2066 THE BELL SYSTEM TECHNICAL JOURNAL,SEPTEMBER 1969 VI.HYBRID Figure 6 shows the directional coupler form of hybrid.The ex- ponentially decaying fields,propagating in the m region of Fig.2, overlap for the two parallel guides of Fig.6,providing continuous distributed coupling.Reference 1 gives approximate expressions for calculating the guide spacing and needed coupling length. Figure 7 shows the partially reflecting mirror form of hybrid;the reflecting line may be a narrow groove coated with a metal film,an empty groove,or a high index dielectric region created by a masking and diffusion or ionic replacement process.A single empty groove,an odd quarter of a wavelength thick,in the direction of propagation would give a coupling loss of about 9 dB. VII.FREQUENCY-SELECTIVE FILTERS Using techniques familiar at lower frequencies,hybrids and resonant circuits can be combined to form filters,a needed component in fre- quency-division multiplex systems.Figure 8 shows such an arrange- ment,where band pass cavities Ci and C2 are used to separate fe from fo and fo;hybrids divide and recombine the energy to form a constant Fig.6-Directional coupler type hybrid.2066 THE BELL SYSTEM TECHNICAL JOTTBNAL, BEFTEMBER 1069 VI. HYBRID Figure 6 shows the directional coupler form of hybrid. The ex￾ponentially decaying fields, propagating in the rh region of Fig. 2, overlap for the two parallel guides of Fig. 6, providing continuous distributed coupling. Reference 1 gives approximate expressions for calculating the guide spacing and needed coupling length. Figure 7 shows the partially reflecting mirror form of hybrid; the reflecting line may be a narrow groove coated with a metal film, an empty groove, or a high index dielectric region created by a masking and diffusion or ionic replacement process. A single empty groove, an odd quarter of a wavelength thick, in the direction of propagation would give a coupling loss of about 9 dB. Vn. FRBQXJBNCY-SELECTIVB FILTERS Using techniques familiar a t lower frequencies, hybrids and resonant circuits can be combined to form filters, a needed component in fre￾quency-division multiplex systems. Figure 8 shows such an arrange￾ment, where band pass cavities Ci and C2 are used to separate fa from /ft and fo; hybrids divide and recombine the energy to form a constant Fig. β —Directiona l coupler type hybrid