WIDE-BAND TRANSMISSION OVER COAXIAL LINES 665 



Stranding 

 Inasmuch as the resistance of the inner conductor contributes a 

 large part of the high frequency attenuation of a coaxial circuit, it is 

 natural to consider the possibihty of reducing this resistance by employ- 

 ing a conductor composed of insulated strands suitably twisted or 

 interwoven.^* Experiments along this line showed that this method 

 is impractical at frequencies above about 500 kilocycles, owing to the 

 fineness of stranding required. 



Characteristic Impedance 



The high-frequency characteristic impedance of a coaxial circuit 

 varies inversely with the square root of the effective dielectric constant, 

 i.e., the ratio of the actual capacitance to the capacitance that would 

 be obtained with air insulation. The impedance of a circuit having a 

 given dielectric constant depends merely upon the ratio of conductor 

 diameters and not upon the absolute dimensions. For a diameter 

 ratio of 3.6, the impedance of a coaxial circuit with gaseous insulation 

 is about 75 ohms. 



Velocity of Propagation 



For a coaxial circuit with substantially gaseous insulation, the veloc- 

 ity of propagation at high frequencies approaches the speed of light. 

 Hence the circuit is capable of providing high velocity telephone chan- 

 nels with their well-recognized advantages. The fact that the ve- 

 locity at high frequencies is substantially constant minimizes the 

 correction required to bring the delay distortion within the limits 

 required for a high quality television band. 



Shielding and Crosstalk 



The shielding effect of the outer conductor of a coaxial circuit is 

 illustrated in Fig. 9, where the transfer impedance between the outer 

 and inner surfaces of the outer conductor is plotted as a function of 

 frequency. There will be observed the sharp decrease in inductive 

 susceptibility as the frequency rises. On this account, the crosstalk 

 between adjacent coaxial circuits falls off very rapidly with increasing 

 frequency. The trend is, therefore, markedly different from that for 

 ordinary non-shielded circuits which rely upon balance to limit the 

 inductive coupling. As a practical matter, less shielding is ordinarily 

 required to avoid crosstalk than to avoid external interference. 



With suitable design the shielding effect of the outer conductor 

 renders the coaxial circuit substantially immune to external inter- 

 ference at frequencies above the lower end of the spectrum. Hence 

 the signals transmitted over the circuit may be permitted to drop 



