COAXIAL IMPEDANCE STANDARDS 



701 



where r is the distance from the axis of the wire or tube to point P. This 

 assumes that the current density on any concentric circle is uniform. 



If the wire and tube are carrying the same current I but in opposite di- 

 rections and if the axes of the wire and tube coincide, there will be no mag- 

 netic field at any external point. Therefore an alternating current in the 

 conductors will not induce a voltage in a pick-up coil placed external to the 

 coaxial. If, however, the two axes do not coincide a voltage will be induced 

 in the pick-up coil and will be a maximum when the pick-up coil is in the 



1.6 



1.4 



1.2 



-J 1.0 



O 



> 



0.4 



1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 

 DEPARTURE FROM CONCENTRICITY IN MILS 



6.0 6.5 



7.0 



Fig. 6— Typical calibration curve in measurements of eccentricity of coaxial standards. 



plane of the two axes. The magnitude of this maximum voltage will be 

 proportional to the distance between the two axes if the measuring distance 

 is large compared to this separation distance. 



In practical use, the maximum field at P was measured in terms of volts 

 relative to a known eccentricity obtained by insulating a coaxial with discs 

 of the known eccentricity. Advantage was taken of the linear relationship 

 of eccentricity and detectable field so that, with the distance to the tube 

 maintained constant, the calibration curve of Fig. 6 was used to evaluate 

 all standard coaxials after final assembly. The sensitivity was such that the 



