352 BELL SYSTEM TECHNICAL JOURNAL 



mission line end of the forward antenna (No. 2) and the larger current 

 (0.167) at the other end reflect the fact that the rhombic antenna is 

 "unidirectional." To a first approximation the current in such an 

 aperiodic antenna accumulates progressively towards the output end. 

 Therefore, the "effective" cross talk current is probably less than 

 (0.16/ + 0.001/)/2 = 0.087; i.e., the effect upon the field radiated in 

 the principal lobe will be altered by less than ten per cent due to the 

 parasitic excitation of the antenna ahead. Antennas farther ahead as 

 well as those behind contribute relatively nothing. 



Since by the reciprocal theorem the directional pattern of any an- 

 tenna is the same for transmitting and receiving, the crosstalk should 

 likewise result in less than 10 per cent effect in the receiving case. 



The measurements of Fig. 8 were made at 18 megacycles. At this 

 frequency the rhombic antennas are proportioned to give maximum 

 radiation approximately end-on. At lower frequencies the crosstalk 

 is probably less. 



The coaxial transmission lines are constructed of 60-foot lengths of 

 one-inch copper plumbing pipe spliced with screw type plumbing 

 unions. The inner conductor is one-fourth inch in diameter and is 

 supported by isolantite insulators. The characteristic impedance of 

 the lines is 78 ohms. The lines extend up the poles where they are con- 

 nected to the antennas through balanced-to-unbalanced matching 

 transformers.^* At the receiving building the lines terminate on a 

 special jack strip. Nitrogen pressure is maintained in all lines to 

 exclude moisture. 



In order to operate the MUSA system it is iiot essential that the 

 velocity of the transmission lines be known. The velocity must be 

 known accurately, however, in order to determine the angle of the 

 waves as they are selected by the steerable lobe. Accordingly, the 

 velocity was calculated (taking the insulators into account) and also 

 measured. The calculated ratio of the line velocity to the velocity of 

 light is 0.941; measurements yielded 0.933 ± 0.004. Using the value 

 of 0.933, angles less than zero have occasionally been measured. 

 A value of 0.937 would have made the lowest indicated angle just zero. 



The longest line is about 1000 meters in length. Its impedance 

 measured at one end when the other end is terminated by a resistance 

 of 78 ohms shows some variation as the frequency is varied. In Fig. 9 

 are shown the results of impedance measurements made by substi- 

 tuting for the line an equivalent parallel combination of resistance and 

 reactance. The two notable variations occurring at approximately 7.7 

 and 15.4 megacycles are believed to be caused by a slight irregularity 

 at each joint, which adds a shunt capacitance of the order of 1.8 micro- 



