs Two cavity Klystron
 Travelling Wave Tube: Basics
Performance parameters:
• Travelling wave tubes are abbreviated as TWT.
• It is majorly used in the amplification of RF signals.
• Basically a travelling wave tube is nothing but an elongated vacuum tube that allows the
movement of electron beam inside it by the action of applied RF input.
• The movement of an electron inside the tube permits the amplification of applied RF
input.
• As it offers amplification to a wide range of frequency thus is considered more
advantageous for microwave applications than other tubes.
• It offers average power gain of around 60 dB.
• The output power lies in the range of few watts to several megawatts.
• A travelling wave tube is basically of two types;
• Helix type TWT
• Coupled cavity type TWT.
Travelling Wave Tube: structure
 Travelling Wave Tube: Construction
• As we can see that the helical travelling wave tube consists of an electron gun and
a slow-wave structure.
• The electron gun produces a narrow beam of the electron. A focusing plate is used that
focuses the electron beam inside the tube.
• A positive potential is provided to the coil (helix) with respect to the cathode terminal.
While the collector is more positive than the coil (helix). In order to restrict beam
spreading inside the tube, a DC magnetic field is applied between the travelling path by
the help of magnets.
• The signal which is needed to be amplified is provided at one of the ends of the helix,
present adjacent to the electron gun. While the amplified signal is achieved at the
opposite end of the helix.
• Attenuator is present along both the sides of the travelling wave tube to eliminate the
backward reflections.
Attenuators are basically formed by providing a metallic coating over the surface of the
glass tube by using Aquadag or Kanthal.
 Travelling Wave Tube: working
• The applied RF signal produces an electric field inside the tube. Due to the applied
positive half, the moving electron beam experiences accelerative force. However, the
negative half of the input applies a de-accelerative force on the moving electrons.
• This is said to be velocity modulation because the electrons of the beam are experiencing
different velocity inside the tube.
• However, the slowly travelling wave inside the tube exhibits continuous interaction with
the electron beam.
• Due to the continuous interaction, the electrons moving with high velocity transfer their
energy to the wave inside the tube and thus slow down. So with the rise in the
amplitude of the wave, the velocity of electrons reduces and this causes bunching of
electrons inside the tube.
• The growing amplitude of the wave resultantly causes more bunching of electrons while
reaching the end from the beginning. Thereby causing further amplification of the RF
wave inside the tube.
• Thus at the end of the tube an amplified signal is achieved.
• The positive potential provided at the other end causes collection of electron bunch at
the collector.
 Travelling Wave Tube: Performance and Applications
Performance characteristics:
• Low Power Amplifier: up to 10 kW
• Frequency Range: 3 G Hz – 50 G Hz
• Wide Band width: about 800 MHz
• Power gain: upto 60 dB
• Efficiency: 20 – 40 %
Applications:
• TWT is used in microwave receivers as a low noise RF amplifier.
• TWTs are also used in wide-band communication links and co-axial cables as repeater
amplifiers or intermediate amplifiers to amplify low signals.
• TWTs have a long tube life, due to which they are used as power output tubes in
communication satellites.
• Continuous wave high power TWTs are used in Tropo-scatter links, because of large
power and large bandwidths, to scatter to large distances.
• TWTs are used in high power pulsed radars and ground based radars.
Wave modes and propagation constantsPropagation
Constants:
Wave corresponding to γ1 is a forward wave and its amplitude
increases exponentially with distance

Wave corresponding to γ2 is also a forward wave but its

amplitude decays exponentially with distance.

Wave corresponding to γ3 is also a forward wave but its amplitude

remains constant with distance.

