Q#1(a): Operating principles: y
y
Klystron: Electrons are formed into a beam that is velocity modulated by the input waveform to produce microwave energy. Magnetron:
Electrons are emitted from cathode and interact with an electric field and a strong magnetic field to generate the microwave energy. Structures: y
A
K lystron: lystron:
klystron consists of four parts: 1. A beam source (electron gun or cathode) 2. Buncher (velocity modulating unit) 3. Accelerator 4. Catcher (beam stop)
Explanation: The
electron gun (cathode) produces the flow of electrons. The bunching cavities regulate the speed of electrons so that t hat they arrive in bunches at the output cavity. The bunch of electrons excites microwaves in the output cavity of the klystron. The microwaves flow into the waveguide, which transports them to the accelerator. The electrons are absorbed in the beam stop or catchers. The collector electrode provides an external path to the electrons to return to the cathode y
Magnetron:
Magnetron
consists of four parts:
1. Anode 2. Filament/cathode 3. Antenna 4. magnets
Explanation: The ANODE (or
plate) is a hollow cylinder of iron from which an even number of anode vanes extends inward. The open trapezoidal shaped areas between each of the vanes are resonant cavities that serve as tuned circuits and determine the output frequency of the tube. The anode operates in such a way that alternate segments must be connected, or strapped, so that each segment is opposite in polarity to the segment on either side. In effect, the cavities are connected in parallel with regard to the output. The FIL AMENT (also
called heater), which also serves as the cathode of the tube, is located in the center of the magnetron, and is supported by the large and rigid filament leads, which are carefully sealed into the tube and s hielded. The ANTENNA
is a probe or loop that is connected to the anode and extends into one of the tuned cavities. The antenna is coupled to the waveguide, a hollow metal enclosure, into which the antenna transmits the RF energy. IC FI E L D The MAGNET
is provided by strong permanent magnets, which are mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode.
Power level
Efficiency
Operating frequency
Klystron
50 MW
30-70 %
Upto 35 GHz
Magnetron
40 MW
40-70%
Upto 95 GHz
1(b): resonant cavity forms a means of storing electromagnetic energy, at a particular frequency within a small bandwidth. In a klystron, the RF circuits are resonant cavities which act as transformers to couple the high-impedance beam to low-impedance transmission lines. A
The y
operation takes place in the following manner: A beam of electrons is produced by a thermionic cathode and accelerated by high-voltage electrodes
y
This
beam is then passed through an input cavity. RF energy is fed into the input cavity at, or near, its natural (resonant) frequency to produce a voltage which acts on the electron beam.
y
The
y
The
y
The
electric field causes the electrons to bunch
RF current carried by the beam will produce an RF magnetic field, and this will in turn excite a voltage across the gap of subsequent resonant cavities. spent electron beam, with reduced energy, is captured in a collector.
Comparison of reflex and multi-cavity klystron: Multi-cavity
klystron:
Multi-cavity
klystrons have additional 'idler' cavities between the input and output cavities and these enhance the velocity modulation of the beam as they cause further acceleration or retardation of the beam.
Reflex klystron:
In a reflex klystron there is just one cavity followed by a reflector that returns the beam back through the cavity. By optimizing the phase of these returned electrons the device will oscillate at the cavity frequency.
1(c): K lystron: y
power amplifier tube used to amplify weak microwave energy (provided by a radio-frequency exciter) to a high power level for a radar transmitter. A
klystron is characterized by high power, large size, high stability, high gain and high operating voltages.
y
A
y
A
y
Klystron provide a coherent transmitted signal
klystron is referred to as a linear beam tube because the direction of the electric field that accelerates the electron beam coincides with the axis of magnetic field, in contrast to a cross-field tube such as magnetron.
Magnetron:
magnetron is a self-excited oscillator used as radar transmission tube.
y
A
y
Magnetrons
y
Magnetrons
y
Magnetron
are characterized by high peak power, small size, efficient operation and low operating voltages. are referred to as cross-field tubes because the direction of electric field that accelerates the electron beam is perpendicular to the axis of magnetic field. is not coherent transmission source.
How
does magnetron uses electric and magnetic field to produce microwave power? The
microwave radiation of microwave ovens and some radar applications is produced by a device called a magnetron. The
magnetron is called a "crossed-field" device because both magnetic and electric fields are employed in its operation, and they are produced in perpendicular directions so that they cross. The applied magnetic field is constant and applied along the axis of the circular device. The power to the device is applied to the center cathode which is heated to supply energetic electrons which
would, in the absence of the magnetic field, tend to move radially outward to the ring anode which surrounds it. Electrons are released at the center hot cathode by the process of thermionic emission and have an accelerating field which moves them outward toward the anode. The axial magnetic field exerts a magnetic force on these charges which is perpendicular to their initially radial motion, and they tend to be swept around the circle. In this way, work is done on the charges and therefore energy from the power supply is given to them. As these electrons sweep toward a point where there is excess negative charge, That charge tends to be pushed back around the cavity, imparting energy to the oscillation at the natural frequency of the ca vity. This driven oscillation of the charges around the cavities leads to radiation of electromagnetic waves, the output of the magnetron.
