ELECTROMAGNETIC THEORY OF LINES AND SHIELDS 571 



the field depends upon the position of the measuring apparatus. The 

 variation is very small, however, unless the source is almost touching 

 the shield and it can be stated that approximately the shielding 

 effectiveness is independent of the position of the source. 



It is interesting to observe from the accompanying tables that 

 while the attenuation loss is greater in iron than in copper, the reflection 

 loss is greater at a copper surface. In fact, at some frequencies the 

 impedances of iron and air nearly match and practically no reflection 

 takes place. Hence, a thin copper shield may be more effective than 

 an equally thin iron shield. And if a composite shield is made of 

 copper and iron, the shield will be more effective if copper layers are 

 placed on the outside to take advantage of the added reflection. 



TABLE I 



The Absolute Value of the Radial Impedance Offered by Air to Cylindrical 

 Magnetic Waves of the First Order (in Microhms) 



/ 



1 cycle 



10 cycles. . . . 



100 cycles. . . . 



1 kilocycle . . 



10 kilocycles. 



100 kilocycles. , 



1 megacycle . 



10 megacycles 



100 megacycles 



Radius = 0.5 cm. 



1 cm. 



0.0395 

 0.395 

 3.95 

 39.5 

 395. 

 3,950. 

 39,500. 

 395,500. 



3.95 ohms 



0.07896 

 0.790 

 7.90 

 79.0 

 790. 

 7,900. 

 79,000. 

 790,000. 



7.9 ohms 



0.1579 

 1.58 

 15 8 

 158. 

 1,580. 

 15,800. 

 158,000. 



1.58 ohms 

 15.8 ohms 



TABLE II 

 The Intrinsic Impedance of Certain Metals (r;)/(V«) in Microhms 



