**1. Introduction**

Planar circuit manufacturing processes have a long-term way. Many of the fabrication techniques of planar circuits can be applied to build 3D structures (substrateintegrated waveguides—SIW [1], empty substrate-integrated waveguides—ESIW [2], ridge empty substrate-integrated waveguides—RESIW [3], double-ridge empty substrate-integrated waveguides—DRESIW [4], substrate-integrated coaxial line— SICL [5] and empty substrate-integrated coaxial line—ESICL [6] or a combination with others as a continuous profile [7]) piling up layers of planar substrates.

In this chapter, some strategies and techniques adapted from planar to planar-3D structures will be explained with examples of manufactured prototypes, this philosophy being ideal for microwave prototypes (300 MHz–300 GHz), because the sizes of cavities are small enough to build heaping up with few small height substrates, of no more than 1.5 mm.

The prototypes can be fed with planar circuits such as coplanar or microstrip lines; because of this, at some point the feeding planar line needs to be connected with the inner waveguide or the connector through transitions, which needs to be also taken into account. One of the most typical transitions to do this are tapers, that is, transitions with a progressive geometrical variation of the guide which replicates the 3D launchers but planarly [8, 9]. Another way to do this is through wires [10].
