On completion of this module, you will be able to:

• Passive and Active Microwave Devices
• Understand directional coupler and attenuator
• Define Microwave Diodes
• Explain Varactor Diode and its working
• Explain Tunnel Diode and its working
• Explain Gun Diode and its working
• Explain Avalanche effect
• Explain MASER
• Understand different types of Microwave tubes

Passive and Active Microwave Devices

• Active and passive microwave devices and components are the essential building blocks of microwave circuits and systems that operate in the frequency range from 300 MHz to 300 GHz.
• Passive Microwave Devices may be composed of lumped elements like inductors, capacitors and resistors or distributed elements like transmission line sections and discontinuities or both.
• Passive Microwave Devices are used extensively in any microwave communication system. It includes Directional Coupler, Power Divider, Attenuator, Resonator etc.
• Active Microwave Devices are used for signal detection, mixing, amplification, frequency multiplication, switching and as sources of RF and microwave signals.
• Microwave Active Components are Microwave Diodes, Transistor, Microwave Tubes etc.

Directional Coupler

• Directional Coupler is a passive network that is used to measure the microwave power delivered to the load. So it samples a small amount of microwave power for measurement purposes.
• It is a device that passes signal through in only one direction to output port and a certain amount of the input signal (usually a very small portions of the input signal) through another port (Coupled port).
• Basic functionality of directional coupler is to pass the signal in only one direction but there are some other types of directional coupler that passes the signal in both directions. This type of directional coupler is called Bi-directional coupler / Dual directional coupler.
• They can be used to measure incident and reflected power, standing wave ratio (SWR), provides a signal path to receiver or to perform other desirable operations.
• Directional Coupler is a 4-port waveguide junction consisting of a primary main waveguide and a secondary auxiliary waveguide. The following figure shows the image of a directional coupler
• Directional coupler is used to couple the microwave power which may be unidirectional or bi-directional.



Fig. : Directional Coupler

Types of Directional Couplers

• There are so many different types of directional couplers and some of the examples are shown below. (A) and (B) are the common types that are frequently used in daily RF testing.
• (C) and (D) are the waveguide types of the directional couples which would be used in very high frequency or high power.



Fig. : Types of Directional Coupler

Properties of Directional Couplers

• The properties of an ideal directional coupler are as follows.
• When the power travels from Port 1 to Port 2, some portion of it gets coupled to Port 4 but not to Port 3.
• As it is also a bi-directional coupler, when the power travels from Port 2 to Port 1, some portion of it gets coupled to Port 3 but not to Port 4.
• If the power is incident through Port 3, a portion of it is coupled to Port 2, but not to Port 1.
• If the power is incident through Port 4, a portion of it is coupled to Port 1, but not to Port 2.
• Ideally, the output of Port 3 should be zero. However, practically, a small amount of power called back power is observed at Port 3.
• The following figure indicates the power flow in a directional coupler.


Fig. 2: Properties of Directional Couplers

• Where
• P_i = Incident power at Port 1
• P_r = Received power at Port 2
• P_f = Forward coupled power at Port 4
• P_b = Back power at Port 3

Coupling Factor C :

• It is defined as the ratio of incident power(P_i) to the forward power(P_f), measured in dB

Directivity D :

• The Directivity of a directional coupler is the ratio of forward power(P_f) to the back power(P_b), measured in dB

Isolation :

• It defines the directive properties of a directional coupler. It is the ratio of incident power(P_i) to the back power(P_b), measured in dB.

Attenuator

• Signals are sent from one place to another through a medium. These signals can be data signals, voltage signals, current signals, etc. When the distance travelled by the signal increases the strength of the signal gradually decreases. This gradual loss of intensity of signals through the medium is called Attenuation.
• A device used to control the amount of microwave power transferred from one point to another point on a microwave transmission is called microwave attenuator


Fig. 2: Attenuator

• So, Attenuator is a two port microwave passive device used to control power levels in a microwave system by partially absorbing the transmitted signal
• Attenuators can be classified as fixed and variable attenuators.
• Resistive films are used in the design of both fixed and variable attenuators.
• Attenuators control the power level in desired direction
• Explain the Reflex Klystron with Applegate diagram

Fixed attenuator

• It provides only fixed amount of attenuation. It consists of dissipative element called pad and it is placed in a waveguide
• Pad is placed in such a way that its plane is parallel to the electric field. For this two thin metal rods are used.

