SHIELDING IN HIGH-FREQUENCY MEASUREMENTS 561 



The principles involved in the application of electromagnetic and of 

 electrostatic shielding are quite different. In the case of electro- 

 magnetic shielding, the methods used have for their object the elimina- 

 tion of all couplings from the unit shielded to all other apparatus; 

 thus, if perfect shielding were possible, the unit would have no coupling 

 to any other parts of the circuit. It is never possible to accomplish 

 this in the case of electrostatic shielding. Any electrical apparatus 

 will have electrostatic coupling to any other apparatus in the vicinity 

 and particularly to ground. The addition of shielding always intro- 

 duces additional electrostatic coupling from the apparatus to the 

 shield and the shield usually has more coupling to other equipment and 

 to ground than the apparatus had before shielding it. Consequently, 

 the principles of electrostatic shielding are mainly a matter of con- 

 trolling this coupling in such a way that it has the least harmful effect 

 in the circuit even at the expense of increasing its actual magnitude, 

 rather than a matter of eliminating it entirely. For this reason 

 electrostatic shielding requires much more extensive consideration and 

 this paper is, therefore, devoted mainly to it, the principles of electro- 

 magnetic shielding being covered only briefly. 



Principles of Electromagnetic Shielding 



The necessity for electromagnetic shielding is limited practically to 

 wound apparatus such as coils and transformers. It may be reduced 

 to a minimum by using high permeability core material wherever 

 possible in coils and transformers, and by using some form of closed 

 core such as the toroidal type. By these means stray fields may be 

 reduced to a relatively low figure. However, there are cases where 

 the remaining coupling may be objectionable and it is then necessary 

 to use shielding to reduce still further the amount of these stray 

 fields. 



Two types of shielding may be used. A high permeability material 

 may be used for the purpose of short circuiting the stray field. The 

 principles of this method of shielding are described fully in another 

 paper and will not be considered further here. 



In the case of air-core coils which are often of the solenoidal type, 

 since the advantage of using the toroidal form is less in this case, and 

 for coils used at very high frequency where heavy magnetic material is 

 not so effective, shields of non-magnetic material may be used to 

 confine the field by the effect of eddy currents. For these shields, a 

 material of high conductivity is used, usually copper, and the principal 

 consideration is the spacing of the shield from the coil rather than the 

 thickness of the shield itself. 



