TRANSMISSION LINES FOR R.IDIO SYSTEMS 437 



effect is negligible. Although these resistances possess appreciable 

 inductive reactance at the higher frequencies the reactance usually 

 may be safely tuned out. 



It has been found practicable to employ continuously variable 

 resistances made from hard drawing pencil lead equipped with spring 

 clip contacts for measurements of high resistances such as (Z2). The 

 high resistivity of graphite makes it possible to obtain several thousand 

 ohms in 5 or 6 inches, free of skin effect and with but little inductance. 



The foregoing resistance substitution method has been found satis- 

 factory for the purpose of measuring the characteristic impedance of 

 lines. Two methods have been employed. One of these is the familiar 

 procedure in which the sending end impedance is measured for the case 

 of the line open and short circuited at the far end. The geometric 

 mean of the two impedances so obtained is the characteristic im- 

 pedance of the line. 



Another scheme producing more precise results is also adapted to 

 the foregoing resistance measuring method in that it requires that the 

 line be some odd number of one-quarter wave-lengths long. Such a 

 line transforms to a different value a terminating impedance which is 

 other than the characteristic impedance. By comparing a variable 

 terminating resistance directly with the value to which it is transformed 

 by the line a setting may be found for which the line functions as a 

 one-to-one transformer. For this condition the value of the variable 

 terminating resistance is the characteristic impedance of the line. 

 Here again drawing pencil leads have been found to be satisfactory 

 termination resistances when set by direct-current measurement 

 methods. 



In practice the foregoing resistance substitution method brings to 

 light many slight irregularities. Variations of apparent characteristic 

 impedance with frequency as much as 5 to 10 per cent have been found 

 for concentric-tube lines equipped with elbows, couplings, and similar 

 fittings. It is believed that impedance variations of this order are to be 

 expected from some such irregularities unless particular care is taken 

 in the construction of the fittings. On the other hand, it has been 

 found that a short straight length of carefully constructed concentric- 

 tube line is so smooth that its characteristic impedance may be em- 

 ployed as a calculable standard. 



VI . Practical Construction Details 

 Open-wire radio-frequency line construction is not very different 

 from that employed in power practice. One outstanding difference is 

 that line supports and insulators must be considered as individual ir- 