• The pad is tapered, which provides a gradual transition from waveguide medium to absorbing medium of pad. It also reduces reflection
• The amount of power that a fixed attenuator can absorb depends on Strength of dielectric field, Location of pad within waveguide, Frequency of operationArea of pad and pad material used for power absorption

Variable Attenuator

• It provides continuous attenuation
• The amount of attenuation is controlled by depth of insertion of absorbing plate inside the waveguide
• For this a knob and gear assembly is used. Knob can be calibrated suitably. The maximum attenuation will be offered when the pad extends all the way across the guide
• Types of variable Attenuators -
  1. Rotary attenuator
  2. Electronically controls( PIN diode , FET attenuator)

Microwave Diodes

• Diodes are two-terminal, nonlinear semiconductors used for generating, mixing, detecting and switching of microwave signals
• The first diodes where point-contact diode used in crystal radios, 100 years ago.
• The schematic signal for diode is shown below -


Fig. 1: Diode and its Schematic Symbol

• In general diode will conduct when the anode voltage is higher (more positive) than the cathode voltage.
• Most diodes used in the microwave industry are made on silicon but in some amplification Gallium Arsenide(GaAs) is better choise.
• Microwave diodes are diodes that mainly work in the microwave frequency band.
• Such as Barrier Injection Transit Time Diode (BARITT), Impact Avalanche Transit Time Diode (IMPATT), Gunn Diode, Trapped Plasma Avalanche Diode (TRAPATT), Tunnel Diode, Varactor Diode etc.
• All of these diodes use the negative resistance effect to directly convert DC electrical energy into radiant microwave energy.

2.2 Varactor Diodes.

• Varactor diode is one kind of semiconductor microwave solid state device and applicationof this diode mainly involve in where variable capacitor is preferred.
• These diodes are also named as Variacap diodes.
• Varactor diodes are specificlly fabricated and optimized such that they permit a high range of changes in capacitance.
• The symbols of varactor diode is shown below that includes a capacitor symbol at one end of the diode that indicates the variable capacitor of the varactor diode.

Working of Varactor Diodes.

• We know that the capacitance of a capacitor is directly proportional to the region of the terminal. As the region of the terminal increases the capacitance of the capacitor increases.
• The depletion area between the P-type and N-type regions can be considered as the insulating dielectric.
• The volume of the depletion region of the diode varies with change in reverse bias.
• If the reverse voltage of the diode is increased, then the size of the depletion region increases and vice-verce.
• Hence by changing the reverse bias of the diode the capacitance can be changed

Characteristics of Varactor Diodes.

• These diodes significantly generate less noisy compared to other diodes.
• The cost of these diode is available at lower and more reliable also.
• These diodes are very small in size and very light weight.
• There is no use when it is operated in forward bias. In reverse bias mode, the varactor diode enhances the capacitances as shown below.

Applications of Varactor Diodes: Main application of Varactor diodes are as folows:

• Pulse generation and pulse shaping.
• Switching circuit and modulation of microwave signal.
• Harmonic generation.
• Tuning stage of radio receiver.
• Low noise amplification.
• Active filter.
• Microwave frequency multiplication.

2.3 Tunnel Diodes

• A Tunnel diode is also known as Eskai diode. it is a highly doped semiconductor that is capable of very fast operation.
• It work on the principle of the Tunneling effect.
• It symbol of tunnel diode is hsown below:-.

