On the correct expansion of leaky modes of planar transmission lines in surface-wave waveguide modes

The proper expansion of the leaky modes of planar transmission lines in the surface-modes of the housing waveguide is discussed and analysed. It is shown that, for a given frequency, all the waveguide modes above cutoff can radiate laterally. The previously published results about leaky modes in planar transmission lines become somewhat different when the criterion shown here is used.