High-power radio broadcast equipment

Describes an equipment developed by the International Standard Electric Corporation for broadcasting on a wave-length range of 280 to 600 metres with a carrier power of 45 kW., the peak power with 100% linear modulation being 180 kW. The system used is one of modulation at low power with subsequent high-frequency amplification, of which there are three stages, the first uses three 250-watt glass valves, the second two 15-kW water-cooled valves and the final stage two banks of six 15-kW water-cooled valves in parallel. The water-cooled valves derive their anode supply of 12,000 V 14 A from a "3-phase double star" thermionic rectifier. The earlier stages derive their power from two motor-generator sets, which are also arranged to deal with certain of the filament supplies and grid-bias voltages. The layout is so arranged that all the power control boards and running machinery are kept separate and distinct from the radio-transmitter circuits. The transmitter including the output circuit for coupling to the antenna is arranged in six isolated free-standing units, each unit being completely enclosed. The speech-input equipment, which comprises variable gain input amplifiers, frequency-correcting networks, studio-signalling equipment and facilities for connecting to outside lines, is mounted in units on a control desk, which also provides for a check receiver and a direct-monitoring position. The performance is such that over a frequency range of from 30 to 10,000 cycles the total variation of output for a constant-audio input is less than 3 decibels. The equipment is provided with a very complete system of protective relays and fault-indicating devices to ensure reliability in operation and rapidity in fault detection. The ratio of antenna power to power taken from the mains is about 20%.