• Based on the classical mechanics theory a particle must acquire energy which is equal to the potential energy barrier height, it has to move from one side of the barrier to other.
• Otherwise energy has to be supplied from some external source, so that N-sided electron of the junction can jump over the junction barrier to reach the P-side.
• If the barrier is thin, then there is a large amount of propability to penetrate the electron through the barrier.
• This process will happen without over energy loss and it is known as tunneling.
• The tunneling phenomenon provides a majority carrier effect.
• This phenomenon is called tunneling and hence the Esaki Diode is known as Tunnel diode.
• The tunnel diode has a region in its voltage current characteristics where the current decreases with increased forward voltage known as its negative resistance region.
• The characteristics makes the tunnel diode useful in oscillators and microwave amplifier.
• The tunnelling effect gives the tunnel diode a negative resistance region and this enable it to be used as an oscillator and also in pre-amplifier amplification at frequency well into the microwave region.

• Due to forward biasing, because of heavy doping condition happens in the diode
• The maximum current that a diode reaches is I_P and voltage applied is V_P.
• The current value decreases when more amount of voltage is applied.
• Current keeps decreasing until it reaches a minimul value. The samll minimul value of current is I_V.
• From the above graph it is seen that from point A to B current reduces when voltage increases. that is the negative resistance region of the diode..
• In this region tunnel diode produces power instead of absorbing it.

Applications of Tunnel Diode: Main application of Varactor diodes are as folows:

• Tunnel diode can be used as a switch, amplifier and oscillator.
• Since it shows a fast response it is used as a high frequency component.
• It is used as a logic memory storage device.
• In satellite communication equipment they are widely used.
• Due to its feature of -Ve resistance it is used in relaxation oscillator circuit..

Gunn Diodes

• Gun diode is a two terminal electronic device which is composed of only one type of doped semiconductor i.e. N-region
• The unique property of Gunn diode is that it work in Negative Differential Resisatance region which means it can be used to genrate microwave frequency of 0 to 100 GHz.
• The Gunn diode is is also known as Transferred Electronic Device(TED) because it is composed of N-region and N-region semiconductor has electron as a majority carrier.
• A negative differential resistance means the relationship between volatage and current is out of phase(180⁰.
• Most widely used standard symbol for Gun Diode is shown below

Construction of Gunn Diodes

• Most widely used material for the construction of the gun diode is Gallium Arsenide(GaAs) and Indium Phosphide(InP).
• The diode is fabricated from a single N-type semiconductor. It has three layers of N-type semiconductor.
• Among the three layer the 1st and IIrd layer is widely doped of N-type semiconductor and IInd or middle layer is lightly doped as compared to Ist and IIIrd layer.

Working of Gunn Diodes

• In Gun diodethere is no P-region and no junction. But still it is called a diode due to the presence of two electrodes in the construction of this diode.
• When external voltage is applied to this diode, the entire voltage appears in the active region. Here active region is referred to as a middle layer of the device.
• Due to which the electron from the Ist layer of the conduction band are transferred into the third layer of the Valance band.
• Mobality of electron in IIIrd layer is less compared to Ist layer.
• When the electron have transferred from the conduction band to the valance band after some threshold value the current through the device start decreasing.
• Due to this the effective Mass of electrons starts increasing and thus mobility start decreasing due to which the current start decreasing.
• This creates the Negative Differential Resistance Region in the Gun Diode.
• In Negative differential Resistance Region the current and voltage have an Inverse Relationship Which means When the current starts to increase the voltage starts to fall and vice-verca.
• Thus it generates pulses with 180⁰ phase reversal and thus this device is able for the operation of Amplifier and Oscillator circuit

Advantages of Gun Diode

• The manufacturing cost of Gun diode is low.
• Gun diode is highly reliable.
• Its installation in circuit is easy
• It exhibits comparatively low operating voltage than normal diodes.

Disadvantages of Gun Diode

• These are lesss stable.
• The efficiency of Gun diode is very low.
• Some times noise effects are more in case of Gun diode.

Applications of Gun Diode

• Oscillators
• Amplifiers
• In ultrasonic detectors.
• Tachometer radio communication systems.
• They are used as Traffic Analyzer sensors.
• They are used in Vehicle ABS system.

Avalanche Effect Diodes

• An avalanche diode is a type of semiconductor diode which is designed to experience avalanche breakdown at a specified reverse bias voltage.
• Avalanche diode consists of two terminals: anode and cathode. The symbol of avalanche diode is same as Zener diode which is shown in below figure.

