RF and Microwave Engineering: Fundamentals of Wireless Communications

4.4 Stripline

A stripline (triplate) transmission line consists of a flat metallic trace centred between two infinite ground planes. The trace width is w and the distance between the ground planes is b (see Figure 4.9). The entire space between the ground planes is filled with a homogeneous dielectric material with a relative permittivity of ε_r. In real striplines the substrate width b_s is finite and the centre strip conductor has a thickness t. The fundamental wave mode is a TEM wave. The transversal magnetic field circles around the centre strip conductor. The electric field points from the strip conductor to ground.

Figure 4.9 Geometry of a stripline and electric and magnetic field distribution of a TEM wave.

4.4.1 Characteristic Impedance

In the literature we find approximate formulas for the characteristic impedance of striplines. A compact formula from [12] gives us

4.34

with an effective trace width w′ of

4.35

Example 4.7 Line Impedance of a Stripline

A stripline is given with the following parameters: substrate height b = 2h + t = 1.6 mm, relative permittivity ε_r = 2.5, trace width w = 1.17 mm and strip thickness t = 10 μm. For the auxiliary parameters we get m = 1.9916 ...