Magnetron operation cont.

As you learned from the previous tutorial when no magnetic field exists, heating the cathode results in a uniform and direct movement of the field from the cathode to the plate, as illustrated in the figure below, view B from the previous tutorial.

However, as the magnetic field surrounding the tube is increased, a single electron is affected, as shown in the next figure below. In view (A), the magnetic field has been increased to a point where the electron proceeds to the plate in a curve rather than a direct path.

Effect of a magnetic field on a single electron.

In view (B) of the figure above, the magnetic field has reached a value great enough to cause the electron to just miss the plate and return to the filament in a circular orbit. This value is the CRITICAL VALUE of field strength. In view (C), the value of the field strength has been increased to a point beyond the critical value; the electron is made to travel to the cathode in a circular path of smaller diameter.

View (D) shows how the magnetron plate current varies under the influence of the varying magnetic field. In view (A), the electron flow reaches the plate, so a large amount of plate current is flowing. However, when the critical field value is reached, as shown in view (B), 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, as shown in view (C), 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. These oscillations are caused by the currents induced electrostatically by the moving electrons. The frequency is determined by the time it takes the electrons to travel from the cathode toward the plate and back again. A transfer of microwave energy to a load is made possible by connecting an external circuit between the cathode and the plate of the magnetron. Magnetron oscillators are divided into two classes: NEGATIVE-RESISTANCE and ELECTRON-RESONANCE MAGNETRON OSCILLATORS.

A negative-resistance magnetron oscillator is operated by a static negative resistance between its electrodes. This oscillator has a frequency equal to the frequency of the tuned circuit connected to the tube.

An electron-resonance magnetron oscillator is operated by the electron transit time required for electrons to travel from cathode to plate. This oscillator is capable of generating very large peak power outputs at frequencies in the thousands of megahertz. Although its average power output over a period of time is low, it can provide very high-powered oscillations in short bursts of pulses.

NEGATIVE-RESISTANCE MAGNETRON

The split-anode, negative-resistance magnetron is a variation of the basic magnetron which operates at a higher frequency. The negative-resistance magnetron is capable of greater power output than the basic magnetron. Its general construction is similar to the basic magnetron except that it has a split plate, as shown in the figure below, views A and B.

View A: Split anode magnetron.

View B: Split anode magnetron.

Movement of an electron in a split anode magnetron.

These half plates are operated at different potentials to provide an electron motion, as shown in the figure above. The electron leaving the cathode and progressing toward the high-potential plate is deflected by the magnetic field and follows the path shown in the figure above. After passing the split between the two plates, the electron enters the electrostatic field set up by the lower-potential plate.

Here the magnetic field has more effect on the electron and deflects it into a tighter curve. The electron then continues to make a series of loops through the magnetic field and the electric field until it finally arrives at the low-potential plate.

Oscillations are started by applying the proper magnetic field to the tube. The field value required is slightly higher than the critical value. In the split-anode tube, the critical value is the field value required to cause all the electrons to miss the plate when its halves are operating at the same potential. The alternating voltages impressed on the plates by the oscillations generated in the tank circuit will cause electron motion, such as that shown in the figure above, and current will flow.

Since a very concentrated magnetic field is required for the negative-resistance magnetron oscillator, the length of the tube plate is limited to a few centimeters to keep the magnet at reasonable dimensions. In addition, a small diameter tube is required to make the magnetron operate efficiently at microwave frequencies. A heavy-walled plate is used to increase the radiating properties of the tube. Artificial cooling methods, such as forced-air or water-cooled systems, are used to obtain still greater dissipation in these high-output tubes.

The output of a magnetron is reduced by the bombardment of the filament by electrons which travel in loops, shown in the first figure above at the beginning of this tutorial, views (B) and (C). This action causes an increase of filament temperature under conditions of a strong magnetic field and high plate voltage and sometimes results in unstable operation of the tube. The effects of filament bombardment can be reduced by operating the filament at a reduced voltage. In some cases, the plate voltage and field strength are also reduced to prevent destructive filament bombardment.

(back) (top) (next) (return to microwaves page)