• A normal p-n junction diode allows electric current only in forward direction whereas an avalanche diode allows electric current in both forward and reverse directions.
• However, avalanche diode is specifically designed to operate in reverse biased condition.
• When forward bias voltage is applied to the avalanche diode, it works like a normal p-n junction diode by allowing electric current through it.
• Avalanche diode allows electric current in reverse direction when reverse bias voltage exceeds the breakdown voltage.
• The point or voltage at which electric current increases suddenly is called breakdown voltage.
• When the reverse bias voltage applied to the avalanche diode exceeds the breakdown voltage, a junction breakdown occurs. This junction breakdown is called avalanche breakdown.
• When the reverse voltage applied to the avalanche diode continuously increases, at some point the junction breakdown or avalanche breakdown occurs.
• At this point, a small increase in voltage will suddenly increases the electric current. This sudden increase of electric current may permanently destroys the normal p-n junction diode.
• However, avalanche diodes may not be destroyed because they are carefully designed to operate in avalanche breakdown region.

Applications of Avalanche Diode

• Avalanche diodes can be used as white noise generators.
• It is used to protect the circuit against unwanted voltages.
• It is used in surge protectors to protect the circuit from surge voltage.

M A S E R

• A MASER is a device that produces coherent Electromagnetic waves through amplification by Stimulated Emission.
• The word MASER stands for Microwave Amplification by Stimulated Emission of Radiation.
• It was invented by the scientists Charles Townes and Arthur Schawlow in 1953.
• It is been used to generate infrared frequency. It works based on Population Inversion and Stimulated Emission
• Population Inversion:Stimulated emission is the basis of the Laser but it need many more atoms in the excited state than the ground state. This is called population inversion and it is achieved by pumping.
• Stimulated Emission:Emission Stimulated by another photon. The emitted photon is in the same direction and in phase with the incident photon.
• The basic principle is the ecitation of atom at the ground state(E₀) to a higher energy state (E₂) with the help of pump source
• When these excited electron simultaneously jump to the ground state, they release the amount of energy they absorb from the pump source.
• Now if the pump frequency is equal to the energy of transition then,
V = (E₂ - E₀)/h
Where h = Plank's constant

• The energy will add up and microwave amplification takes place such as MASER are called two level MASER.

Microwave Tubes

• For the generation and amplification of microwaves, there is a need of some special tubes called as microwave tubes.
• The basic principal of operation of the microwave tubes involves transfer of power from a source of ac voltage by means of a current density modulation electron beam.
• The same is achieved by accelerating electron in a static electric field and retarding them in ac field.
• The various types of tubes are
  1. Klystrons
  2. Reflex Klystron
  3. Travelling wave tube
  4. Magnetron

Klystron

• A klystron amplifier is a specific linear beam Vaccum tube, which is used as an amplifier for high resonant frequencies.
• A klyston is a vaccum tube that can be used either as a generator or as an amplifier of power at microwave frequencies.
• A klystron include one or more cavity, which control the electric field around the axis of the tube
• Based on the resonant cavity, klystron amplifier classified into to types:
  1. Two cavity klystron amplifier
  2. Reflex klystron amplifie

Fig. 1.1: Klystron Tube

• A grid is placed in the middle of these cavities to allow the electrons to flow.
• It operates by the principle of velocity and current modulation.
• It consists of two cavity as shown above in figure.
• The cavity closed to the cathode is known as buncher cavity or input cavity, which velocity modulates the electron beam the other cavity is known as catcher cavity or input cavity it catches energy from the bunched electron beam.

