A Unique Method of Modulation for High-Fidelity Television Transmitters

Present-day high-fidelity 441-line television demands modulation frequencies as high as 4 megacycles. Tube capacitance and the flywheel effect of resonant circuits make such modulation difficult and inefficient when conventional methods are used. The author describes a system called "transmission-line modulation" in which modulation is effected between the radio-frequency generator and the antenna by means of a variable impedance connected across the radio-frequency transmission line. This impedance, consisting of a quarter-wave line terminating in the modulator tubes, is controlled by the voltage applied to the grids of these tubes. At high video frequencies the plate efficiency and degree of modulation compare favorably with the conventional systems employed in sound broadcasting. A 1-kilowatt experimental television transmitter employing this system, which may be modulated 80 per cent at frequencies up to 5 megacycles, is described. For demonstration purposes a 200-megacycle oscillator, modulated at frequencies up to 20 megacycles, is shown.