TRANSATLANTIC RADIO TELEPHONY 325 



that the method allows, involving as it does, differences between 

 measurements of high-frequency impedance. 



All of the impedance measurements were made using a high- 

 frequency bridge designed and constructed by Mr. C. R. Englund of 

 Bell Telephone Laboratories. This bridge is similar to that described 

 by Shackelton ^^ except that the standards used consist of a calibrated 

 condenser and a decade resistance. Impedances haviijg capacitive 

 reactance are measured by direct comparison with the standards, 

 while impedances having inductive reactance are tuned with the stand- 

 ard condenser to parallel resonance and the resonant combination 

 compared with the decade resistance. Impedances involving ex- 

 tremely small reactances, either positive or negative, are built out 

 with a condenser in parallel to a value that may be measured con- 

 veniently. 



2. Characteristic Impedance and Propagation Constant. Since the 

 early days of transmission line study, the characteristic impedance 

 and the propagation constant have been determined by two impedance 

 measurements at the near end of the line with the far end of the line 

 open- and short-circuited, respectively.^*^ For two reasons, this method 

 has not been used in our determination of the fundamental antenna 

 constants: first, it is impossible to apply a short to the real terminals 

 of the wave-antenna due to the presence of the ground-connection 

 impedance; and, second, with lines multiple quarter wave-lengths 

 long the input impedance, as a result of resonance in the line when it 

 is open-circuited or grounded, attains either extremely large or ex- 

 tremely small values which could not be measured accurately with 

 the available testing equipment. 



To obviate these difficulties, Mr. C. R. Englund, of Bell Telephone 

 Laboratories, developed a method of determining the characteristic 

 impedance and the propagation constant of the wave-antenna by 

 measuring the input impedance with two known finite terminations 

 at the far end. Under this condition it may be shown that the 

 characteristic impedance is given by the expression : 



T^ _ \{S, - G,){S. - c.'i)(Zi - Zo) + (Zi + c.o)(Z, + o'.,)(5., - S7 ) .... 

 ^ " V (5. -50 + (^.-^2) ^^^^ 



and that the propagation constant is given by: 



'* W. J. Shackelton, "A Shielded Bridge for Inductive Impedance Measurements 

 at Speech and Carrier Frequencies," Bell System Tech. Jour., 6, 142; Jan., 1927. 



"* Bela Gati, "On the Measurement of the Constants of Telephone Lines," The 

 Electrician, 58, 81, Nov. 2, 1906. 