Two Cavity Klystron

• A klystron is a vaccum tube that can be used either as a generator or as an amplifier or as an oscillator at microwave frequencies.
• The klystron is a linear beam device that is the electron flow is in a straight line focused by an axial magnetic field.
• The velocities of electrons emitted from the cathode are modulated to produce a density modulated electron beam

Fig. 2: Two Cavity Klystron Tube

Working

• The first grid next to the cathode controls the number of electrons in the electron beam and focuses the beam
• The velocity accelerating the DC electron beam to a high velocity before injecting it into the grids of the buncher cavity
• The grids of the cavity enable the electron to pass through, but they confine the magnetic fields within the cavity.
• The space between the grids is referred to as the interaction space, or gap
• Electron traversing the interaction space when the RF potential on grid 3 is positive with respect to grid 2 are accelerated by the field
• The decelerated electrons give up energy to the fields inside the buncher cavity, while those that have been accelerated absorb energy from its fields.
• Upon leaving the interaction gap, the electrons enter a region called the drift, or bunching, space, in which the electrons that were speeded up overtake the slower-moving ones. This causes the electrons to bunch and results in the density modulation of the beam
• The RF signal to be amplified is used for exciting the input buncher cavity thereby developing an alternating voltage of signal frequency across the gap A.
• Let us now consider the effect of this gap voltage on the electron beam passing through gap A. The situation is best explained by means of an Applegate diagram shown in Fig.

Fig. 2: Two Cavity Klystron Tube

• At point B' on the input RF cycle, the alternating voltage is zero and going positive. At this instant, the electric field across gap A is zero and an electron which passes through gap A at this instant is unaffected by the RF signal.
• Let this electron be called the reference electron eR which travels with an unchanged velocity

  v_o =√ 2eV ⁄ m

  where V is the anode to cathode voltage.
• Similarly, an early electron ee that passes the gap 'A' slightly before the reference electron eR is subjected to a maximum negative field.
• Hence this early electron is decelerated and travels with a reduced velocity vo. This electron ee falls back and reference electron eR catches up with the early electron.

Fig. 3: Apple gate diagram of klystron amplifier

Performance Characteristics

• Frequency – 250 MHz to 100 GHz (60 Hz normally)
• Power – 10 KW to 500 KW(CW) 30 MW (Pulsed)
• Power gain – 15 dB – 70 dB (60 dB normaly)
• Bandwidth – limited to 10 – 60 MHz
• Noise figure – 15 to 20 dB
• Theoretical efficiency – 58 %

Applications

• As power output tubes – In UHF TV transmitters, In satellite communication ground Station, Radar Transmitters
• A power oscillator – 5 0 50 GHz is used as a Klystron Oscillator

Reflex Klystron

• Reflex klystron is single cavity variable frequency microwave oscillator.
• Reflex klystron that works on reflection and oscillations in a single cavity.

Construction and Working of Reflex Klystron

• Reflex Klystron consists of an electron gun, a cathode filament, a cavity with a pair of grids and repeller plate.
• Accelerating grid are present which insists the electron to travel in linear beam.

• The cathode emits electrons which are accelerated forwarded by an accelerating grid with a positive voltage on it and focused into a narrow beam.
• The electron pass through the cavity and undergo velocity modulation which produces electron bunching and the beam is repelled back by a repeller plate kept at a negative potential with respect to cathode.
• On return the electron beam once again enters the same grids which acts as a buncher.

• Let us assume that a reference electron er crosses the anode cavity but has no extra velocity and it repels back after reaching the Repeller electrode, with the same velocity.
• let's say ee which has started earlier than this reference electron, reaches the Repeller first, but returns slowly, reaching at the same time as the reference electron.
• The late electron el, which starts later than both er and ee, however, it moves with greater velocity while returning back, reaching at the same time as er and ee.
• Now, these three electrons, namely er, ee and el reach the gap at the same time, forming an electron bunch. This travel time is called as transit time, which should have an optimum value.
• The optimum transit time is represented as.
T = n + 3/4 , Where n is an integer.
• This transit time depends upon the Repeller and anode voltages.

Applications of Reflex Klystron

• Reflex Klystron is used in applications where variable frequency is desirable, such as −.
• Radio receivers.
• Portable microwave links.
• Parametric amplifiers.
• Local oscillators of microwave receivers.
• As a signal source where variable frequency is desirable in microwave generators.

Travelling Wave Tubes (TWT)

• TWT is a high gain low noise wide bandwidth microwave amplifier.
• The unique feature of the TWT is a helix or coil that surround the length of the tube and the electron beam passes through the center or axis of the helix.
• Surrounding the tube are either permanent magnets or electromagnets that keep the electrons tightly focused into a narrow beam.

