A 1-Kilowatt Frequency-Modulated Magnetron for 900 Megacycles

The method of Smith and Shulman has been used for the frequency modulation of a 1-kilowatt continuous-wave magnetron. This tube is of the "vane" type, having twelve resonant cavities, and it is mechanically tunable over a range from about 720 to 900 megacycles by a cylindrical element which varies the interstrap capacitance. At the applied magnetic field required for frequency modulation without change in amplitude, 1 kilowatt output at 900 megacycles is obtained with an anode voltage of 2.5 kilovolts and an efficiency of about 55 per cent; the efficiency rises with decreasing frequency or with increasing magnetic field. At 900 megacycles, electron beams in nine of the magnetron resonant cavities give a frequency deviation of 3.5 megacycles (a total frequency swing of 7 megacycles) at an output of 1 kilowatt, rising to 4 megacycles at an output of 750 watts. The frequency deviation is reduced when the tube is tuned to lower frequencies. The modulator power required would be very low, since the grid-cathode capacitance of the frequency-modulation guns is small and the grids draw no current. It would be practicable to increase the frequency deviation of this tube by about 15 per cent through an increase in beam current, and by an additional 20 per cent through the use of eleven beams. A change in the type of beam cathode would effect an even greater deviation.