Shielding In High-Frequency Measurements^ 



By J. G. FERGUSON 



In this paper the purpose and usefulness of shielding in high-frequency 

 measurement are outlined. General principles of electrostatic shielding are 

 developed as applied to simple impedances and to networks of impedances, 

 particularly to bridge networks. Practical applications of these principles 

 to the shielding of adjustable impedances, and in the construction of actual 

 bridge circuits, are described. 



SHIELDING of high-frequency measurement apparatus has for 

 its immediate object the control of certain electromagnetic and 

 electrostatic couplings unintentionally introduced in the usual high- 

 frequency circuit. These couplings are represented by stray ad- 

 mittances between the various parts of the system, either direct or 

 to ground, and mutual impedances resulting from stray magnetic 

 fields. In general, the control of these couplings is exercised for the 

 purpose of attaining an accuracy of test that cannot be obtained so 

 readily in other ways. 



When we speak of electromagnetic and electrostatic coupling, it 

 should be understood that these are simply component parts which 

 together make up the total coupling which exists. They cannot be 

 considered as existing independently of each other, and we cannot 

 consider the shielding for one of these components without taking 

 into consideration the effect on the other. 



However, for the frequencies and impedances ordinarily used in 

 communication work, at least, this interdependence is small enough 

 to allow us to consider the shielding problem for each type by itself, 

 without getting into practical difficulties due to this connection. As a 

 result, two types of shielding which are known as electromagnetic, and 

 electrostatic shielding have been developed. 



It may be argued that by extensive separation of the physical parts 

 of circuits and apparatus, any couplings can be decreased in value and 

 in consequence errors caused by them can be reduced, thus eliminating 

 any need for shielding. But there are obvious limits to the extent to 

 which this method can be employed practically. In the case of 

 electrostatic coupling to ground, it is scarcely of any value, and in any 

 case excessive separation of the parts of a circuit introduces other 

 errors due to the length of the wiring involved. Accordingly, it is 

 usually necessary, where the maximum accuracy is desired, to have 

 recourse to shielding. 



1 Presented before the A. I. E. E. Summer Convention, June 24-28, 1929. 



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