Construction of TWT

• Travelling wave tube is a cylindrical structure which contains an electron gun from a cathode tube.
• It has anode plates, helix and a collector. RF input is sent to one end of the helix and the output is drawn from the other end of the helix.
• The figure of TWT is shown below -

• A electron gun focuses an electron beam with the velocity of light.
• A magnetic field guides the beam to focus without scattering.
• Applied RF field propagated in helix, produces an electric field at the center of the helix.
• The resultant electric field due to applied RF signal, travels with the velocity of light multiplied by the ratio of helix pitch to helix circumference. The velocity of electron beam, travelling through the helix, induces energy to the RF waves on the helix.
• Thus, the amplified output is obtained at the output of TWT. The axial phase velocity Vp is represented as.
V_p=V_c(Pitch/2πr)
• Where r is the radius of the helix.

Working of TWT

• Helix is O-type tube which works on progressive application method.
• In case of helix, RF is fed by using inductive structure with pitch of Lambda which indicates existence of one complete RF cycle in every pitch.
• As electron beam passes through first pitch, it undergoes velocity modulation process but depth of modulation in first pitch is low because strength of RF is low in first pitch.
• As this dispersive electron bunch passes through second pitch it again undergoes velocity modulation process improving depth of modulation.
• So, at the end of helix, we get sharp electron bunch.
• As this electron bunch passes through helix, it delivers the power to every pitch improving amplification factor in every pitch.
• Amplifier gain can be controlled by controlling number of turns in the helix.

Specification of TWT

• Frequency of operation : 500 MHz – 95 GHz
• Power outputs: 5 mW (10 – 40 GHz – low power TWT)
• 250W (CW) at 3 GHz (high power TWT)
• 10 MW (pulsed) at 3 GHz
• Efficiency : 5 – 20 % ( 30 % with depressed collector)
• Noise figure : upto 25 dB

Applications of TWT

• Low noise RF amplifier in broad band microwave receivers.
• Repeater amplifier in wide band communication links and long distance telephony.
• Continuous wave high power TWT’s are used in troposcatter links (due to larger power and larger bandwidths).
• Used in Air borne and ship borne pulsed high power radars.
• A TWT is a vaccum tube that is used in wireless communication or satellite system to amplify radio frequency in the microwave range.

Magnetron

• A magnetron is a device that generates high power electromagnetic wave. It is a cross field tubes in which the electric and magnetic field cross. i.e. run perpendicular to each other.
• There are three main types of Magnetron.
1. Negative Resistance Type
2. Cyclotron Frequency Type
3. Travelling Wave or Cavity Type
• It is basically considered as a self-excited microwave oscillator and is also known as a crossed field device.
• The reason behind calling it so is that the electric and magnetic field produced inside the tube are mutually perpendicular to each other thus the two crosses each other.
• Negative Resistance:
• It makes use of negative resistance between two anode segments but has low efficiency.
• It is useful only at low frequencies( < 500 MHz ).
• Cyclotron Frequency Type:
• It depends upon up on synchronism between an alternating component of electric and periodic oscillation of electrons in a direction parallel to this field.
• These are useful only for frequency greater than 100 MHz.
• Travelling Wave or Cavity Type:
• It is a diode usually of cylindrical configuration with a thick cylindrical cathode at the center and a coaxial cylindrical block of copper as anode.
• In the anode, the blocks are cut a number of holes and slots which act as resonant anode cavities.
• The space between the anode and cathode is the interaction space and to one of the cavities is connected a coaxial line or waveguide for extracting the output.
• The permanent magnet is placed such that the magnetic lines are parallel to the vertical cathode and perpendicular to the electric field between cathode and anode.

• The cavity magnetron shown in Fig. has 8 cavities that are tightly coupled to each other. We know, in general that a N-cavity tightly coupled system will have N-modes of operation each of which is uniquely characterized by a combination of frequency and phase of oscillation relative to the adjacent cavity.

