You want the substrate to be suspended halfway between the top and bottom cover, and you want it bounded to the left and right by a channel wall. If you want only to propagate energy in the trace itself, the dimensions of the channel have to be below a certain width.
An approximation, assuming the width is much wider than the cover to cover height, is that the cuttoff frequency for the lowest waveguide mode will be:
Fc = C/2A, where A is the width in cm, and C is the speed of light 30 cm/ns.
For a cuttoff frequency of 22 GHz, you need A = 30 [cm/ns] / (2 * 22 GHz) = 0.68 cm.
**broken link removed**
Rich