702 



BELL SYSTEM TECHNICAL JOURNAL 



When the iris is constructed of material of finite thickness, r, the expression 

 for the susceptance is more compUcated,^" • " and the equivalent circuit 

 becomes a four-terminal network with both shunt and series elements. 

 The equivalent shunt susceptance of this network can be obtained experi- 

 mentally by measuring the insertion loss of the iris, from which a curve such 

 as shown in Fig. 16 can be computed. These data* were taken for irises 

 .050" thick in waveguide having internal dimensions of 0.872" X 1.872" in 

 the frequency range around 4000 mc. The ordinate is a parameter, K, from 

 which the normahzed susceptance is calculated: 



B 



VANES 



■<i)- 



(43) 



-he ^ TTd 



Fig. 13 — One type of inductive obstacle in rectangular waveguide. 



d b B=^ LOGeCScf^ 



Fig. 14— One type of capacitive obstacle in rectangular waveguide. 



B.5^ 



LOGe 



Vnde2/ 



Fig. 15 — Another type of inductive obstacle consists of a cylindrical post. 



Along the abscissa is plotted the ratio of iris opening to width of the wave- 

 guide. 



It can be demonstrated that for values of K from — 1 to — 20, the equiv- 

 alent iris opening is approximately the actual opening less the thickness of 

 the metal sheet. For practical purposes, when the susceptance lies be- 

 tween — 1.5 and —30, it is often sufficient to use the approximation, 



a \ 2a / 



(44) 



where t is the thickness of the iris. 



* Data supplied by Mr. L. C. Tillotson of Bell Telephone Laboratories. 



