330 THE BELL SYSTEM TECHNICAL JOURNAL, APRLL 1951 



inside the shield is still constant. In the case of band-pass filters we can use 

 disjoint contours as in Fig. 9. These must be symmetric about the ^-axis 

 and again we shall find it advantageous to have them symmetric also about 

 the co-axis. In all cases the net charge on the shield must be positive, and 



A^ 



^> 



POSITIVE CHARGE 



DISTRIBUTED ON A 



CONDUCTING SHIELD 



\ 



\ 



L NEGATIVE 

 r CHARGES 



NEGATIVE 

 CHARGES 



Fig. 8 — Negative charges outside shield have no effect on potential inside. 



we can state as a general filter principle that: The natural oscillation con- 

 stants '^shield'' pass-bands from infinite loss points. 



7. Gain Invariant and Phase Invariant Transformations 



We have just seen that we can transfer positive charges (or poles) from 

 the right half of the />-plane to the left without changing the value of the 

 potential (or gain) on the real frequency axis. Similarly, there are trans- 



