A Spiral-Beam Method for the Amplitude Modulation of Magnetrons

A new method is described for the amplitude modulation of magnetrons. Although the method has been applied to the modulation of an existing cw magnetron at 900 Mc, scaling to higher frequencies should prove to be perfectly feasible. In principle, a beam of electrons spiraling in a longitudinal magnetic field varies the conductance presented by a resonant cavity coupled to the magnetron, and so varies the power delivered to the load. The peak power output of the system is at present 400 to 500 watts, while the necessary modulating power is only about 1/2 watt. The linearity of the system is reasonably good, and the bandwidth is at least 20 Mc. The depth of voltage modulation realized is 85 to 90 per cent, while the frequency variation during the amplitude-modulation cycle is only ±15 kc. A television picture free from obvious defects has for the first time, it is believed, been produced by the electronic amplitude modulation of a magnetron.