The eigenvalue analysis for the upper-hybrid/Langmuir waves trapped in cylindrical density structures is carried out. Under physical parameters typical for kilometric continuum (KC) source region, it is found that the frequency spacing between individual fine structure spectrum is within a reasonable observed range of 0.1~1 kHz. It is also found that the quasi field-aligned modes are associated with density cavities, while the radial modes are excited within enhanced density column, thus showing that density irregularities of various types can support discrete wave spectrum featuring wide-ranging frequency gaps.