Microstrip or stripline? That choice has been faced by high frequency designers for decades. Both transmission-line technologies are widely used in both active and passive microwave circuits, with excellent results. Is one approach better than the other? Before tackling such a question, it might help to know how each transmission-line technology works and what kind of demands each place on a printed circuit board (PCB) material.
Microstrip is a transmission-line format in which the conductor is fabricated on a dielectric substrate which itself has a bottom ground-plane layer. Conductors are usually formed by etching away unwanted metal from a conductor layer, such as copper.
Stripline is often compared to a flattened coaxial cable in that, like the cable, it consists of an inner conductor completely surrounded by dielectric material which is itself surrounded by a ground braid or foil. Of course, stripline circuits are planar, so that they appear as a sandwich of conductors in the middle, surrounded by dielectric layers, which in turn have parallel ground planes on the top and bottom.