**4.1 Feeding lines**

3D structures based on planar layers must include planar feeding lines such as microstrip lines or coplanar-grounded waveguides to feed the prototypes and connect them to a vector network analyzer. Moreover, prototypes fed with these lines can be accurately calibrated with TLR (Through-Reflect-Line) calibration kits. That allows to move the reference plane to the beginning of the waveguide and removes the losses and effects of the feeding line and connectors. It is recommended to design the prototypes with the same length as the calibration kits, so the same kit can be used to measure multiple prototypes. For different frequency bands, feeding lines with different lengths and widths must be used, and obviously, different calibration kits will be required.

Depending on the connectors used to measure, the design of the feeding line changes slightly. Usually, these connectors are either typical removable coaxial RF connectors or soldered ones. In **Figure 12**, some calibration kits can be seen with different connectors.
