This line includes a fixed-width trace on the inner layer and solid ground areas above and below.
The conductor can be located in the middle of the ground area (Figure 2) or have a certain offset (Figure 3). This method is suitable for the inner radio frequency routing.
Figure 2. Stripline (end view)
Figure 3. Offset stripline. A variant of stripline for PCBs with different layer thicknesses (end view)