316 



BELL SYSTEM TECHNICAL JOURNAL 



The transmission lines from the antennas to the receiver are of the 

 coaxial type made of copper tubing \\" in inside diameter surrounding 

 a central conductor of |" outside diameter. Ceramic insulators \" 

 thick are spaced every 16" and a locking insulator is placed every 250 

 feet to prevent creeping of the inside pipe. The velocity of propaga- 

 tion of the line is 0.980 of that in space. The attenuation amounts to 

 about 1 db per 1,000 feet at 20 megacycles. 



The lines are buried to protect them from mechanical injury and 

 to prevent phase errors due to differences in expansion. Bacterial 

 growth in the marsh makes the soil extremely corrosive and it was 

 necessary to protect the lines by coatings of tar and asbestos tape. 

 The lines are kept under gas pressure at all times. 



In a musa receiving system a saving of nearly one half of the trans- 

 mission line can be made if the receiving equipment is located at the 

 center of the antenna system rather than at one end. Furthermore, 

 since the average length of transmission line will be cut in half, the 

 diameter of the coaxial transmission line also could be cut in half and 

 still maintain the same signal loss. The economic, and to a lesser 

 extent the locational, advantages of the center position were so great 

 that the equipment was so placed in spite of certain technical dis- 

 advantages. 



Theory of Phase Shifter System with Center 

 Location of Receivers 

 At this point some of the theory of the musa phase shifting system 

 will be reviewed and extension made to cover the situation of the 

 receiver being located near the middle of the antenna system. 



Fig. 5 — Diagram of wave front approaching antenna. 



Assume a plane wave front progressing toward the earth from the 

 Kennelly-Heaviside layer at an angle 6 with respect to the horizon 

 and impinging upon the receiving antennas indicated as points 7, 8, 9, 

 10, 11, etc. (Fig. 5). Let the receiving station be located at antenna 9. 

 Let the time of arrival at the receiving station of the voltages induced 



