Propagation constant evanescent modes

The discrete set of bound-mode propagation constants means that the fields of the waveguide in the spatial steady state are given by the finite sum over all bound modes in Eq. (11-2). In contrast, each radiation and evanescent mode can take any of the continuum of propagation-constant values given in Table 25-1, and thus an integration over all values of is necessary. However, like bound modes, the total radiation field requires a summation over the subscript j of Eq. (25-1) to account for the transverse fields of different modes. However, rather than use / as the continuum variable, when P is real for radiation modes and imaginary for evanescent modes, we use instead the modal parameter Q, defined below, in order to simplify the notation. We take / to be the positive root of the inverse relation whence... 

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