ELECTROMAGNETIC FIELD ANALYSIS 



501 



discriminate against some frequencies. A capacitor in parallel with the 

 wave guide or an inductor in series with it will favor transmission of low 

 frequencies at the expense of high frequencies. These discontinuities are 

 deliberately designed to be sufficiently large to produce noticeable effect. 

 A frequency filter is a more elaborate structure made up of capacitors and 

 inductors designed to achieve desired frequency discrimination. 



Discontinuities in high-frequency wave guides are also either accidental, 

 unavoidable or intentional. The principal difference is in the order of 

 magnitude — any irregularity of apparently small physical dimensions may 

 represent a large virtual reservoir of energy. Among the simplest types 

 of intentional discontinuities in wave guides are '^irises", Fig. 7. Local 

 fields are created in the vicinity of the irises. Under the influence of a 

 wave traveling along the guide, electric charge and current are induced in 

 the metal partition. On either side of the partition the complete field is 

 the result of the superposition of fields representing various transmission 

 modes. The cutoff frequencies of these modes may be arranged in an 



(a) (b) (c) 



Fig. 7 — Inductive, capacitive, and resonant irises. 



increasing sequence. If the operating frequency is between the lowest 

 cutoff frequency and the next higher, the propagation constants of all 

 modes except the dominant are real and the corresponding fields will not 

 extend very far from the iris. During one-half cycle the local field with- 

 draws energy from the dominant wave — this being the only source of energy 

 — and during the remaining half this energy is returned. The local field 

 acts as a virtual source of power — "virtual" since it operates on borrowed 

 power. On account of symmetry the dominant waves generated by this 

 virtual source and traveling in opposite directions will be of equal intensities. 

 The scattered wave traveling toward the source of the incident wave is 

 called the wave reflected from the iris; on the other side the scattered and 

 incident waves merge into the transmitted wave. The storage of energy 

 in the local field depends on the frequency — hence, the frequency selec- 

 tivity. 



In the case shown in Fig. (7a) the flow of current in the partition is un- 

 impeded and there is no tendency for any local concentration of charge in 

 the partition; the local field is largely magnetic and the iris represents an 



