698 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



the length of the wire to better than 0.01 ju^f- The 'bringing effect" at an 

 open-end of a coaxial is considered negligible. 



The individual center wires of the standards have been cut 0.500" longer 

 than the tubes and the extra length utilized at the test end for connection 

 to the test equipment. In computing the impedances, the length of the tube 

 has been used. The 0.500" center wire projection is considered as part of 

 the test-equipment leads and is separately accountable. 



Disc Short Circuiting of Coaxial Standards 



A shortcoming of coaxial standards previously developed has been in the 

 use of solder to attach a short circuiting disc at the far end when using the 

 length to provide calculable series impedance (Zgh). The use of a disc is the 

 best means to short-circuit the inner and outer conductors with a minimum 

 and calculable terminal impedance, and the accomphshment of this objec- 

 tive by repeatable mechanical means is an important feature of the new 

 standards. 



A very thin disc pressed against the end of a coaxial is effectively con- 

 tacted along two concentric rings representing the peripheral edges of wire 

 and tube of diameters d and D respectively. The d-c. resistance of the 

 metallic area between the rings in terms of its resistance, r, in ohms per 

 square is: 



D/2 



= J^f-^ (6) 



(6) reduces to : 



R 



d/2 



i? = ^lnD/d. (7) 



Figure 4 shows a cress-section of the short-circuiting device developed 

 for the standards. Figure 3 includes a view of the assembled device. It con- 

 sists of a stainless steel housing carrying a trapped silver disc and other 

 parts to effect a repeatable, minimum-impedance short circuit when the 

 housing is screwed to the end fitting of the coaxial. The operation is as 

 follows: 



(a) When the housing is screwed on the end fitting, the disc is forced 

 against the end of the copper tube, with pressure equalized by and 

 derived from the rubber grommet. 



(b) Assuming the housing is always screwed on the end fitting as far as 

 it will go, the total pressure on the disc is regulated on initial as- 

 sembly by contTol of the amount of projection of the tube beyond 

 the end fitting. 



