Principle And Operation Of Magnetrons And Travelling Wave Tube
Magnetron is a grouping of a simple diode vacuum tube together with built in cavity resonators and an exceptionally powerful magnet. There are three types of magnetrons:
- Negative resistance type
- Cyclotron frequency type
- Travelling wave or Cavity type
Negative resistance magnetrons make use of negative resistance between two anode segments.
Cyclotron frequency magnetrons depends upon synchronism amid an alternating component of electric field and periodic oscillation of electrons in a direction parallel to this field.
Cavity type magnetrons depends upon the interface of electrons with a rotating electromagnetic field of constant angular velocity.
A magnetron consist of a cathode which is used to release the electrons and number of anode cavities and a permanent magnet is placed on the flipside of cathode and the space between the anode cavity and the cathode is called interacting space.
The electrons which are emitted from the cathode moves in diverse path in the interacting space depending upon strength of electric and magnetic fields applied to the magnetron.
Depending upon the relative strength of the magnetic and electric field the electrons released from the cavity move towards the anode will navigate through the interacting space.
- Effect of electric field only
- In the nonexistence of magnetic field (B=0) ,the electrons move directly from the cathode to the anode due to the radial electric field force acting on it .
- If the magnetic field strength increases vaguely (i.e for moderate value of B),it will apply a lateral force bending the path of electron .
- If the strength of the electrons is made adequately high enough so as to avert the electrons from reaching the anode ,the anode current becomes zero.
- The magnetic field required to return electrons back to cathode just gazing the surface of the anode is called the critical magnetic field or cut-off magneticf field(Bc)
- If the magnetic field is larger than critical Magnetic field(B>Bc),the electrons experiences a greater rotational force and may return back to cathode quite faster .This results in heating of cathode.
Effects of magnetic fields only
- The magnetic field is applied perpendicular to electric field hence is is called axial magnetic field, therefore the magnetrons are called cross field device because of radial electric field and axial magnetic field are perpendicular to each other.
- If the magnetic field strength is more than the electrons emitted will return back to cathode with high velocity which may destroy the cathode cavity this effect is called back heating of cathode.
Combine effect of electric field and magnetic field
- When both fields are existing in that case the electron may have dissimilar path depending upon the strength of E and H.
- If E>H ,in that case the electrons reach at the anode but the path will be bend because of small magnetic field.
- If H>E,in this case the electrons return back to the cathode.
MECHANISM OF OSCILLATION IN MAGNETRON ( ∏ mode of Oscillation)
- It is understood that the oscillation are capable of starting a device because of high quality factor.
Q=fc/BW(For oscillator BW→0,Hence Q→∞)
The self oscillation can be maintained if the phase dissimilarity between anode cavity is N∏/4 and the best result can be obtained for N=4.
Hence,it is called ‘∏’ mode oscillation .
TWT( TRAVELLING WAVE TUBE)
The travelling wave tube is a new sort of tube which displayed considerable promise as a board band amplifier proposed by Pierce and others in 1946.
TWT AS AMPLIFIER
- The travelling wave tube is an amplifier which makes use of a distributed interaction between an electron beam and travelling wave.
- In case of TWT the interacting space is extended for the total length of the tube so that bunching takes place continuously.
- The basic structure of a TWT consist of a cathode to emit the electrons and input waveguide and the output waveguide so as it give input signal and to take out the RF output.
- A magnetic field is applied so as to focus the electrons along the center of the helical structure.
- Attenuators are provided to absorb the reflected electrons so as to avoid occurrence of any oscillation along the tube due to impedance variance.
SLOW WAVE STRUCTURE
- Generally the phase velocity (Vp) of input signal (RF signal which is a microwave signal) is larger than the speed of light ‘c’ , therefore for bunching to be possible both must move in the same direction with the same velocity. Thus the Rf signal is made to pass through slow wave structure so the axial component of the phase velocity almost equal to Ve.
- TWT being an linear beam device mainly used in microwave amplifier in which the bunching occurs continuously and to extend the interaction we introduce slow wave structure so that the electrons beam RF signal move with same velocity in the same direction .
- The electrons which move against the negative half cycle of the RF signal will bunch together and transported energy to the RF and the bunching process is continues and we obtain the amplified output across the output waveguide which is shown below
GAIN CHARACTERISTICS OF SLOW WAVE STRUCTURE
The output gain is defined as,
Ap=10 log|output power/input power|²
Ap=-9.54 + 47.3 NC db
where ,N=electric wavelength
C= gain parameter=(Io Zo/4Vo)^1/3
where ,Io=dc beam current
vo=dc beam voltage
Zo= characteristics impedance
QUESTIONS AND ANSWERS
Q)A travelling wave tibe (TWT) has the folliwing characteristics dc beam current(Io)=4mA, beam Voltage(Vo)=2KV ,characteristics impedance(Z0)=20Ω,frequency f=8GHz.Calculate the gain parameter and the power in db given the circuit length N=50?
Ans) gain parameter ,C=(Io Zo/4Vo)^1/3=(4×10^-3×20/4×2×10³)=2.16×10^-2
power in db= -9.54 +47.3× (50)×(2.16×10^-2)=41.46 db
Q)Distinguish between between TWT and klystron?
|In case of TWT the field travels along with the electron beam.||Field is stationary only electron beam travels.|
|The interaction of the electron beam and the RF field is continous over the entire length.||In this case the interaction take place only at the gaps of afew resonant cavity.|
|The input wave in TWT is propagating wave||The input waveis not propagating wave|
|The TWT is a non resonating microwave circuit.||Klystron is a resonating microwave circuit.|
Q)Explain bunching Process?
Ans:Bunching is a process by which the electrons take energy from the cavity at different time and delivered to the cavity at the same time as shown in fig
Q:Comparison between reflex klystron and Magnetron?
- Klystrons are linear tube in which the magnetic field is applied to focus the electron and the electric field is applied to drift the electron.But,in case of magnetron both the electric and magnetic fields are perpendicular to each other ,hence is called a cross field device.
- In klystron the bunching takes place only inside the cavity which is very small width hence ,generate low power and low efficiency.But,in magnetron the interacting space or the bunching space is extended ,so efficiency can be increased.
- Magnetron are mainly used as oscillator in radar communication for telemery operation and also used in missiles.
Q:Why magnetron is called cross field device?
Ans:In case of magnetron both the electric field and magnetic field are perpendicular to each other ,hence is called a cross field device
Q:What are the applications of TWT?
- Repeater amplifier in wideband communication link.
- Medium and high power satellite transponder output.
- Work as CW (continous Wave)radar &radar jamming system
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