274 BELL SYSTEM TECHNICAL JOURNAL 



sembly is as much as five eighths of a wave-length at the highest fre- 

 quency in its operating range, which is 9000 to 21,000 kilocycles. Each 

 transmitter is required to operate at several assigned frequencies within 

 this range and to change in a few minutes from one to another. This is 

 done by changing coils and varying condensers in the oscillator and 

 amplifier circuits and switching to different quartz crystals. Except in 

 cases where two assigned frequencies are in harmonic relationship, it is 

 necessary to provide a crystal for each of the frequencies. The crystals 

 are mounted in an oven and continuously maintained at 50 deg. ± 

 0.05 deg. cent, by recording regulators. In order to avoid long inter- 

 ruptions to service in the event of a crystal failure or other circum- 

 stance requiring the opening of the oven and the subsequent re- 

 establishment of temperature equilibrium, the ovens and crystals are 

 provided in duplicate. 



The electrical problems which are encountered by the engineer 

 designing a power amplifier for these high frequencies arise largely from 

 the inherent stray or distributed capacities and inductances which are 

 far less important at lower radio frequencies. For example, between 

 the anodes of the amplifier circuit there exist capacities, which are 

 composed of capacities within the tube itself, the direct capacities be- 

 tween the tube water jackets, the mounting plates and the like. The 

 total value of this composite capacity in the last stage is approximately 

 100 m.m.f. This value cannot be appreciably reduced by any change 

 in design which now seems desirable. The reactance of 100 m.m.f. at 

 20,000 kilocycles is about 80 ohms. Thus the engineer is confronted 

 at the outset with a generator (the tubes) which has an internal impe- 

 dance in the order of 2000 ohms but across whose terminal is shunted 

 inherently an 80-ohm reactance. Fortunately, this obstacle can be 

 surmounted by introducing resonance effects but nevertheless it places 

 very important limitations on the design of the associated circuits. 

 These problems become more difficult with increase of either power or 

 frequency. Increase in power requires higher voltages and currents 

 and thus larger elements, spaced farther apart. The augmented bulk 

 increases both stray capacities and unwanted inductance of leads. 

 Higher frequencies increase the magnitude and therefore the relative 

 importance of these effects. 



The power control board has nine panels equipped with the necessary 

 instruments and apparatus for controlling and distributing all power 

 to the transmitter. The motor-generators, pumps, fans, oil circuit 

 breakers, and other apparatus are remotely controlled from this point. 

 A system of relays and signal lamps provides protection and indicates 

 the location and general nature of any trouble. With the exception of 



