312 BELL SYSTEM TECHNICAL JOURNAL 



have plugs in jacks during operation. In order to avoid cords, which 

 would be in the way, the jacks to be connected regularly are mounted 

 adjacent to each other and connected together by two plugs mounted 

 in a shell similar to that commonly used for terminating resistances. 

 Alternating current for cathode heating is supplied to conduit outlets 

 near each panel. Flexible cords with plugs complete the circuit to the 

 panels. All audio frequency, bias, and signal wiring is made into 

 cables in the usual telephone manner. Wires having a potential of 

 over 150 volts to ground are placed in conduit and safety switches are 

 provided to remove the voltage from a panel when the panel cover is 

 removed. 



Antenna System 



The degree of vertical resolution and the signal-to-noise improve- 

 ment of a musa antenna system are functions of the overall length 

 and number of unit rhombic antennas used. The decision to build a 

 sixteen-antenna system was based on experience with the six-antenna 

 system and took into consideration the land necessary, the cost of 

 antennas and transmission lines, and the complexity of the receiving 

 equipment, as well as the resolution which it would be practical to 

 use and still have it possible for the operator or automatic equipment 

 to follow changes in the direction of signal arrival. 



When the spacing between unit antennas of a musa system is 

 several wave-lengths there will be more than one vertical angle at 

 which the phase shifters will simultaneously phase the antenna outputs. 

 The spacing between antennas is so chosen that the range traversed by 

 the lowest of these angles will be the range covered by useful signals. 

 Fortunately the angle of useful signals varies with frequency in such a 

 manner as to permit a variation in frequency over the range desired 

 with a fixed spacing. The unit rhombic antenna is designed to have a 

 null at the position of the second phasing maximum and reception is 

 thus confined essentially to the lowest phasing maximum. 



Extensive study did not disclose a better unit rhombic antenna than 

 the one used in the experimental system and consequently an antenna 

 590 feet (180 meters) long, 60 feet high, and having each side angle 

 equal to 140 degrees was used. The spacing is 656 feet (200 meters) 

 between corresponding parts of adjacent antennas. 



A representative directive diagram of a unit rhombic and the 

 16-unit array in a vertical plane in the line of the antennas is shown 

 plotted in rectangular coordinates in Fig. 3. A polar diagram of the 

 major lobe in three different positions, corresponding to three possible 

 angles of diversity reception, is shown on Fig. 4a. In this figure the 



