SPECTRAL-ANALYSIS CONSIDERATIONS RELEVANT TO RADIATION AND LEAKY MODES OF OPEN-BOUNDARY MICROSTRIP TRANSMISSION-LINE

The continuous radiation spectrum of open microstrip transmission line and its nonspectral leaky modes are conceptualized through a transfor m-domain integral-operator formulation and relevant spectral analysis. Two complex (transform-variable) wavenumber planes are implicated by Sommerfeld-integral representations of associated Green's functions an d the necessary axial inverse transformation to the space domain. The radiation spectrum is identified with branch cuts in the axial wavenum ber plane, which constrain the migration of branch-point singularities in the transverse wavenumber plane. Leaky-wave modes occur only when the branch cuts in the axial wavenumber plane are violated, allowing b ranch points in the transvere wavenumber plane to migrate across the ( initial) real-axis integration path. The relation between spectral rad iation modes, nonspectral leaky-wave modes and branch cuts in the axia l wavenumber plane is discussed. The influence of branch cuts in the a xial wavenumber plane upon the location of branch points in the transv erse wavenumber plane is detailed, and a rationale is offered for the choice branch cuts in the latter plane. Although the formulation is de veloped specifically for the microstrip line, it is applicable more ge nerally to a wide class of open conducting or dielectric waveguides. I t is believed that the ideas presented here are new and significant, a nd provide perhaps the first general method for conceptualizing the co ntinuous spectrum of practical open waveguides.