RADIO BROADCASTING TRANSMITTERS 



131 



These machines are slot wound and employ composition brushes, a 

 filter consisting of a 1-mh. inductance and four 1,000-juf electrolytic 

 condensers being used to suppress commutator and slot ripples. Grid 

 bias voltages are obtained from a 2-kw., 300-volt unit, which is also 

 installed in duplicate. The only other rotating apparatus is that 

 associated with the water-cooling system. The tubes are cooled by 

 means of distilled water which is conducted to the anodes of the 

 amplifier tubes through insulating hose coils. The total heat trans- 



Fig. 6 — Antenna coupling and tuning unit for SO-kvv radio transmitter. 



ferred by the cooling water is approximately 175 kw. A flow of 75 

 gallons per minute is maintained. Four 56-in. by 58-in. radiator units 

 are employed, each consisting of a bank of copper tubes with spiral 

 fins over which air is blown by a 37-in. fan. Ample radiator capacity 

 is provided to maintain the water below 180 deg. F. under all atmos- 

 pheric conditions. 



To promote antenna efficiency and to reduce the intensity of the 

 electric field in the station building, the equipment is arranged to 

 deliver its output to the antenna through a radio-frequency trans- 

 mission line approximately 500 ft. long. The line is balanced to 

 ground and is designed for a characteristic impedance of 600 ohms. 

 The antenna coupling and tuning unit is shown in Fig. 6. It is in- 



