A Magnetron Using an Additional External Reactive Load for Frequency Tuning: Theory Features and Experiment

This article presents the results of the theoretical and experimental studies of frequency tuning in the ${X}$ -band low-voltage magnetron with two energy outputs. The principal condition of applying the magnetron is the application of the short-circuit piston as a reactive load of the second (passive) energy output. For theoretical investigation, we used two mathematical models of the magnetron: the simple analytical model for simulation of the “cold” anode block in one-wave approximation using the equivalent circuit method and the 3-D model of the magnetron using the particle-in-cell (PIC) method. It is shown that the necessary condition for frequency tuning in such magnetrons is a request to design the anode block when the coupling of the passive RF output with the anode block exceeds the analogous coupling between the anode block and the active RF output. The numerical experiment shows the more complicated nonlinear mechanism for limiting the frequency tuning band. It has been found that a reason for such limitation is conditioned by competition of operating $\pi $ -mode with the closest in the frequency of additionally excited resonant high- and low-frequency $\pi $ -modes. For the experimental investigation, we used the prototype of the magnetron having the following operating parameters: the central frequency is 9.38 GHz, the anode voltages are ${530}, \ldots,{580}$ V, the axial magnetic field is 0.21 T, the indirect heater cathode and the forced-air cooling. As a result, at maximum output power of ~18 W and efficiency of ~37%, the frequency tuning full range was ~320 MHz.