. The Bell System technical journal . form of single or multi-mode transmission SECTION A-A (a) Cb) Fig. 50 — Examples of structures in which flat transfer loss may be obtaineddespite coupling loss variations. In either dominant-mode directional couplers or in multi-mode cou-pled-wave devices such as the one illustrated in Fig. 1, one may obtainmuch more frequency selecti^•ity than occurs incidentally due to thefrequeue}^ sensitivity of the coupling elements used. This may be doneby coupling two transmission lines which have the same phase constantat one frequenc}^ but unequal phase con
. The Bell System technical journal . form of single or multi-mode transmission SECTION A-A (a) Cb) Fig. 50 — Examples of structures in which flat transfer loss may be obtaineddespite coupling loss variations. In either dominant-mode directional couplers or in multi-mode cou-pled-wave devices such as the one illustrated in Fig. 1, one may obtainmuch more frequency selecti^•ity than occurs incidentally due to thefrequeue}^ sensitivity of the coupling elements used. This may be doneby coupling two transmission lines which have the same phase constantat one frequenc}^ but unequal phase constants at other , as shown by equation (31), the midband transfer loss may be setat any desired \alue by adjusting the integrated coupling strength exat midband (where /3i — (So = 0), and at other fre(iuencies where (^i —02) ^ 0, the transfer loss will increase. For the particular case of ex = 7r/2(fixed) for which complete power transfer occurs when /3i = 02 (and as-suming ai = ao or both as are negligible). Fig. 51 shows the shape ofthe filter characterist
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