WIDE-BAND TRANSMISSION OVER COAXIAL LINES 661 



lead sheath, as shown, to prevent the entrance of moisture. One or 

 more of the copper tape structures without individual lead sheath may- 

 be placed with balanced pairs inside a common cable sheath. 



Another flexible structure is shown in Fig. 5. The outer conductor 

 in this case is a lead sheath which directly surrounds the inner conduc- 

 tor with its insulation. Since lead is a poorer conductor than copper, 

 it is necessary to use a somewhat larger diameter with this construction 

 in order to obtain the same transmission efficiency. Lead is also in- 

 ferior to copper in its shielding properties and to obtain the same de- 

 gree of shielding the lead tube of Fig. 5 must be made correspondingly 

 thicker than is necessary for a copper tube. 



The insulation used in the structure shown in Fig. 5 consists of hard 



RUBBER WASHER INNER CONDUCTOR 



(copper) 



LEAD OUTER 

 CONDUCTOR 



Fig. 5 — Coaxial structure with rubber disc insulators. 



rubber discs spaced at intervals along the inner wire. Cotton string 

 or rubber disc insulation may be used with either form of outer tube. 

 The hard rubber gives somewhat lower attenuation, particularly at the 

 higher frequencies. 



Another simple form of structure employs commercial copper tubing 

 into which the inner wire with its insulation is pulled. Although this 

 form does not lend itself readily to a continuous manufacturing process, 

 it may be advantageous in some cases. 



Transmission Characteristics 

 Attenuation 



At high frequencies the attenuation of the coaxial circuit is given 

 closely by the well-known formula: 



where R, L, C and G are the four so-called "primary constants" of the 

 line, namely, the resistance, inductance, capacitance and conductance 



