Radio doings (Dec 1930-Jun1932)

January, 1931 RADIO DOINGS Page Twenty-seven WE/TCCN ELECTRIC <SC EILOVACE EECACCAJT JTAEICN Excerpts frot/i a paper presented before the Los Angeles Chapter of the Institute of Radio Engineers, December 15, by E. H. Schreiber, Program Service Supervisor, Southern California Telephone Company There are two important considerations which determine the strength of signal and the service area of a broadcast transmitter. The first of these is the POWER of the transmitter, which is considered as the energy contained in the unmodulated carrier wave. The second is the DEGREE OF MODULATION, or comparative amplitude of the signal with which the carrier is modulated. It is expressed in percent and is defined as the ratio of the signal or voice frequency amplitude to the carrier frequency amplitude. The power of the transmitter is limited by law, being assigned by the Federal Radio Commission. Operating with the maximum allowable power, it then follows that service area of the station may be increased by use of the maximum modulation, or 100 per cent. With 100 per cent modulation, the power rises to peaks four times as great as with 50 per cent modulation, and gives a signal twice as loud. The transmitter to be described is rated at 50,000 watts and 100 per cent modulation. Under this maximum modulation, the equipment must provide for an output of 200,000 watts. Western Electric 50 K.W. Transmitter The equipment layout of the transmitter proper consists of seven panel units with a screen inclosure. The first is an oscil-modulator of ^0 watts output and 100 per cent modulation. This is followed by three pushpull stages, amplifying the modulated RF power. The fust power amplifier unit using two 250 watt tubes occupies the second unit. Third and fourth units contain the second power stage and associated output and has water-cooled tubes. The fifth unit houses the final power stage incorporating six water-cooled tubes each capable of a peak out put of 40 K.W. The last two units constitute the front of an electrically screened inclosure housing the output circuits for this latter stage. There is a full complement of meters, the cases of which are either grounded or mounted behind glass for protecting the operating personnel. The power panel which is separate from the others is made up of three units with the control arrangements on the first. A power supply of 250 K.W. at 440 volts is re quired. The left panel houses the control arrangement, which is such that the trans, can be started by a single set of push buttons, the various circuits being energized in the proper sequence by means of suitable relays and contactors. The central unit is a three phase, half wave, rectifier supplying power at 1600 volts to the plates of the air-cooled tubes. The six-tube rectifier on the right supplies plate power at 17,000 volts for the water-cooled tubes. The filament and plate transformers for the latter are located in the power room on the floor below. The filter consists of two units, one for each side of the push-pull circuit, employing a 6 mfd condenser and a 12 henry inductance. Two 24 volt, 550 ampere D.C. generators (one spare) supply power to the filament circuits. Grid bias voltages are obtained from a 2 K.W., 300 volt unit which is also installed in duplicate. The large power tubes are cooled by means of distilled water which is conducted to the anodes of the amplifier tube; through insulating hose coils. The total heat transferred by the cooling water is approximately 175 K.W. A flow of 75 gallons per minute is maintained. Four 56 in. by 58 in. radiator units are provided which maintain the water below 1 80 degrees F, under all atmospheric conditions. A tract of land about 1200 by 1200 feet is required for a 50 K.W. transmitter. A two-story building is erected near one edge of the property which contains the transmitter equipment. The basement contains the generators, transformers and filters. The main floor has a radiator and fan toom, a transmitter room housing the equipment units, a speech input control room, and miscellaneous rooms for the attendants. The actual design of the antenna for this transmitter is determined by the customer's requirements. The tendency has been toward higher structures in an effort to increase the total radiation and localize the signals to some extent. In one instance an 800 foot flat top suspended between two 400-foot towers was used very effectively. Still greater efficiency is anticipated from the use of a single insulated tower over 900 feet in height for a frequency of about 600 Kc The tower is guyed at a single point at about two-thirds of its full height. In either case a small copper shielded coupling unit is located directly beneath the antenna. This connects with the transmitter building by means of a 500-foot transmission line, balanced with respect to ground, so as to reduce radiation from it. The maximum deviation from the prescribed wave length that is permitted by the existing government regulations is 500 cycles — somewhat beyond the capability of the ordinary wave meter. In the absence of adequate frequency control apparatus, very serious beat-note interference has been of frequent occurrence. Considerable improvement has been brought about by the extensive adoption of piezo-electric reference oscillators and automatic piezo-electric control. This system of controls is capable of maintaining the assigned frequency to ± 30 cycles for long periods. These crystal control circuits, which are now in general use, are prof Continued on Page 41)