144 BELL SYSTEM TECHNICAL JOURNAL 



tion in emergency. Further, it practically restricts the transmitting 

 set to one in which the power tubes are used as amplifiers, and it 

 was known that some difficulty might be experienced in operating a 

 number of 250 watt tubes in parallel if it should be necessary to 

 transmit at wave lengths as low as 300 meters. For these reasons and 

 after some development work it was decided that the proper system 

 to use in the first experiment was one in which modulation is carried 

 on by the constant current method which requires about an equal 

 number of modulator tubes and power tubes and sends out all com- 

 ponents of the modulated wave. 



The simultaneous transmission of three channels from the land 

 station may be accomplished in several ways. It is possible, for ex- 

 ample, to carry on such multi-channel operation from one antenna, 

 which is multi-tuned, or from three separate antennas. The antenna 

 power may be supplied by one system of tubes carrying all three 

 conversations or the power tube system may be split into three parts. 

 Also, using a single antenna simply tuned it would be possible to 

 transmit the three channels from one system of tubes by a system of 

 double modulation which had been installed by the Western Electric 

 Company on United States battleships two or three years earlier. 

 The difficulties which are likely to arise in these various schemes are as 

 follows: The use of multi-tuned antennas involves loss of power in 

 the circuits used to give the antenna three degress of freedom. The 

 use of a single system of power tunes for three channels requires that 

 the tube system be capable of handling a large overload at times 

 without impairment of quality, since it is possible that the peaks of 

 three channels may occur simultaneously. It was expected that 

 under conditions of this kind there would be inter-modulation of the 

 channels due to the modulating action in the plate circuits of the 

 power tubes. The use of three separate antennas located very close 

 to one another and tuned to frequencies differing by three or four 

 per cent, might lead to such close coupling of the three channels that 

 cross-talk and modulation of one channel by another would result, 

 the latter by plate modulation of one set of tubes by the currents 

 induced in its antenna. The use of the double modulation system — 

 altho requiring but one radiated carrier — is open to the objection of 

 overloading and cross modulating of channels and also to the objection 

 that the receiving apparatus aboard ship must be more complicated. 

 An analysis ol lhe.se and other proposed methods ol operation resulted 

 in the decision lo employ at that lime three separate but closely 

 adjacent antennas and three separate transmitting sets using the 

 constant current modulation system. This choice was made because 

 f conditions peculiar to this particular problem and to the vacuum 



