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BELL SYSTEM TECHNICAL JOURNAL 



identical band reflection filters, and two quarter wavelengths of line. Each 

 of the hybrids is analogous to a low-frequency hybrid coil and operates as 

 follows. A wave in Hhe C (See Fig. IV-4) incident on the hybrid is divided 

 equally and with equal phase into A and B but does not appear in D or 

 reappear in C. If waves in A and B are incident on the hybrid a wave pro- 

 portional to their vector sum will appear in C, a wave proportional to their 

 vector difference will appear in D but nothing will reappear in A or B. 

 A wave in the input line incident on the channel dropping circuit will thus 

 be divided by the input line into the lines leading to the two band reflection 

 filters. These filters are designed to reflect frequencies lying within their 

 band and pass all other frequencies. If the frequency is outside of the re- 

 flected band the two waves will travel on to connections A and B of the 



HYBRID 

 JUNCTION ^^ C 



^) 



4 



IDENTICAL 

 BAND 

 REFLECTION ^ 



FILTERS ' 



DROPPED CHANNEL 



/ig (90° PHASE SHIFT] 

 4 



DUMMY 

 : LOAD 



Q 



HYBRID " 

 JUNCTION 



OTHER 

 CHANNELS 



ADVANTAGES: 



ISOLATION OF DESIGN 



PROBLEMS. 

 CIRCUIT FLEXIBILITY 



Fig. IV-4. — Schematic diagram of a constant impedance channel dropping filter 

 using hybrid junctions and band reflection filters. 



output hybrid. Here they will have equal phase and amplitude, their 

 vector difference wiH be zero and the wave appearing in C of the output hy- 

 brid and consequently in the output line will contain all the power. If 

 the frequency lies within the band of the reflection filters the two waves will 

 be reflected by them and will travel back to the connections A and B of 

 the input hybrid. The two waves strike these connections with opposite 

 phase since one of them has traveled twice over an extra quarter wavelength 

 of line. Their vector sum will consequently be zero and the wave which 

 appears in terminal D of the input hybrid and consequently in the dropped 

 channel line will contain all the power. The circuit of Fig. IV-4 is therefore 

 a constant resistance channel dropping network which diverts energy lying 

 within the band of the reflection filters but allows all other energy to pass 

 through without disturbance. Conversely, by the law of reciprocity, this 



