From a few hundreds of MHz to 10 GHz, microstrips are widely used because of its simplicity and its low fabrication cost. Microstrips consist of a metal strips and a ground metallic sheet separated by a layer of dielectric as shown in Figure 4. Once the PCB material is fixed, i.e., the dielectric constant and thickness h of the board are chosen, the characteristic impedance of the mircostrip is determined by the width w of the line. Wide strip has a lower impedance than that of narrow strip.
Figure 4: Microstrip line geometry