Wave corresponding to γ4 is a backward wave but its amplitude

remains constant with distance.
Example problem
 Travelling Wave Tube: Slow wave structures
• In microwave tube to generate or to amplify the RF and microwave signal at larger
power over a wider bandwidth non resonant periodic circuits or slow-wave structures
are used.
• These structures are used to reduce the wave velocity in a certain direction so that the
slow moving electrons of electron beam can interact with the fast moving
electromagnetic wave.
• It is well known that the phase velocity of the EM wave in the waveguide is larger than
velocity of the light in vacuum.
• Electron beam used in the vacuum tube can be accelerated only to the velocities that are
far less than the velocity of the light.
• Due to this electrons of electron beam does not get sufficient time to interact with the
wave.
• In order to reduce the velocity of the wave so that the electrons can get sufficient time
to interact with wave, wave is passed through the slow wave-structure and thus the
phase velocity of the wave is reduced.
 Slow wave structures-types
Examples of SLOW WAVE structures;
Amplifiers: Travelling Wave Tube VS Klystron structures
 CROSSED-FIELD TUBES (M-TYPE TUBES)
• In linear beam tubes like Klystron or Travelling wave tube (TWT) , the dc Magnetic
field parallel to the dc Electric field is used to focus the electron beam .
• Crossed-field tubes derive their name from the fact that the dc magnetic field is
perpendicular to the dc electric field . These tubes are also called M-Type tubes.
• In a crossed-field tube, the electrons emitted by the cathode are accelerated by the
electric field and gain velocity.
• If an RF field is applied to the circuit , those electrons entering the circuit during
retarding field are decelerated and give up some of their kinetic energy to the RF
field.
• Consequently , their velocity is decreased and these slower electrons will then travel
the dc electric field far enough to regain essentially the same velocity as before.
• Those electrons entering the circuit during the accelerating field are accelerated by
means of receiving enough energy from the RF field and are returned back towards
the cathode.
• This back bombardment of the cathode produces heat in the cathode and decreases
the operational efficiency.
Magnetron Oscillators: M-Type tube: Magnetron
• Hull invented magnetron, but it was only on interesting laboratory device.
• During the world war II an urgent need for high power microwave generators for
RADAR transmitters led to the rapid development of Magnetron
• Magnetrons provide microwave oscillations of very high frequency
• All magnetrons consists of some form of anode & cathode operated in dc Magnetic
field between cathode & anode.
• Because of cross field between cathode & anode , the electrons emitted from
cathode are influenced by the cross field to move in a curved path.
• If the dc magnetic field is strong enough the electrons will not arrive at in the
anode but return to the cathode, consequently anode current is cutoff.
 CROSSED-FIELD TUBES (M-TYPE TUBES)
Magnetrons can be classified in to three types as follows,
1. Negative resistance Magnetrons or Split-Anode Magnetron :
Make use of static negative resistance between two anode segments. Low
efficiency and are useful only at low frequencies (< 500 MHz).
2. Cyclotron-frequency Magnetrons :
Operates under the influence of synchronism between an alternating component
of electric field and periodic oscillation of electrons in a direction parallel to this
field.
Useful only for frequencies greater than 100 MHz
3. Cavity or Traveling-wave Magnetrons :
Depends upon the interaction of electrons with a traveling electromagnetic field of
linear velocity.
These are customarily referred as Magnetrons
Provide oscillations of very high peak power and hence are useful in radar
applications
 MICROWAVE TUBES
The detailed diagram of cavity magnetrons is,
 MICROWAVE TUBES
Construction:
• Each cavity in the anode acts as an
inductor having only one turn and the
slot connecting the cavity and the
interaction space acts as a capacitor.
• These two form a parallel resonant
circuit and its resonant frequency
depends on the value of L of the
cavity and the C of the slot.
• The frequency of the microwaves
generated by the magnetron oscillator
depends on the frequency of the RF
oscillations existing in the resonant
cavities. Cross sectional view of
anode assembly can be viewed as,
 working

If the magnetic field is absent, i.e. B = 0

If the magnetic field is absent, i.e. B > 0 and B < Bc

If the magnetic field is absent, i.e. B > 0 and B = Bc If the magnetic field is absent, i.e. B > 0 and B > Bc
 Forms of the anode block in a magnetron
• It generally consists of an even
number of microwave cavities
arranged in radial fashion.
• The form of the cavities varies,
shown in the figure .
a) slot- type
b) vane- type
c) rising sun- type
d) hole-and-slot- type
 Magnetron modes of oscillations
• The total phase shift around the ring of this cavity
resonators should be 2nπ where n is an integer.
• If ϕv represents the relative phase change of the
AC electric field across adjacent cavities, then
ϕv=2πn/N
• Where n=0,±1,±2,±(N/2−1),±N/2
• Which means that N/2 mode of resonance can
exist if N is an even number.
• If, n=N/2 then ϕv=π
• This mode of resonance is called as π−mode.
• If n=0 then ϕv=0
• This is called as the Zero mode, because there will
be no RF electric field between the anode and the
cathode.
• This is also called as Fringing Field and this mode
is not used in magnetrons.
Strapping rings: Mode strapping
• The frequency of the π mode is separated
from the frequency of the other modes by
strapping to ensure that the alternate
segments have identical polarities.
• For the π mode, all parts of each strapping
ring are at the same potential; but the two
rings have alternately opposing potentials.
• For other modes, however, a phase
difference exists between the successive
segments connected to a given strapping
ring which causes current to flow in the
straps.