34 BELL SYSTEM TECHNICAL JOURNAL 



At the terminal we have as before : 



li + Ir = It, Zo+Ii - Zo-Ir = ZJt. 



Hence the more general expressions for the reflection and transmission 

 coefficients are : 



33 ^°^ — Zt p _ Zo~ p 



^ _ Zq' -\- ZfT n^ _ Zt rj^ 



J / -7 _ I ^— ' 1 V — T^-T i- I- 



These reflection and transmission coefficients can be expressed in terms 

 of the ratios of the line impedances to the terminal impedance. 



Shielding 



When a source of electromagnetic waves is enclosed in a metallic 

 box, the field outside the box is substantially weaker than it would 

 have been in the absence of the box. The box is said to act as a 

 "shield." Under some conditions, transmission of electromagnetic 

 waves in free space and in the metallic shield is governed by equations 

 of the form (1). In those cases the shielding effect can evidently be 

 regarded as due to a reflection loss at the boundaries of the shield and 

 to an attenuation loss in the shield itself. A schematic representation 

 of a single layer shield is shown in Fig. 3. The source of disturbance 



Fig. 3 — Transmission line representation of a shield. The generator represents 

 the source of the electromagnetic disturbance, the section OP the space surrounding 

 the source, the section PQ the shield, and the impedance Zt the space outside the 

 shield. 



is shown as a generator, the space around this source is represented 

 by a piece of a transmission line OP, the shield by a piece PQ and the 

 space outside the shield by the impedance Zt. 



The simplest case to consider is that of an electrically thick shield, 

 in which the attenuation between P and Q is so great that waves 

 reflected at Q do not affect appreciably the situation at P. In such a 

 case the impedance at P looking toward Q equals the characteristic 

 impedance Zq" and the same is true of the impedance at Q looking 