Operating Principle :

• The operating principle of a magnetron is such that when electrons interact with electric and magnetic field in the cavity then high power oscillations get generated.

Construction of Magnetron :

• The figure here shows a magnetron with 8 cavities.

• A cylindrical magnetron has a cylindrical cathode of a certain length and radius present at the centre around which a cylindrical anode is present. The cavities are present at the circumference of the anode at equal spacing.
• Also, the area existing between anode and cathode of the tube is known as interaction space/region.
• It is to be noted here that there exists a phase difference of 180⁰ between adjacent cavities. Therefore, cavities will transfer their excitation from one cavity to another with a phase shift of 180⁰.
• Thus we can say that if one plate is positive then automatically its adjacent plate will be negative.

Working of Magnetron

• The excitation to the cathode of the magnetron is provided by a dc supply which causes the emergence of electrons from it.
• Here in this section, we will discuss the working of magnetron under two categories.
1. Without RF input.
2. With the application of RF input.
• When RF input is not present:
• When the magnetic field is absent then the electron emerging from the cathode radially moves towards the anode. This is shown in the figure below:

• This is so because the moving electron does not experience the effect of the magnetic field and moves in a straight path.
• When a small magnetic field exists inside the magnetron then the electron emerging from the cathode will slightly deviate from its straight path. And this will cause a curvy motion of the electron from cathode to anode as shown in the figure:

• When the magnetic field is further increased then electrons emerging from the cathode gets highly deflected by the magnetic field. And graze along the surface of the cathode, as shown below:

• This causes the anode current to be 0. The value of the magnetic field that causes the anode current to become 0 is known as the critical magnetic field.
• If the magnetic field is increased beyond the critical magnetic field. Then the electron will bounce back to the cathode itself without reaching the anode.
• It is known as back heating. So to avoid this the electric supply provided to the cathode must be cut-off after oscillations have been set up in the tube.
• When the RF field is present:
• In case an active RF input is provided to the anode of the magnetron then oscillations are set up in the interaction space of the magnetron. So, when an electron is emitted from the cathode to anode then it transfers its energy in order to oscillate. Such electrons are called favoured electrons.
• In other cases, the emitted electron from the cathode while travelling takes energy from the oscillations thereby resultantly increasing its velocity.
• So despite reaching the anode, the electrons will bounce back to the cathode and these electrons are known as unfavoured electrons.
• When the RF input is further increased then the electron emitted while travelling increases its velocity in order to catch up the electron emitted earlier with comparatively lower velocity.
• All these favoured electrons form electron bunch or electron cloud and reaches anode from the cathode.
• The formation of electron bunch inside the tube is known as phase focusing effect.
• The movement of these favoured electrons inside the tube enhances the field existing between the gaps in the cavity. This leads to sustained oscillations inside the magnetron thereby providing high power at the output.

Specifications of Magnetron

• Frequency range : - 0.5 – 75 GHz
  
• Efficiency : - 40 % – 70 %
  
• Range of anode voltage:- 10 – 100 KV
  
• Anode current : - 10 – 100 A
  
• Cross Magnetic field :- 10 – 50 mWb/m2
  
• Duty Cycle : - 0.1 %
  
• Power output : - In excess of 250 KW(pulsed mode), 10mW( UHF band), 2 mW( band), 8 KW(at 95 GHz)
  

Advantages of Magnetron

• Magnetrons are a highly efficient device used for generation of the high power microwave signal.
  
• The use of magnetrons in radar can produce radar system of better quality for tracking purpose.
  
• It is usually small in size thus less bulky.
  

Disadvantages of Magnetron

• It is quite expensive.
  
• Despite producing a wide range of frequency, there exists a drawback in controllability of the generated frequency.
  
• It offers average power of around 1 to 2 kilowatts.
  
• Magnetrons are quite noisy.
  

Applications of Magnetron

• A major application of magnetron is present in a pulsed radar system in order to produce a high-power microwave signal.
  
• Magnetrons are also used in heating appliances likes microwave ovens so as to produce fixed frequency oscillations.
  
• Tunable magnetrons find their applications in sweep oscillators.