888 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



Fig. 9 — Distribution of jsmall tangential electric fields at interior ferrite faceJ 



Fig. 10 — Resistance configuration. 



spect to height. A field null in this construction therefore extends across 

 the entire face of the full height ferrite and all of this face is then "active" 

 in the construction of an isolator. This field situation no longer accurately; 

 applies to the partial height slab. The departure of the ferrite from the 

 top wall creates large fringing fields extending from the ferrite edges, and 

 large electric fields may exist tangential to the ferrite face close to these 

 edges. We would therefore expect the null condition to persist only in a 

 small region about the vertical center of the ferrite face. We may, how- 

 ever, also expect longitudinally fringing modes (TM-like) to be scattered! 

 at the input edge of the ferrite slab so that a longitudinal field maximum 

 will exist at the central region of the ferrite. However, this is a higher: 

 mode, so that this maximum decays rapidly past the leading edge. 



Considering all the effects, the distribution of small tangential electric 

 fields at the ferrite face may be expected to appear as shown in Fig. 9. 

 Experimentally, we have utilized this low loss region and have avoided 

 the decay region of the higher TM-like modes by using the resistance 

 configuration shown in Fig. 10. The resistivity is uniform and about 75 



POLYSTYRENE- 



COPPER PLATE 



RESISTANCE STRIP 

 FERRITE 



V/M///////^^^^J/^^^.';^/^J^^//^/////^^^^?^??j//?^/^9r»'/ 



m 



Fig. 11 — Elimination of longitudinal components. 



