446 



BELL SYSTEM TECHNICAL JOURNAL 



radio-frequency resistance will be described. The scheme utilizes the 

 resonant properties of a section of concentric-tube line of which the 

 inner conductor is one-half wave-length long. The required resistance 

 is obtained by a connection to the proper position on the inner con- 

 ductor. The device is illustrated schematically on Fig. 23. It may be 

 seen from the curves on this figure that the device is a means for ob- 

 taining a variable resistance which for most practical purposes is non- 

 reactive. Additional advantages are that the device is rugged and that 

 it may be designed to dissipate an appreciable amount of power. 



For the purpose of testing transmitters and for other purposes in 

 which the terminating network is required to dissipate several kilo- 



6 8 10 20 



FREQUENCY-MEGACYCLES 



60 80 100 



Fig. 24 — The curve gives the attenuation in decibels for a balanced 600-ohm line 

 constructed from No. 6 B & S iron wire. 



watts, an iron wire line has been found to be of considerable utility. 

 An iron wire line has the advantage that its impedance is almost inde- 

 pendent of the frequency provided that the length of the line is suffi- 

 cient. This impedance is very closely the characteristic impedance of 

 the line. The far end of the line may be left either opened or closed. 

 For operation at some one frequency the input impedance may be made 

 more nearly equal to the characteristic impedance by means of a ter- 

 mination at the far end. For this purpose the scheme employing a 

 short length of line as a parallel transforming impedance has been 

 found very convenient. 



