Electron Resonance Magnetron

In the electron resonance magnetron , the plate is constructed to resonate and function as a tank circuit. Thus, the magnetron has no external tuned circuits.

Power is delivered directly from the tube through transmission lines, as shown in view A. The constants and operating conditions of the tube are such that the electron paths are somewhat different from those in the last figure from the previous tutorial. Instead of closed spirals or loops, the path is a curve having a series of sharp points, as illustrated in view B, below. Ordinarily, this type of magnetron has more than two segments in the plate. For example, the figure view B, illustrates an eight-segment plate.

View A: Plate tank circuit of a magnetron.

View B: Electron path in an electron resonance magnetron.

The electron-resonance magnetron is the most widely used for microwave frequencies because it has reasonably high efficiency and relatively high output. The average power of the electron-resonance magnetron is limited by the amount of cathode emission, and the peak power is limited by the maximum voltage rating of the tube components. Three common types of anode blocks used in electron-resonance magnetron's are shown in the figure below.

Common types of anode blocks.

The anode block shown in the figure above, view (A), has cylindrical cavities and is called a HOLE- AND-SLOT ANODE. The anode block in view (B) is called the VANE ANODE which has trapezoidal cavities. The first two anode blocks operate 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, as shown in the next figure below. This also requires an even number of cavities.

Strapping alternate segments.

The anode block illustrated in the third figure above, view (C), is called a RISING-SUN BLOCK. The alternate large and small trapezoidal cavities in this block result in a stable frequency between the resonant frequencies of the large and small cavities.

The figure below, view A, shows the physical relationships of the resonant cavities contained in the hole-and-slot anode (the third figure above, view (A)). This will be used when analyzing the operation of the electron-resonance magnetron.

View A: Equivalent circuit of a hole and slot cavity.

View B: Equivalent circuit of a hole and slot cavity.

Electrical Equivalent

Notice in the figure above, view A, that the cavity consists of a cylindrical hole in the copper anode and a slot which connects the cavity to the interaction space.

The equivalent electrical circuit of the hole and slot is shown in view B. The parallel sides of the slot form the plates of a capacitor while the walls of the hole act as an inductor. The hole and slot thus form a high-Q, resonant LC circuit. As shown in the third figure above, the anode of a magnetron has a number of these cavities.

An analysis of the anodes in the hole-and-slot block reveals that the LC tanks of each cavity are in series (assuming the straps have been removed), as shown in the next figure below. However, an analysis of the anode block after alternate segments have been strapped reveals that the cavities are connected in parallel because of the strapping. The last figure shows the equivalent circuit of a strapped anode.

Cavities connected in series.

Cavities in parallel because of strapping.

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