154 BELL SYSTEM TECHNICAL JOURNAL 



antenna and terminates adjacent to the output terminals of the 

 antenna. 



Large terminated rhombic antennas, built with number 12 A.W.G. 

 wire, present a terminal impedance, balanced to ground, of nearly 800 

 ohms resistance. Where the ratio of the inner diameter of the outer 

 pipe to the outer diameter of inner pipe of the transmission line is the 

 optimum value ^ of 3.6, the characteristic impedance of the line is 

 about 77 ohms. The problem is, therefore, to transform properly a 

 balanced resistance of about 800 ohms into an unbalanced resistance of 

 about 80 ohms without appreciable losses over the entire operating 

 range of the antenna. 



Figure 19 is a photograph of a simple type of coupling circuit 

 developed for experimental use. Figure 20 is the measured loss versus 

 frequency curve obtained when the device is connected between the 

 stated impedances. The broad frequency range is primarily the result 

 of a high coefficient of coupling. Balance is obtained by symmetrically 

 disposing two primary windings in series about two secondary windings 

 in parallel. A still greater operating range may be obtained through a 

 more elaborate arrangement. Note in Fig. 19 that the boxes and pipe 

 line are gas tight so that nitrogen pressure can be applied, if desired, to 

 prevent moisture absorption through "breathing." 



Conclusion 

 The original paper ^ on rhombic antennas, in discussing the situation 

 regarding short-wave communication, stated that the main limitations 

 in this field were three in number: 



(a) Inherent receiver noise. 



{h) External noise (static, man-made noises, etc.) 



{c) Signal fading. 



That paper attempted to show that the design of the receiving antenna 

 system has an important bearing upon overcoming all three of these 

 difficulties. 



In the present paper, the authors have indicated how progress in 

 overcoming the first two limitations, through refinements in the 

 horizontal rhombic type of antenna, has been made. 



Improvements in the selection of the receiving antenna dimensions 

 have greatly helped the noise situation. Termination and coupling 

 circuit developments now allow many of these antennas to operate one 

 circuit or several circuits simultaneously within a four-to-one frequency 

 range. 



' E. J. Sterba and C. B. Feldman, "Transmission Lines for Short-Wave Radio 

 Systems," Proc. I. R. E., July, 1932; Bell Sys. Tech. Jour., July, 1932. 



