422 BELL SYSTEM TECHNICAL JOURNAL 



apparatus is provided by means of relays and contactors in the high- 

 voltage supply circuits which prevent the high voltages from being 

 applied if the filament or grid circuits are not closed or if the doors of 

 the transmitter are open. 



An illustration of the ship's transmitter is shown in Fig. 12. The 

 picture is somewhat out of perspective owing to difficulty in taking the 

 photograph in the limited space available on shipboard. 



The receiving problem on shipboard is complicated by a number of 

 factors. The transmitting and receiving frequencies must be within a 

 few per cent of each other, if the best transmission conditions for the 

 time and place are to be utilized and if the frequencies are to remain in 

 the bands assigned internationally to the mobile services. This re- 

 quirement, as well as the noivSe conditions on shipboard, calls for a 

 receiver of high selectivity, which is obtained, in the present instance, 

 by the use of a double-detection set. The over-all selectivity is accom- 

 plished both by having a number of highly selective circuits ahead of 

 the first detector and by using tuned circuits in the intermediate fre- 

 quency stages, the high-frequency selectivity being used primarily to 

 prevent overloading of the first tube and the intermediate frequency 

 circuits being used to obtain the final selectivity required. 



A reduction of the disturbances due to stay noises and better dis- 

 crimination against the transmitted carrier is obtained if the trans- 

 mitting and receiving antennas are widely spaced. On the other hand, 

 for operating reasons, it is desirable to have the transmitter and receiver 

 located in the same room. In the case of the Leviathan installation, 

 the transmitting antenna is located directly above the radio room, 

 between the second and third stacks, and the receiving antenna is 

 placed as far as possible behind the third stack. The receiving antenna 

 is connected through a suitable step-down circuit to a shielded trans- 

 mission line, the other end of which is connected to the receiver, the 

 receiver itself being very thoroughly shielded to avoid direct interfer- 

 ence from the transmitter. On account of limited space, only two 

 antennas are provided to handle the four frequencies, each antenna 

 representing a compromise between the most efficient antennas which 

 could be put up to handle the separate wavelengths. 



As stated above, the receiver itself is of the double-detection type, 

 using heater type tubes throughout. Screen-grid tubes are used for 

 the first detector and intermediate frequency amplifiers and three- 

 element tubes in the remaining positions. A photograph of the re- 

 ceiver and associated voice-frecjuency equipment, as it is installed on 

 the Leviathan, is shown in Fig. 13, and a diagrammatic representation 

 of the receiver is shown in Fig. 14. The high-frequency selective sys- 



