Magnetrons function as self-excited microwave oscillators. Crossed electron and magnetic fields are used in the magnetron to produce the high-power output required in radar equipment. These multicavity devices may be used in radar transmitters as either pulsed or cw oscillators at frequencies ranging from approximately 600 to 30,000 megahertz. The relatively simple construction has the disadvantage, that the Magnetron usually can work only on a constructively fixed frequency.
Physical construction of a magnetron
The magnetron is classed as a diode because it has no grid. The anode of a magnetron is fabricated into a cylindrical solid copper block. 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 cathode is indirectly heated and is constructed of a high-emission material. The 8 up to 20 cylindrical holes around its circumference are resonant cavities. The cavities control the output frequency. 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.
| resonant cavities | anode |
Figure 2: Magnetron МИ 29Г of the Bar Lock |
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| filament leads |
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cathode pickup loop |
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| Figure 1: Cutaway view of a magnetron | ||||
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.
Figure 3: forms of the plate of magnetrons |
The form of the cavities varies, shown in the Figure 3. The output lead is usually a probe or loop extending into one of the tuned cavities and coupled into a waveguide or coaxial line. | |
| a) slot- type b) vane- type c) rising sun- type d) hole-and-slot- type |
Basic Magnetron Operation
As when all velocity-modulated tubes the electronic events at the production microwave frequencies at a Magnetron can be subdivided into four phases too:
- phase: Production and acceleration of an electron beam
- phase: Velocity-modulation of the electron beam
- phase: Forming of a „Space-Charge Wheel”
- phase: Giving up energy to the ac field
Figure 4: the electron path under the influence of the varying magnetic field. |
1. Phase Production and acceleration of an
electron beam When no magnetic field exists, heating the cathode results in a uniform and direct movement of the field from the cathode to the plate (the blue path in figure 4). The permanent magnetic field bends the electron path. If the electron flow reaches the plate, so a large amount of plate current is flowing. 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. However, when the critical field value is reached, as shown in the figure as a red path, 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, the plate current drops to zero. 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. |
2. Phase: velocity-modulation of the electron beam
The electric field in the magnetron oscillator is a product of ac and dc fields. The dc field extends radially from adjacent anode segments to the cathode. The ac fields, extending between adjacent segments, are shown at an instant of maximum magnitude of one alternation of the rf oscillations occurring in the cavities.
Figure 5: The high-frequency electrical field |
In the figure 5 is shown only the assumed
high-frequency electrical ac field. This ac field work in addition
to the to the permanently available dc field. The ac field of each
individual cavity increases or decreases the dc field like shown in
the figure. Well, the electrons which fly toward the anode segments loaded at the moment more positively are accelerated in addition. These get a higher tangential speed. On the other hand the electrons which fly toward the segments loaded at the moment more negatively are slow down. These get consequently a smaller tangential speed. |
3. Phase: Forming of a „Space-Charge Wheel”
On reason the different speeds of the electron groups a velocity modulation appears therefore.
Figure 6: Rotating space-charge wheel in an eight-cavity magnetron |
The cumulative action of many electrons returning
to the cathode while others are moving toward the anode forms a
pattern resembling the moving spokes of a wheel known as a
„Space-Charge Wheel”, as indicated in figure 6. The space-charge
wheel rotates about the cathode at an angular velocity of 2 poles
(anode segments) per cycle of the ac field. This phase relationship
enables the concentration of electrons to continuously deliver
energy to sustain the rf oscillations. One of the spokes just is near an anode segment which is loaded a little more negatively. The electrons are slowed down and pass her energy on to the ac field. This state isn't static, because both the ac- field and the wire wheel permanently circulate. The tangential speed of the electron spokes and the cycle speed of the wave must be brought in agreement so.
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4. Phase: Giving up energy to the ac field
Figure 7: Path of an electron |
Recall that an electron moving against an E field is accelerated by the field and takes energy from the field. Also, an electron gives up energy to a field and slows down if it is moving in the same direction as the field (positive to negative). The electron gives up energy to each cavity as it passes and eventually reaches the anode when its energy is expended. Thus, the electron has helped sustain oscillations because it has taken energy from the dc field and given it to the ac field. This electron describes the path shown in figure 7 over a longer time period looked. By the multiple breaking of the electron the energy of the electron is used optimally. The effectiveness reaches values up to 80%. |
Modes of Oscillation
The operation frequency depends on the measurements of the cavities and the interaction space between anode and cathode. But the single cavities are coupled over the interaction space with each other. Therefore several resonant frequencies exist for the complete system. Two of the four possible waveforms of a magnetron with 8 cavities are in the figure 8 represented. Several other modes of oscillation are possible (3/4π, 1/2π, 1/4π), but a magnetron operating in the π mode has greater power and output and is the most commonly used.
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Strapping
Figure 9: cutaway view of a magnetron, showing the strapping rings and the slots. Figure 8: Waveforms of the magnetron |
So that a stable operational condition adapts in the optimal pi mode, two constructive measures are possible:
- Strapping rings:
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 pi 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.
- Use of cavities of different
resonance frequency
E.g. such a variant is the anode form „Rising Sun”.
Magnetron coupling methods
Energy (rf) can be removed from a magnetron by means of a coupling loop. At frequencies lower than 10,000 megahertz, the coupling loop is made by bending the inner conductor of a coaxial line into a loop. The loop is then soldered to the end of the outer conductor so that it projects into the cavity, as shown in figure 1, view (A). Locating the loop at the end of the cavity, as shown in view (B), causes the magnetron to obtain sufficient pickup at higher frequencies.
 Figure 10: Magnetron coupling, view (A) |
 view (B) |
The segment-fed loop method is shown in view (C) of figure 2. The loop intercepts the magnetic lines passing between cavities. The strap-fed loop method (view (D), intercepts the energy between the strap and the segment. On the output side, the coaxial line feeds another coaxial line directly or feeds a waveguide through a choke joint. The vacuum seal at the inner conductor helps to support the line. Aperture, or slot, coupling is illustrated in view (E). Energy is coupled directly to a waveguide through an iris.
 Figure 11: Magnetron coupling, view (C) |
 view (D) |
 view (E) |
Magnetron tuning
A tunable magnetron permits the system to be operated at a precise frequency anywhere within a band of frequencies, as determined by magnetron characteristics. The resonant frequency of a magnetron may be changed by varying the inductance or capacitance of the resonant cavities.
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Tuner frame |
Figure 12: Inductive magnetron tuning |
inductive tuning elements |
An example of a tunable magnetron is the
M5114B used by the ATC- Radar
Figure 13: Magnetron M5114B of the ATC-radar
Figure 13: Magnetron VMX1090 of the ATC-radar
